Seal Weld Symbol: How to Read, Specify, and Inspect Seal Welds on Any Drawing

A fabrication shop in Ohio welded exactly what the print showed: a nice, neat 1/8 in. fillet running all the way around a column base. Twelve months later, rusty water was weeping down the painted steel and the owner wanted to know who was paying. The drawing had the weld, the size, and the length — but the two words that actually mattered, SEAL WELD, got dropped from the tail of the symbol when the detail was redrawn. Nothing on that sheet was technically wrong. It was simply incomplete, and nobody on the shop floor could see what wasn’t there.

Why a Tiny Note in the Tail of a Welding Symbol Can Sink a Project

A fabrication shop in Ohio welded exactly what the print showed: a neat 1/8 in. fillet running all the way around a column base plate, ground smooth in places, dressed in others, the kind of weld a welder is happy to sign. Twelve months later, rusty water was weeping down the painted steel and staining the concrete below, and the owner wanted to know who was paying for the remediation.

The drawing had the weld. It had the size. It had the length. What it did not have was the phrase SEAL WELD in the tail of the welding symbol, because the detail had been redrawn during a value-engineering round and the note got dropped in the translation. Nothing on that sheet was technically wrong. It was simply incomplete, and nobody on the shop floor could see what wasn’t there.

That is the strange, expensive problem with seal welds: the requirement to seal is invisible unless someone writes it down.

The Stakes Are Bigger Than the Weld

It is easy to dismiss a seal weld as a minor item, a courtesy pass around a joint. But consider what actually goes wrong when a sealing requirement is missed or misread:

• Water intrusion and corrosion in structural steel. Moisture and oxygen-laden air creeping into an unsealed cavity is the classic structural failure mode. In mechanical and structural work, seal welds exist mainly to keep the outside environment out of a hollow section, lap joint, or box member where corrosion starts invisibly and progresses for years.
• Leaks in process piping. Under ASME B31.3 and similar codes, a seal weld is a specified supplement to a threaded or partial-penetration joint in services where tightness matters. Miss it, and you have a fugitive emissions problem, a safety hazard, or a contaminated product.
• Failed galvanized assemblies. Trapped liquids or gases expand violently in molten zinc, blowing out welds and creating porosity. A seal weld that was supposed to be porosity-free turns porous, and the Class 1 joint it was protecting fails inspection after coating, not before.
• Disputed change orders. Nobody budgeted for the extra pass. The fabricator says the print did not call for it. The engineer says the intent was obvious. Now the project has a paper trail longer than the weld itself.

Why There Is No Seal Weld Symbol

Here is the central tension this article exists to untangle: a seal weld is a function, not a weld type. AWS A3.0, Standard Welding Terms and Definitions, defines a seal weld as “any weld designed primarily to provide a specific degree of tightness against leakage.” Notice the wording. It says nothing about process, nothing about a specific joint geometry, nothing about a shape you could draw. ASME Section IX (QW/492) echoes the same definition, and the tightness language of ASME B31.3 process piping rules matches it, so the codes agree with each other on the concept.

Because a seal weld is a function, it can be delivered by SMAW, GMAW, FCAW, or GTAW, with a fillet symbol, a groove symbol, or a sealing run on the root side. AWS A2.4:2020 recognizes this directly, devoting clause 7.12 specifically to seal welds while clause 6.12 covers the tail of the welding symbol, the little V-shaped fork where notes like SEAL WELD, a WPS number, or a specification reference live. The symbol itself stays a fillet or a groove. The sealing intent rides along in the tail.

That small design decision has enormous field consequences, because it means a seal requirement can be dropped as easily as a comment in a PDF. And unlike a missing fillet, a missing word rarely triggers a weld map discrepancy. It just triggers a leak twelve months later.

Improvised Seals Are Their Own Problem

The opposite mistake is just as common. A welder on the floor notices a gap that “ought to be sealed” and runs a quick bead around it. No drawing line, no note, no approval. AWS D1.1 clause 6.5.1 puts the burden squarely on inspection: the inspector must verify that no unspecified welds have been added without approval. An unapproved seal weld is an unapproved weld, full stop, and it can invalidate the structural assumptions behind a connection designed to move.

So both directions of the error hurt. Forget the seal and you get corrosion and leaks. Improvise the seal and you get code violations, distorted connections, and disputes.

What You Will Be Able to Do After Reading This

By the end of this guide, you will be able to locate where a seal requirement actually lives on a drawing, read the six most common seal weld scenarios the way an inspector does, understand where AWS A2.4, AWS D1.1, and the ASME codes collide on sealing practice, spot the hidden risks from unintended load paths to galvanizing blowouts, and specify seal welds so nothing critical depends on anyone’s memory. If you are still building your foundation, start with our welding symbol chart and the guide to how to read welding symbols, then come back here for the part the chart cannot teach you: the meaning that hides in eleven characters in a tail.

What Exactly Is a Seal Weld? Definitions, Purpose, and the Weld vs. Symbol Confusion

Now that we know how a missing note can lead to leaks, it’s important to understand exactly what a seal weld is. The confusion starts here for many welders and detailers alike, as the term is often thrown around without a true understanding of its meaning. In reality, a seal weld isn’t a specific weld bead shape or a proprietary process, but rather a job that the weld is tasked to perform.

Defining It by the Codes

To clear up any ambiguity, it’s best to look towards the industry codes that define it. Across multiple governing bodies, the wording is nearly identical. AWS A3.0 defines a seal weld as one that is designed primarily to provide tightness against leakage. This mirrors ASME Section IX QW-492, which defines a seal weld as “any weld designed primarily to provide a specific degree of tightness against leakage.” Additionally, ASME B31.3 defines a seal weld as “a weld intended primarily to provide joint tightness against leakage in metallic piping.”

The AISC also acknowledges this function in building construction, noting that seal welds commonly assume a fillet weld profile, and that any aesthetic requirements must be spelled out within the contract documents. With all of these definitions agreeing with one another, the takeaway is simple: tightness against leakage is the priority.

The Two Core Purposes

While the definition stays consistent, the reasoning behind needing a seal weld can vary drastically from job to job. These purposes typically fall into two categories:

• Containing a fluid: Used when a joint must hold a gas or liquid inside of a vessel or pipe. This is most common in pressure systems.
• Excluding a fluid: Used to keep moisture, air, or other substances out of a cavity. This is by far the more common structural use, preventing long term corrosion from water intrusion.

Regardless of which purpose your job calls for, the function remains the same. This is what separates a seal weld from a structural strength weld, as strength may be secondary in some cases, yet tightness is always the goal.

The Weld Symbol vs. The Welding Symbol

One of the largest sources of confusion stems from terminology. As laid out by AWS A2.4, there is a formal distinction between the two:

• Weld symbol: The small shape that represents the type of weld. For example, the triangle representing a fillet weld, or the groove shape representing a groove weld.
• Welding symbol: The complete assembly. This includes the arrow, reference line, weld symbols, dimensions, supplementary symbols, finish marks, and the tail. It is the full set of instructions for the welder.

This distinction is crucial for seal welds. Since a seal weld is a function, not a weld type, there is no dedicated seal weld symbol. Instead, the same fillet or groove symbol is utilized, with the requirement of “SEAL WELD” layered on top, typically within the tail of the welding symbol.

Function Over Process

Another important clarification is that sealing is not tied to any specific welding process. A seal weld can be deposited by SMAW, GMAW, FCAW, or GTAW, so long as the resulting weld is capable of providing the required tightness. The procedure, not the process title, is what will determine if that tightness is achieved.

Seal Weld vs. Seam Weld

Lastly, we must kill the most common misconception: a seal weld is not the same as a seam weld. A seam weld is its own weld type, most often produced through resistance welding processes, and it has its own distinctive symbol. Seal welds, on the other hand, rely on fusion welding to seal a joint. To dive deeper into the various joints and processes, take a look at our Pipe Welding Procedures Guide, which breaks down these methods even further.

Anatomy of a Welding Symbol: Arrow, Reference Line, Tail, and Supplementary Symbols

Before you can chase a seal weld requirement across a drawing, you need to know where it hides. A complete welding symbol is compact technical grammar: a few marks that, read in the right order, tell a welder what to weld, on which side, how big, how long, and to what specification. The Miller Welds guide to reading weld symbols treats the symbol as an instruction rather than decoration, and that framing matters. Learn the parts once and you can decode almost any sheet you are handed.

The Arrow: Defining the Joint and the Arrow Side

The arrow is the leader that points to the joint being welded, and it does more than point: it fixes which surface is the arrow side. The surface the arrow touches is the arrow side; the surface on the far side of the joint is the other side. Every placement decision that follows depends on this single distinction, so get it wrong and the weld ends up on the wrong face of the plate. On joints where both sides receive the same weld, a broken (zigzag) leader is used.

The Reference Line: The Backbone of Every Instruction

The reference line is the horizontal backbone from which the arrow extends, and everything that instructs the welder hangs off it: the weld symbol itself, the dimensions, the supplementary symbols, and the tail. Nothing floats free. In AWS A2.4, a weld symbol placed below the reference line applies to the arrow side, and a symbol placed above it applies to the other side. ISO handles this differently, a contrast we return to shortly because it is a frequent source of confusion on mixed drawings.

Basic Weld Symbols and Dimensions

The basic weld symbol is the small shape that identifies the weld type: a triangle for a fillet, a V or bevel for groove welds, a circle for plug or slot. Around it sit the numbers that make it buildable — leg size or effective throat, weld length, pitch for intermittent patterns, and the number of welds when several share a run. A triangle with no size next to it is an incomplete instruction, not an invitation for the welder to choose.

Supplementary Symbols: Modifiers That Change Meaning

Supplementary symbols sit at the junction of the arrow and reference line or along the reference line itself, and they modify the instruction:

• Weld-all-around circle — weld the full perimeter of the joint.
• Field weld flag — weld on site rather than in the shop.
• Contour and finish marks — flush, convex, or concave finishing, with a letter specifying the method.
• Melt-through — required root reinforcement on the back side, used with single-sided groove welds.
• Backing weld or back weld — a supplementary run deposited to support or follow the main weld.
• Spacer — a strip of metal placed in the joint.
• Consumable insert — a preplaced strip that melts into the root.

Worth noting for anyone reading international drawings: AWS A2.4 contains four symbols that have no ISO equivalent — the spacer, back weld or backing weld, melt through, and consumable insert. If you see them, you are almost certainly looking at an AWS-formatted symbol.

The Tail: Where SEAL WELD Lives

The tail is the small V-shaped fork, drawn like a greater-than or less-than sign, at the end of the reference line opposite the arrow. It exists for one purpose: to carry information for which the standard makes no other provision. That includes a WPS number, a welding process designation, a governing specification, or a plain-language note such as SEAL WELD. The tail is optional — when there is nothing extra to say, it is simply omitted. ISO 2553 describes the same content as complementary information: non-symbolic information relevant to the weld that may be placed in the tail. The triangle tells you the weld shape; the tail tells you the weld’s job, which is why reading it is non-negotiable.

AWS A2.4 vs. ISO 2553: Two Systems, Never Mixed

ISO 2553:2019 recognizes two placement systems. System A uses a dual reference line, one continuous and one dashed, and places the arrow-side weld on the continuous line — the opposite convention to AWS. Under ISO 2553, the dashed line always corresponds to the arrow side, which is why the two standards read as mirror images. System B places symbols above and below a single reference line in a way that reverses System A again. The rule that prevents disasters is simple: systems A and B must never be mixed on a single drawing, and a drawing set should state which convention it uses, as flagged in this ANSI note on welding symbol changes.

Read the symbol in order — arrow, reference line, basic symbol, dimensions, supplementary marks, then the tail — and the instruction resolves every time. It is at that last stop that a seal weld either appears or quietly disappears.

Reading a Seal Weld Requirement on Real Drawings: Six Common Examples

Symbol charts are clean; prints never are. On a real drawing the seal requirement arrives as a mix of geometry, dimensions, and prose typed into a detail five revisions ago. These six scenarios show up most in structural and piping work: find the arrow, find the reference line, then read the tail before you fall in love with the triangle.

Example 1: The Fillet Triangle With SEAL WELD in the Tail

The arrow points at a lap or T-joint, the reference line carries a fillet symbol on the arrow side sized, say, 1/8 in., and the V-shaped tail reads SEAL WELD. The triangle gives the shape; the tail gives the job. AWS A2.4 reserves the tail for a process designation, a specification, a WPS number, or a note no standard symbol can express. Drop that tail during a redraw and the shop welds an ordinary fillet.

Example 2: Weld-All-Around Circle Plus a Seal Note

Here the fillet symbol is paired with a circle at the arrow-and-reference junction, and the tail still says SEAL WELD. One symbol covers the whole perimeter, and that is also the risk: the AISC engineering FAQ notes that the weld-all-around symbol can drive excessive work on short edges and awkward contours, and prefers defining the specific joints needing welds.

There is history too: up until 2015, AWS D1.1 prohibited tying welds around out-of-plane corners, and when that ban was deleted it was replaced with a caution, partly to permit the seal welds needed on galvanized assemblies. A seal weld around a corner is hard to deposit continuously, and undercutting there is likely.

Example 3: Square-Groove Butt Joint With a Backing or Sealing Run

Square groove symbols on a butt joint sometimes carry a second symbol on the other side of the reference line: a backing or back weld run along the root. That root pass is what actually seals. AWS A2.4 includes four symbols ISO 2553 does not use, and the back weld is one of them, so drawings mixing conventions can quietly lose the sealing pass. When the seal is the point, the tail should name the sealing run’s size and side. Procedure detail matters, which is why pipe welding procedures qualify root passes as their own operation.

Example 4: Single-V Groove With the Melt-Through Symbol

When a groove is welded from one side and the design demands complete joint penetration, the drawing may carry a melt-through symbol: a heavy parallel bar on the side of the reference line opposite the main weld symbol. That bar proves intent: full penetration and visible root reinforcement on the far side, often the sealing surface itself. A single-V symbol without melt-through leaves penetration to interpretation, and interpretation is where leak paths are born.

Example 5: Continuous Versus Intermittent Seal Welds

Intermittent welds carry a leg size, a length, and a pitch: 1/4-2 in., 3 in. staggered. That notation means gaps, and gaps are capillary highways for water, vapor, and product. A seal weld must be continuous, so a compliant symbol carries a length but no pitch, and the tail or a note adds CONTINUOUS. If you see a pitch near a seal requirement, stop and resolve it before metal is cut.

Example 6: Pipe Branch Connection With an All-Around Circle

A branch outlet on a header with a fillet symbol and an all-around circle can be a structural attachment or a seal, and the difference comes down to one word in the tail. Structurally, the fillet carries load; functionally, it keeps fluid out of the joinery. The Lincoln Electric design file and ASME B31.3 wording both hinge on that distinction, and a gas or process line makes the stakes obvious. The procedure matters too, as covered in gas line welding code and purging guidance.

The Cautionary Tale: When “SEAL” Was Just Decoration

On one project, an engineer printed the word SEAL in the tail of a square butt weld symbol. The fabricator read the groove symbol itself as the sealing requirement, ran a single-pass root, and left the joint with the very porosity the note existed to prevent. Nobody was wrong about the geometry; everyone was guessing at intent.

Want a faster way to spot these patterns? Our welding symbol chart lays the anatomy out side by side, and Miller’s guide to reading weld symbols walks through the arrow-side and other-side conventions. Read the tail first, every time.

AWS A2.4, D1.1, and ASME: Where Seal Weld Symbols Collide With the Codes

Three code bodies govern a single seal weld, and none of them govern the same thing. AWS A2.4 controls how the requirement is drawn, AWS D1.1 controls how it is welded in structural steel, and the ASME codes control how it is executed and tested in pressure piping and vessels. Write SEAL WELD in a symbol tail without checking what D1.1 or ASME already require at that exact joint, and the drawing can demand something the code forbids.

What A2.4 Actually Controls

The American Welding Society originated the standard for welding symbols, and ANSI approved it as ANSI/AWS A2.4, now in its 2020 edition. Part A covers welding symbols, Part C covers nondestructive examination symbols, clause 6.12 defines the tail as the place for process designations, WPS references, specification callouts, and other notes, and clause 7.12 is where the standard recognizes the SEAL WELD note as legitimate notation.

Crucially, A2.4 permits the notation but does not size it, qualify it, or exempt it from anything. It is a drafting language; once the drawing leaves the detailing office, D1.1 or ASME is the referee.

Conflict 1: Welds on Opposite Sides of a Common Plane

Seal welds run continuously around corners, which is the whole point, but AWS D1.1 clause 2.4.7.5 requires welds on opposite sides of a common plane to be interrupted at the corner. A continuous seal wrapped around a corner violates that rule directly. Add a weld-all-around circle and a SEAL WELD note to a cap plate detail and you have produced a drawing the shop cannot weld as detailed.

Conflict 2: Stiffener Welds and the 4T-to-6T Rule

AWS D1.1 clause 2.4.7.4 requires stiffener welds to be cut short not less than four times and not more than six times the web thickness from the weld toe of the web-to-flange weld. That gap preserves flexibility at the flange and avoids a brittle triaxial restraint zone. Running a seal weld straight through it defeats the provision and puts the stiffener back into exactly the restraint condition the code was written to avoid.

Conflict 3: Flexible Connections

Seated beam connections and framing angles depend on the flexibility of the outstanding angle legs, and D1.1 restricts welding that would destroy it. Seal the outstanding leg and the connection becomes a rigid moment-resisting joint nobody analyzed.

Conflict 4: Minimum Fillet Weld Sizes

AWS D1.1 Table 5.8 sets minimum fillet weld sizes from the thickness of the thinner part joined, and the reason is metallurgical, not bureaucratic. An undersized weld deposits too little heat for the joint thickness, the cooling rate shoots up, and the heat-affected zone can transform to hard, brittle martensite. The result is hydrogen cracking risk in a weld meant only to keep water out. An exemption from Table 5.8 must be written into the contract documents; those minimums carry a purpose a casual note cannot override.

Conflict 5: Bearing Connections Cannot Share Load

AWS D1.1 clause 2.6.3 does not allow bolts and welds to share load in bearing connections. A small seal weld added around a bolted connection typically fractures first, before the bolts load up, and the seal is lost. The bolts keep working; the seal quietly stops sealing. AISC’s engineering FAQ on welding makes the same point from the design side.

When the Codes Prohibit Seal Welds Outright

ASME rules govern pressure piping and vessels, and in some services seal welding is restricted or prohibited outright. The National Board is explicit: where threaded joints are seal welded, thread sealing compound must not be used, because the compound burns, outgasses, and undermines the very joint it was meant to help. Nuclear service is stricter still, so confirm the service allows a seal weld before specifying one on threaded pipe. Execution guidance lives in our pipe welding procedures guide and this piece on gas line welding codes and purging rules.

Where Seal Welds Show Up: 8 Industries That Depend on Them

Seal welds rarely make headlines, but they show up almost everywhere fluids, weather, or contaminants could cause trouble. Knowing which industry you are working in tells you what the seal is supposed to exclude, what it must contain, and what typically goes wrong when it is treated as an afterthought.

1. Process Piping and Refinery Work

Under AWS A2.4 and ASME B31.3, seal welds in piping support joint tightness in threaded and socket connections carrying flammable or hazardous service. Here the goal is containment: keeping the product inside the run. What goes wrong is almost always root-side — a seal run laid over a dirty thread, or a thread sealing compound applied before welding. Pipe Welding Procedures covers the WPS and inspection side of this work in detail.

2. Pressure Vessels and Tanks

In vessels, the seal weld is genuinely a pressure boundary item. Nozzle attachments, nameplate pads, and external lugs must not become leak paths. The usual failure is a start-stop crater at a corner, or a fillet run too small to bridge a fit-up gap, and the leak shows up only during hydrostatic test — the most expensive time to find it.

3. Structural Steel Buildings

Here the purpose flips. Nobody is holding pressure in a wide-flange column. The job is excluding moisture from an enclosed cavity so it cannot sit against an unpainted surface and rust. The subtlety is that a seal weld added casually to a bolted connection can change load paths, which is a risk we unpack in the next section.

4. Hot-Dip Galvanized Fabrications

This is where seal welds carry the most hidden risk, because molten zinc finds every gap. The American Galvanizers Association classifies joints three ways:

• Class 1: held together by a full seal weld, described by the AGA as the highest degree of corrosion protection attainable for a welded assembly.
• Class 2: uses seal welds but requires venting through an escape hole when the overlapping area exceeds 16 square inches (100 square centimeters), so expanding trapped gas can escape during immersion.
• Class 3: not seal welded, relying on coatings and drainage.

A Class 2 detail can be upgraded to Class 1 by plugging the vent hole after galvanizing. The classic failure is skipping the vent and cracking the joint as trapped air expands in the 840°F zinc bath. Our guide to galvanized steel welding walks through the venting and repair sequence.

5. Food, Beverage, and Pharmaceutical Stainless

The fluid being excluded here is product and cleaning solution, and the enemy is the crevice. An unsealed lap joint traps residue, breeds bacteria, and drives crevice corrosion in stainless. The weld failure is usually overlap or lack of fusion at the toe, which creates exactly the crevice the seal was meant to eliminate.

6. Water and Wastewater Treatment

Basins, weirs, and clarifier structures seal against constant, low-pressure water contact. What goes wrong is undercut at corners and crater pits on termination, both of which hold water against bare steel after the coating is breached. Details about that defect are covered in Welding Defects: Undercut.

7. Automotive, Rail, and Marine Assemblies

These are weather-sealing applications: excluding road spray, ballast water, and salt air from body cavities. Automotive joining leans heavily on resistance processes, which is why the spot welder physics distinctions matter — a spot weld is not a seam, and neither is a seal.

8. HVAC, Ductwork, and Nuclear Threaded Joints

Duct seal welds keep conditioned air from escaping lap seams. At the far end of the spectrum, nuclear and high-energy services often restrict or prohibit seal welding of threaded joints altogether, which is why you must verify service category before the symbol ever reaches the shop floor.

Across all eight, the pattern holds: the seal is a function, the industry defines what that function must survive, and the detailer who understands both writes a drawing nobody has to interpret.

The Hidden Risks: Unintended Load Paths, Galvanizing Explosions, and Inspection Blind Spots

A seal weld is easy to underestimate: small, fast, and apparently harmless beside a full-penetration groove weld. That is exactly why it does outsized damage when it goes wrong. Shops size seal welds by eye, add them on the fly, and treat them as a courtesy instead of an engineered feature. Each habit has cracked connections, ruptured assemblies in a galvanizing kettle, defeated ultrasonic scans, and started change-order arguments nobody budgeted for.

A Load Path Nobody Drew

Take a bolted bearing connection where someone decides a seal weld around the plate edge is a good idea. The connection was designed so the bolts carry the load. A continuous fillet along the same faying surface quietly installs a parallel load path the calculations never assumed.

Because a seal fillet is small, it is stiff for its size. As the joint deforms under load, the weld picks up stress the bolts should have carried, fractures first, and the seal is lost, with a crack now sitting inside a connection nobody is inspecting. AWS D1.1/D1.1M is explicit that bolts and welds may not share load in bearing-type connections. The seal weld does not strengthen the joint; it decides who fails first.

Flexible Connections Stop Being Flexible

Seated beam connections, framing angles, and end plate connections are detailed to rotate slightly. That movement is a design feature, absorbed by slip at bolted interfaces and gentle bending of outstanding angle legs. Seal welding those interfaces makes them rigid, so the rotation demand lands in the weld or the angle legs, neither sized for it. The predictable outcome is a fractured seal weld or a cracked leg, right where water collects.

The Ultrasonic Blind Spot

Then there is inspection interference, the quietest risk of all. When backing or plates are seal welded to a member, the seal weld metal sits directly in the ultrasonic beam path. As the James F. Lincoln Foundation design guidance describes, this can result in the inability to transmit ultrasound through the entire cross-section of the groove weld.

Galvanizing: When Trapped Zinc Turns Violent

Hot-dip galvanizing is where seal welds stop being a nuisance and become a safety issue. Overlaps and closed cavities trap air and moisture. Lower the assembly into molten zinc at roughly 450 °C (840 °F) and trapped water flashes to steam, expanding more than a thousandfold in volume. A cavity that cannot vent builds pressure until it ruptures, spraying molten zinc out of the kettle.

The American Galvanizers Association requires a vent hole whenever the overlapping area exceeds 16 square inches (100 square centimeters), with hole sizes and locations in its venting guidance. The seal weld itself must be porosity-free on Class 1 joints, because a porous bead lets zinc seep into the overlap and stay trapped in service. Crews near the kettle also need full splash protection, as covered in our welding PPE guide.

The Workmanship Slide

Seal welds are typically made at higher travel speed and lower heat input than structural welds, yet long continuous runs still twist and bow a piece after it is squared and painted. A fast, cold, undersized bead is the classic recipe for a hard heat-affected zone and cracks at terminations, the same patterns seen in undercut and related weld defects. Leave the size to the welder and “seal weld” means whatever leg length felt right that afternoon.

The Code Trap and the Contract Trap

There is also the compliance trap: a required seal weld can force a violation of another provision, such as minimum fillet sizes tied to base metal thickness or stiffener welds that must stop clear of the web-to-flange toe. Under D1.1, inspectors must confirm no unspecified welds have been added without approval, so improvised seal welding is an acceptance issue, not a cosmetic one. Contractually, seal welds added after fabrication and coating are work nobody pays for, because they were never on the approved drawing.

Failure Modes at a Glance

None of this is an argument against seal welds. It is an argument for treating a one-word note in a symbol tail with the same discipline as the groove weld beside it. Small weld, real consequences: every one of these failures traces back to a drawing that left something unsaid.

How to Specify Seal Weld Symbols Correctly: Best Practices for Engineers and Detailers

Everything before this has been about reading seal weld requirements; this is about writing them. The shop in the story at the start of this article did nothing wrong; the instruction was simply missing. Put the detail on the sheet and the fabricator can price, weld, and inspect it. Leave it off and you get a nice fillet and a leak.

First, Ask Whether You Need a Seal Weld at All

The American Galvanizers Association and AISC both note that many sealing problems are solved more cheaply with a gasket, sealant, drain hole, or vent. AISC’s engineering FAQs are blunt: seal welds in buildings rarely hold internal pressure and are frequently replaced with clear silicone caulk after painting. Welding adds heat, distortion, residual stress, and inspection burden, so specify it only when joint geometry, service fluid, or galvanizing genuinely requires a metallic seal, and write down why.

Put the Requirement in the Tail, and Give the Tail a Number

There is no seal weld symbol, so the requirement lives in the tail of the welding symbol, as AWS A2.4 provides for. The two reliable notations are SEAL WELD written in the V-shaped tail, or a general note such as “SEAL ALL JOINTS AS NOTED” with a leader to the affected locations. The word SEAL floating alone near the joint is the ambiguity that burned the fabricator in section 4.

Better still, put a procedure reference in that same tail. AWS A2.4 explicitly recognizes a reference to the approved welding procedure specification as an accepted way to convey a joint’s full welding requirements. A tail reading SEAL WELD, WPS PQR-14, 1/4 in. fillet, continuous removes almost every remaining question. If the sealing run is a bare back or backing weld rather than a fillet, AWS’s system allows BACK WELD or BACKING WELD in the tail of the reference line. Our guide to pipe welding procedures and WPS documentation covers that paperwork.

Define Size, Length, Continuity, Profile, and Acceptance Criteria

Never leave size to the welder. A seal weld sized on the floor runs at high travel speed with the smallest acceptable throat, which is exactly the profile that leaks. Specify all five:

• Size leg length or throat, stated explicitly.
• Length continuous, or the extent of the sealed run.
• Continuity seal welds are continuous by definition; an intermittent pattern defeats the purpose.
• Profile seal welds commonly take a fillet profile, and AISC advises putting any exposed-edge aesthetic requirement into the contract documents.
• Acceptance criteria the visual acceptance class, and whether undercut is permitted, a classic seal killer at corners and terminations. Limits are covered in our guide to undercut causes and acceptance limits.

The Minimum-Size Question: An Exemption Must Be Written Down

Drawings often carry a note reading “minimum weld size does not apply to seal welds.” It is an engineer’s written exemption from the AWS D1.1 minimum fillet weld sizes, not a code default. Those minimums exist for a metallurgical reason: thicker welds carry enough heat input to slow cooling and prevent a hard, crack-prone heat-affected zone, so undersizing a seal weld on thick material can trade a leak for a crack. Want the exemption? Write it into the contract documents, name the joints it covers, and give the minimum size you will accept.

Venting, Sequencing, and the Weld-All-Around Shortcut

For hot-dip galvanized assemblies, venting is part of the specification: trapped liquid or gas expands on immersion in molten zinc, so a poorly vented cavity is a deformation or explosion waiting to happen. Confirm vent and drain provisions before the sealing weld goes down; the American Galvanizers Association publishes the guidance for these joints.

Sequence matters: seal weld before coating, never through paint. A seal weld burned over painted or galvanized filler is a quality defect and a fume and fire hazard in a coating system, which is why OSHA’s 1910.252 hot-work rules are strict about coatings and combustibles. The weld-all-around circle is no substitute for the words: it gives the extent of the weld, not the sealing function.

Treat the Seal Weld Like Any Other Production Weld

Seal welds need an approved WPS, qualified welders, and an inspection plan. AWS D1.1 also has the inspector confirm that no unspecified welds were added without approval, so a seal weld that appears without a drawing note is an approval problem. Any ground or dressed finish must be priced into the contract.

Get those items right and everyone reads the same instruction.

Inspection and Leak Testing: Proving a Seal Weld Actually Seals

A seal weld can look perfect and still leak. That is the uncomfortable truth behind ASME Section IX’s definition, which describes a seal weld as a weld designed primarily to provide a specific degree of tightness against leakage rather than to carry structural load. The acceptance question, therefore, is not “does it meet a fillet size table?” It is “does it hold the fluid out, or in?” Proving that takes a different inspection mindset than checking a structural fillet.

Start With the Eyes: Visual Examination

Ninety percent of seal weld failures are visible before any instrument comes out. The defect list that actually defeats a seal is short and consistent:

• Porosity and pinholes, especially crater pits at the stop-start points where the welder broke the arc.
• Incomplete fusion at the root or along the joint face, leaving a hairline path straight through the seal.
• Overlap (cold lap), where weld metal sits on the surface without actually wetting into it.
• Excessive undercut at corners and terminations, which creates a channel right where the seal is most vulnerable.
• Inadequate terminations, where the run simply stops short of the required continuity.

All of these are surface or near-surface conditions, which is why a disciplined visual examination is the first and cheapest filter. Undercut deserves particular attention: even on an exempt seal weld, engineers generally expect profile and undercut acceptance criteria to still apply, precisely because a leak-tight result depends on them. Our guide to welding undercut causes and acceptance limits covers how to judge those limits in practice.

The Leak-Detection Toolkit

Once visual inspection passes, the method you choose depends on sensitivity, geometry, and what you are allowed to do to the assembly.

Dye penetrant testing is the workhorse for shop inspection, and it pairs naturally with pressure-based methods for a complete picture. For piping work, the pressure test is the definitive statement, and our pipe welding procedures guide walks through how inspection fits into the overall WPS and pass sequence.

When Ultrasonic Testing Fails You

There is a specific instrumentation trap here that designers routinely miss. The commentary to AWS D1.1 notes that when a contract requires seal welds on all steel backing, that seal weld can prevent ultrasonic examination of the full groove weld cross-section. Sound waves cannot read through a bond they were never calibrated to cross. Where penetration must be verified, specify radiographic testing or surface methods instead, and say so in the documents rather than hoping the contractor figures it out.

Documentation: Criteria Exist Even When Size Does Not

A seal weld exempt from minimum fillet sizes is not exempt from inspection criteria. Write down the profile requirements, the undercut limits, the continuity expectation, and who makes the reject-or-repair call. A “technical reject” decision with no written basis behind it becomes an argument on the shop floor, and those arguments rarely end well. Note also that qualification does not disappear: welders and procedures are still qualified under ASME Section IX or the applicable AWS requirements even for non-structural work, and the National Board reinforces that tightness is an engineered outcome, not an accident of workmanship.

The Seal Weld Symbol Checklist: Quick Reference and Final Takeaways

You have made it through symbol anatomy, code collisions, six drawing scenarios, and the leak-testing toolkit. Now let’s compress all of it into something you can actually carry to the drawing table or the shop floor. Whether you are a detailer checking a release package or a welder reading a fit-up before striking an arc, this closing checklist is the part of the article worth printing out.

The Nine-Point Seal Weld Checklist

1. Ask whether the seal weld is needed at all. Before anyone picks up a torch, ask what is being contained or excluded. A caulk bead, a gasket, a weep hole, or a vent detail often solves the problem at lower risk and lower cost than a weld. AISC’s engineering FAQ is blunt about how often building seal welds are replaced by silicone caulk after paint.
2. Check for a note elsewhere on the drawing. General notes, detail bubbles, and schedule columns often carry the sealing requirement that the symbol itself omits. Read the whole sheet before concluding that a seal weld is missing, and before concluding that one is required.
3. Read the tail before you read the triangle. The fillet triangle tells you a weld is required. Only the tail, referenced per AWS A2.4 clause 6.12, tells you that the weld is a seal. A triangle alone is never, ever a seal weld.
4. Confirm whether minimum size and inspection criteria apply. AWS D1.1 minimum fillet sizes carry a metallurgical purpose: enough heat to slow cooling and avoid a hard, crack-prone heat-affected zone. Any exemption must be written into the contract documents, not assumed.
5. Confirm continuity. A seal weld is continuous. An intermittent pattern defeats the entire purpose, no matter how the symbol looks on paper.
6. Check corner conditions. If the seal weld runs around a corner, verify it against D1.1’s provisions on interrupting welds on opposite sides of a common plane, and against stiffener and flexible-connection rules.
7. Check venting for galvanized assemblies. If the overlap exceeds 16 square inches (100 square centimeters), a vent is required regardless of how well the seal weld is made.
8. Verify the coating sequence. Welding through paint is not acceptable practice. Seal welds on structures headed for paint or galvanizing are made before coating application, full stop.
9. Confirm a WPS and a qualified welder. Even a non-structural sealing run is a welded joint. Procedures and welders remain qualified under AWS or ASME Section IX requirements. Our guide to pipe welding procedures and WPS qualification covers what that paperwork should actually contain.

Four Takeaways Worth Remembering

Strip away the details and the article reduces to four sentences. A seal weld is a function, not a weld type. AWS A3.0 defines it as any weld designed primarily to provide a specific degree of tightness against leakage, which is why no dedicated symbol exists. The requirement lives in the tail or a note, never in the triangle. If the tail was dropped in a redraw, the sealing instruction simply vanished with it. Small welds carry real consequences — structural load paths, galvanizing pressure buildups, blocked ultrasonic readings, and disputed change orders all trace back to a bead the size of a fingertip. And the cheapest defense is a clearly specified drawing, because every dollar spent on a legible tail saves hundreds downstream.

The Eleven Characters That Cost Thousands

Back to that column base in Ohio. The corrective work involved cutting away sound weld metal, drying out a saturated connection, re-detailing the joint, re-inspecting it, and repainting a section of a building the owner had already accepted. The bill ran into four figures and consumed weeks of a project schedule that had no slack to spare.

The words that would have prevented all of it were SEAL WELD. Eleven characters. Roughly a second and a half of typing when the detail was originally drawn, and another second of reading before the shop released the piece.

That is the real lesson of the seal weld symbol: it is not a drafting technicality. It is a communication problem wearing a metallurgy costume. Learn to read the tail, insist on writing it, and the next twelve months of weeping rust will belong to someone else.

Frequently Asked Questions

Is there an official seal weld symbol in AWS A2.4 or ISO 2553?

No. Neither AWS A2.4 nor ISO 2553 defines a unique symbol shape for a seal weld, because a seal weld is a weld function rather than a weld type. The sealing requirement is conveyed instead through the tail of the welding symbol, a general note on the drawing, or a sealing run, backing run, or melt-through detail. You will still use the normal fillet, groove, or other basic weld symbol to describe the geometry.

Where do you write SEAL WELD on a welding symbol?

It goes in the tail, the forked or V-shaped element at the end of the reference line opposite the arrow. The tail is reserved for information that has no other designated place, such as a WPS number, a welding process, a specification reference, or a note like SEAL WELD. If a note on the drawing applies to every seal weld in the project, a general note is equally acceptable and often cleaner.

What is the difference between a seal weld and a seam weld?

A seal weld is any weld whose primary job is providing a specific degree of tightness against leakage, and it can be a fillet or groove weld made by common arc processes. A seam weld is a distinct weld type with its own symbol (a circle with two parallel lines) describing a continuous or intermittent weld along an overlapping joint. The similar names cause frequent confusion on the shop floor.

Does a seal weld need a welding procedure specification (WPS)?

Best practice is yes. Seal welds are frequently small, fast, and treated casually, but they are still permanent welds that transfer some load and affect heat input, distortion, and the heat-affected zone. Giving them a WPS, a qualified welder, and defined inspection criteria removes ambiguity and prevents the common metallurgical problems caused by undersized, fast-travel welds.

Can a seal weld be smaller than the AWS D1.1 minimum fillet weld size?

Only if the engineer and the contract documents say so. AWS D1.1 Table 5.8 minimum fillet weld sizes exist for a metallurgical reason: enough heat input to slow the cooling rate and avoid a hard, crack-prone heat-affected zone. Many drawings carry a note stating that minimum weld size does not apply to seal welds, but that is an explicit project exemption, not a code default.

Why do galvanized assemblies with seal welds need vents?

Because trapped liquid or gas inside a sealed overlap will vaporize and expand rapidly when the part is dipped into molten zinc, creating an explosion and quality risk. The American Galvanizers Association requires a vent whenever the overlapping area exceeds 16 square inches (100 square centimeters). The vent can often be plugged after galvanizing to restore full corrosion protection.

Will a seal weld affect ultrasonic or radiographic inspection?

It can. AWS D1.1 commentary notes that seal welds on steel backing can prevent ultrasound from passing through the entire cross-section of a groove weld, making ultrasonic testing unreliable for that joint. Where full penetration must be verified, radiographic or surface examination methods are usually substituted, and the inspection method should be specified on the drawing.

Can silicone caulk be used instead of a seal weld?

In many building construction applications, yes. AISC notes that caulking exposed joints with a clear silicone after painting is a recognized alternative to seal welding structural steel, where the goal is simply keeping water out of a cavity rather than containing pressure. Caulking avoids the unintended load paths, distortion, and inspection complications that seal welds can introduce.

Are seal welds allowed everywhere?

No. Some codes and service conditions prohibit or restrict them. For example, ASME B31.1 prohibits socket welds and seal welds in specified service categories and forbids thread sealing compound where threaded joints are to be seal welded. Always check the governing code and project specification before adding a seal weld note to a drawing.