Arc Spot Weld: The Complete Guide to Puddle Welds, Symbols, Settings, and Strength

What Is an Arc Spot Weld? Defining the Process and Its Many Names

An arc spot weld is a small, circular weld that joins two pieces of metal where one sits on top of the other. You strike an arc on the top sheet, let it burn through, and hold the puddle just long enough for the molten metal to fuse into the thicker member underneath. When it cools, a round nugget of fused steel locks the two layers together. That is the whole idea in one sentence: melt through the top, fuse into the bottom, move on.

Now the technical version. AWS D1.3/D1.3M defines an arc spot weld as a spot weld made by an arc welding process in which the weld is made without preparing a hole in either member. Read that last part twice, because it is the heart of the process. No drilled hole, no punched hole, no prep of any kind. The arc itself does the hole-making. You simply lay the top sheet on the supporting member, and the electrode melts through everything in its path.

If you have spent any time on a job site, you have heard these welds called puddle welds. That is the trade name, and it is a good one, because the visible result is exactly that: a puddle of metal sitting on the deck. But around 1975, research from the Canadian Sheet Steel Building Institute gave the weld its more precise name, arc spot weld, to reflect the fact that it is made by an arc welding process. The code bodies followed. So when the structural drawings say arc spot weld, older hands say puddle weld, and they mean the same thing. And remember, every one of these little circles is a structural fastener, not a cosmetic tack.

Here is where it gets confusing. Both arc spot welds and resistance spot welds carry the word spot, yet they share almost nothing. Resistance spot welding, the kind that builds car bodies, passes current through electrodes that squeeze the work from both sides. No arc, no filler, no shielding gas. An arc spot weld is nearly the opposite: an open arc, often with filler wire or a covered electrode, melting from one side only. Same nickname, completely different physics.

AWS D1.3 also restricts arc spot welding to the flat welding position, and to sheet steel welded to a supporting structural member. Don’t confuse it with an arc plug weld, which needs a prepunched or predrilled hole that the arc then fills. The spot version makes its own hole through the top sheet; the plug version is handed one. If you’re new to the craft, building a feel for arc control starts with deliberate welding practice, because these welds live or die on timing.

The term travels well beyond steel deck. Automakers use arc spot modes to tack thin panels, HVAC shops use them to fasten duct and hanger straps, and appliance manufacturers rely on them for cabinet and chassis assembly. Wherever thin sheet meets a heavier base metal, an arc spot weld is one of the fastest ways to join them.

From here, this guide digs into the details that separate a weld that holds from one that pops off underfoot. We’ll compare arc spot welds to resistance spot, plug, and seam welds, walk through how melt-through and fusion actually work, cover the processes and equipment that make it happen, then break down drawing symbols, WPS requirements, and the strength equations in AISI S100. We’ll finish with real-world uses, common defects, safety and fume control, and hands-on tips to sharpen your technique.

Arc Spot Weld vs. Resistance Spot Weld vs. Plug and Seam Welds

“Spot weld” gets used loosely for anything that looks like a round dot holding metal together, but the processes behind that dot are genuinely different animals. Mixing them up leads to the wrong symbol on a drawing, the wrong equipment on a truck, and welds that fail inspection. Here’s how they actually compare.

Heat and Filler: The Core Difference

Resistance spot welding clamps two sheets between copper electrodes and passes current through them. Resistance at the faying surface generates heat, and pressure forges a nugget with no filler metal added at all. The nugget forms internally between the sheets, a “blind” weld you cannot see from the outside, which is why RSW is notoriously hard to inspect visually.

An arc spot weld works the opposite way. The arc melts through the top member and fuses it to the member below, filler metal being added to build the puddle. Because the weld is created on the accessible face, it is inspectable right there. That visible-face advantage is a big reason arc spot welding dominates field work.

A plug weld is essentially the same arc process, except the top sheet has a prepunched or drilled hole that gets filled with filler metal. That hole adds a step but lets you join thicker material and control the fusion zone more precisely. A seam weld takes the arc spot concept and stacks overlapping spots into a continuous joint, giving a stronger, more airtight connection without a full groove.

Accessibility and Where Each Fits

RSW needs electrodes pressing from both sides, and the weld is essentially finished before anyone can change a position. Arc spot welding needs access to one side only, which is exactly what you have when you are kneeling on a deck over a joist. For readers dealing with factory-style sheet metal joints, the spot weld cutter bit guide covers how to drill out RSW nuggets safely. Rivets and mechanical fasteners sidestep heat entirely, but they add weight, leave protruding heads, and don’t seal against moisture the way a fused weld can.

Reading the Symbols: AWS A2.4

The distinction on drawings matters as much as the process itself. According to AWS A2.4, only RSW and RSEW (resistance seam) carry no side significance and sit centered on the reference line. Everything else has an arrow side or other side.

• RSW symbol: centered on the line; weld diameter goes to the left of the circle and pitch to the right, with the number of spots in parentheses.
• Arc spot weld: placed above or below the line, with the process named in the tail (GMAW, GTAW, FCAW, SMAW).

That tail requirement is not optional, because the same circle means something different depending on the process. Since arc spot welds on steel deck fall under AWS D1.3 rather than AWS D1.1, getting the symbol and process callout right keeps you aligned with the correct code. For process-specific setup, the polarity guides and consistent welding practice pull it all together.

How Arc Spot Welding Works: Melt-Through, the Puddle, and Fusion

A puddle weld looks like almost nothing happening. In reality, five quick events fire off in under two seconds, and each one has to finish before the next can succeed.

Step by Step: From Arc Strike to Filled Puddle

1. Strike the arc on the top sheet. The electrode touches the upper sheet and the arc ignites on its surface.
2. Burn through. The arc melts a keyhole straight through the thin upper sheet in a fraction of a second.
3. Heat the base member. With the hole open, the arc plays directly on the exposed lower member and raises it to fusion temperature.
4. Fill in a circular motion. The electrode moves in small circles, depositing metal that fuses to both the hole edge and the base plate.
5. Extinguish and self-quench. Because there is no travel, the surrounding cold steel pulls heat away fast and the puddle freezes almost instantly.

That circular motion is not decoration. It ties the whole perimeter of the hole into the base metal and builds a small dome of weld metal that overlaps the sheet on all sides. Even passes that look identical can differ underneath, which is why consistent technique matters as much here as anywhere — the same reason welding practice pays off in every process.

Why the Base Member Has to Fusion First

The thin sheet melts almost instantly. The thicker base member does not. Fill the hole before that lower surface is molten and you get a cold weld that sits on top instead of fusing into it. Fusion at the interface is what actually carries shear load across the connection. The circular motion buys the base member a little extra heating time while the puddle grows, which is why rushing the fill is one of the most common ways to weaken a puddle weld.

A Little Metallurgy, Kept Simple

Two terms explain most of the behavior. Dilution is base metal mixing into the weld metal, changing its composition slightly near the fusion line. The heat-affected zone is the thin band around the weld where steel was not melted but its microstructure was altered by the heat. Neither is dramatic on thin deck steel, but both matter when you specify minimum weld sizes.

The sheet itself remains the weak link. Because it is thin, it tends to tear around the weld before the weld metal ever shears — the weld is usually stronger than the material it joins.

The Two Failure Modes (and a Third)

The failure equations in the AISI specification trace back to research by Blodgett (1978) and Pekoz and McGuire (1980), summarized in the AISI RP16-1 report. They identified two governing modes:

• Weld shear failure governs at low diameter-to-thickness ratios, where the weld is small relative to the sheet.
• Sheet tear failure governs at high ratios, where a large weld tears out of a thin sheet.

Blodgett also observed that as the average diameter-to-sheet ratio grows, the sheet can buckle behind the compression side of the weld. Good design lands between shear and tear so neither governs alone.

Arc Time Is the Variable That Rules Everything

Of all the settings on the machine, arc time drives the result. Virginia Tech testing found full-time arc spot welds were on average 11% stronger than 2/3-time welds and 44% stronger than 1/3-time welds across all sheet thicknesses and nominal diameters. The visible diameter shrinks directly as arc time drops, and capacity shrinks with it.

Here is the subtle part: reduced-arc-time welds still matched AISI strength predictions, but only when the measured visible diameter — not the nominal diameter on the drawing — was used in the equation. That is the practical takeaway. Arc time sets the real diameter, and the real diameter sets the strength.

Processes and Equipment: SMAW, GMAW, FCAW, and GTAW Spot Modes

You do not need exotic gear to make an arc spot weld. AWS D1.3, the structural welding code for sheet steel, permits shielded metal arc, gas metal arc, flux cored arc, gas tungsten arc, and submerged arc welding for these joints. What matters is matching the process to the job, the weather, and the sheet thickness.

SMAW: The Traditional Puddle Weld

Stick welding built the roof-deck trade. With a 1/8 in. E6010 electrode you strike an arc, push through the top sheet, and wash a puddle onto the member below until it reaches the specified diameter. Virginia Tech arc spot weld tests used exactly this setup, varying current between 105 and 200 A depending on sheet steel thickness. Thin deck wants the low end; heavier sheet or welding into a thick joist flange needs more heat to fuse both members.

E6010 is the classic choice because it digs through mill scale, rust, and paint. But when the mercury drops, that changes. For temperatures below 32°F (0°C), a Canadian task group recommended switching from E6010 to a low-hydrogen electrode such as E7018 to cut the risk of cold-cracking. If you go that route, dial in your heat with our 3/16 in. E7018 amperage guide before you start production.

GMAW: Timed Spot Mode

Gas metal arc welding is the fast, repeatable option in a fab shop or a sheltered deck. Many power sources offer a spot or “stitch” mode: you hold the gun still, press the trigger, and the machine runs a preset weld-time cycle — from a fraction of a second to a couple of seconds — then stops. The timer decides how much metal lands on the joint, which keeps hundreds of welds consistent without depending on operator rhythm. Keep your wire inside the gas shield and mind your contact-tip-to-work distance, because a wandering CTWD changes how that weld time translates into penetration.

FCAW: Wind-Proof and Field-Ready

Self-shielded flux cored wire carries its own shielding inside the electrode, so it shrugs off the wind that blows a GMAW gas cloud away on an exposed deck. That makes it a favorite for outdoor work, and it slots neatly into the same tapered-puddle motion as stick. Watch your polarity: self-shielded wires usually run electrode-negative, a distinction covered in our guide to DCEN vs. DCEP for flux core welding. Gas-shielded GMAW runs electrode-positive (DCEP). For wind-exposed work with no gas bottle, our gasless MIG and flux-cored guide walks through the setup.

GTAW: Precision on Thin Stock

Gas tungsten arc welding is the surgical option. It fuses thin sheets, delicate brackets, and cosmetic joints with no spatter and a tidy nugget. The tradeoff is speed, so it belongs on small assemblies, not a 20,000-square-foot roof. Because TIG depends heavily on current direction, brush up on DCEN, DCEP, and AC in TIG welding before you dial in a spot.

Settings: Melting Rate and the Peel Test

Rather than guess, experienced crews set the machine by melting rate. A recommended field technique runs about 13 in. per minute of electrode melt to produce a 0.75 in. specified-diameter arc spot weld in sheets from 0.030 in. to 0.06 in. thick. In practice you hang the arc until the right length of electrode is consumed for the time you held the trigger. Voltage, wire feed speed, and shielding gas flow (typically 20–30 cfh for gas-shielded modes) then fine-tune the puddle. Whichever process you choose, the goal is identical: full fusion through the top sheet into the member below.

Confirming those settings takes seconds. A peel test is the field’s quick check:

Get the process and the settings right once, and the rest is repetition. Build that muscle with our welding practice guide before you take it onto a production deck.

Reading the Drawing: Symbols, WPS, and Code Requirements

Structural drawings almost never spell out “arc spot weld” in words. They use a symbol, and misreading it means welding the wrong size, pitch, or side of the joint. The notation is compact and logical once you learn the rules—and the rules are backed by code, not tradition.

The AWS A2.4 Spot Weld Symbol

Per AWS A2.4, a spot weld is drawn as a plain circle sitting on the reference line. That bare circle says “spot”—no fillet legs, no groove. Everything else stacks around it:

• Diameter goes to the left of the circle.
• Pitch (center-to-center spacing) goes to the right.
• Number of spots in a run sits in parentheses.
• Process designation in the tail is mandatory—it tells the fabricator whether to grab the GMAW gun or an electrode holder.

Placement follows the three-way rule. A symbol on the arrow side means weld where the leader touches. Opposite the arrow means the far side. Centered on the reference line means both sides. Without that tail entry the symbol is incomplete, because the process is never optional information. A seam weld reuses the same circle but adds two parallel lines flanking it on the reference line, signaling a continuous weld instead of discrete puddles. Resistance spot welds are generally shown on a plan view and tied to a schedule rather than called out with the same field leaders.

WPS and PQR: The Paperwork Behind the Arc

A Welding Procedure Specification (WPS) is the written recipe: process, filler metal, electrode size, amperage, voltage, polarity, travel speed, and base-metal thickness range. It’s supported by a Procedure Qualification Record (PQR)—the test data proving that recipe actually produces sound welds. AWS D1.3 allows a prequalified WPS for certain common joints and thicknesses, but anything outside those limits needs a qualified PQR. A welding inspector or engineer signs off, and the WPS travels with the job. Field welding still demands a written WPS because wind, fit-up, and access change the variables. Treat it as your shop’s baseline, whether that means a fresh contact tip or a torch set correctly.

Edge, End, and Spacing Limits

D1.3 specifies arc spot welds by their minimum effective diameter at the faying surface (d_e), with a minimum allowable value of 3/8 in [9 mm]. The critical geometric rule is edge distance: the minimum center-of-weld-to-edge distance is not less than 1.5d (1.5 times the visible diameter). AISI uses the same 1.5d threshold. Undersize this and the sheet tears out before the weld ever fails. End distance and spacing do the same job—keeping puddles far enough apart to avoid net-section failure while close enough to build a working diaphragm. Pitch is set by the design, but it’s the drawing and WPS that govern, not field habit.

Weld Washers: The Burnback Fix

Thin sheet is where arc spot welding gets fussy. Too little material under the arc and the puddle burns through, leaving a hole instead of a weld. That’s why weld washers exist.

The washer acts as a sacrificial heat sink, letting the arc form a proper puddle without punching through. Keep a bin of the correct grade and hole size at each station. Getting these details right starts long before the arc strikes; a well-organized welding facility keeps the right washers, WPS, and filler staged so the job runs clean.

Strength and Design: AISI S100 Equations, Shear vs. Tension, and Testing

Here’s a detail that surprises a lot of welders: the strength of an arc spot weld is not really an AWS number. The American Welding Society’s D1.3 Structural Welding Code—Sheet Steel covers how to make the weld, the procedure qualification, and the inspection, but the actual nominal capacities live in the North American Specification for the Design of Cold-Formed Steel Structural Members, AISI S100. The 2025 D1.3 revision even deleted its own strength provisions and points designers straight to AISI S100. So when an engineer sizes a puddle weld, they’re working from Section E2.2.

AISI S100 gives separate equations for the two ways an arc spot weld can fail. In shear, Section E2.2.1 treats failure as the weld itself shearing off, so capacity scales with the effective weld area and the electrode’s tensile strength, Fxx. In tension, Section E2.2.2 covers the weld pulling straight out, but that’s only one limit; the surrounding sheet usually gets a vote too.

The Two Failure Modes and Their Limits

That introduces three different diameters. Visible diameter d is what you measure with a gauge after the weld cools. Average diameter da is the mean of the weld at the faying surface. Effective diameter de is the one you actually plug into the equation:

de = 0.7d − 1.5t, not less than 0.55d

where t is total combined sheet thickness. The 0.7 and 1.5 factors account for the weld’s rounded, tapering shape, and the 0.55d floor stops thin-sheet geometry from collapsing capacity too far. This is why combined thickness and consistent technique matter so much: thicker stack-ups shrink de fast.

The second failure mode is sheet tearing or buckling around the weld. There the controlling ratio is da/t. When that ratio grows large, the sheet fails before the weld does, so AISI caps capacity with a term built on the sheet’s ultimate strength, Fu. Tension design in AISI S100 applies firm limits that keep the joint in its intended range:

• tdaFu ≤ 3 kips
• emin ≥ d
• Fxx ≥ 60 ksi and Fu ≥ 60 ksi
• Fxx > Fu

Designers rarely load welds in pure shear or pure tension. Combined loading uses an interaction equation that keeps the sum of the utilized shear and tension ratios at or below 1.0. That sounds tidy, but the tension side hides a catch. An AISI study (RP16-1, 2016) reviewed 1,193 arc spot weld specimens, narrowed to 450 applicable tests, and concluded the shear equations were conservative while the tension weld-failure provisions could substantially overpredict real strength. Later research at Missouri University of Science and Technology reached the same conclusion, so engineers should apply the updated tension factors with caution rather than treating them as generous.

Sheet-to-sheet welds behave differently from sheet-to-structural ones, because there’s no thick substrate to anchor the puddle and draw heat away. The Steel Deck Institute’s Diaphragm Design Manual handles this with a simple allowance: the shear capacity of a sheet-to-sheet arc spot weld may be taken as 75% of the equivalent sheet-to-structural connection. The reduction reflects the softer load path: a thin top sheet and a puddle with little substrate behind it offer less reserve, so the code discounts the connection accordingly.

Verification and Testing in Practice

Verification splits along industry lines. Structural deck work is confirmed by visual inspection plus destructive peel or chisel testing on a sampling basis. Automotive leans on ultrasonic testing for non-destructive evaluation of blind resistance spot welds, a method that maps the nugget without cutting the part apart. Arc spot welds, being fusion puddles with visible crowns, still depend on the older mechanical checks, so judging a sound weld by eye and by chisel is a skill worth sharpening.

Where Arc Spot Welding Is Used: Steel Deck, Automotive, HVAC, and More

Walk across any commercial roof under construction and you are literally standing on the answer: a grid of small circular welds tying steel deck panels to the structure below. That is arc spot welding’s home turf, and it spreads far beyond the roofline.

Steel Deck: The Biggest Single Application

The dominant use of arc spot welding is attaching cold-formed steel roof and floor deck to hot-rolled open-web joists and structural shapes. Those connections turn a stack of corrugated panels into a structural diaphragm that resists lateral and uplift loads. As the Cold-Formed Steel Engineers Institute documents, these welds are used extensively to join deck and panels to bar joists or hot-rolled members.

Corrugated sheet steel diaphragms typically run from 22 to 16 gauge (0.028 in to 0.060 in / 0.72 mm to 1.52 mm) thick, transferring load to the frame through arc spot welds or mechanical fasteners. Beyond the simple panel-to-joist weld, crews also make side-lap and corner-lap connections where multiple sheet layers stack together. That is why deck drawings show the welds on a fixed grid, spaced along the ribs at every support.

The 2025 edition of AWS D1.3 is a meaningful shift here. It revised arc spot welding provisions from “double thicknesses” to multiple sheet thicknesses, explicitly permitting welds through four thicknesses at deck corner laps. In practice, that widens what you can join in a single puddle without changing the drawing.

Automotive and Appliance: Resistance Spot’s Home Turf

Automotive manufacturing uses spot welding at enormous scale, but the process is almost always resistance spot welding (RSW), not the arc version. RSW accounts for approximately 85% of car-body joining work, and a standard car body carries roughly 4,000 to 6,000 resistance spot welds; some estimates for a full body run 5,000 to 7,000, with RSW averaging about 60% of total welds (AHSS Insights).

Keep the two straight: RSW squeezes the sheets and passes current through them with no filler and no arc. Arc spot welding melts through a top sheet into the one below. Appliances lean on RSW for the same reason: speed and repeatability on thin, clean, galvanized steel. In a car plant, robot arms cycle through thousands of RSW tips a day, an entirely different world from a welder on a roof deck striking an arc.

HVAC, Trailers, Racks, and More

HVAC duct and enclosure fabrication is a quieter but steady user of arc spot welding:

• HVAC and enclosures: attaching duct liners, hangers, and panel skins
• Trailer and truck bodies: fastening side skins to frames
• Metal buildings: deck, purlins, and secondary framing
• Storage racks and agricultural equipment: brackets, guards, and panel assemblies

These industries share one trait: lots of thin-gauge connections needing to be fast, strong, and cheap. A puddle weld fits where a bolt would need a hole and a rivet would need access. If you are building those fundamentals, our guide to welding practice is a good next stop.

Why Contractors Keep Choosing It

The economics are simple. Arc spot welding needs no prepunched holes, so layout is faster. It works from one side of the joint, which matters when the far side is closed off or overhead. Each weld takes seconds, and consumables are minimal: a stick electrode, a little flux-cored wire, or a short burst of shielding gas. Compare that to drilling and bolting or riveting, and the labor savings stack up across thousands of connections per job. Wire-fed setups benefit from knowing your polarity for flux core welding and keeping a fresh contact tip, since both affect arc stability on short, hot welds.

Common Mistakes, Defects, and How to Fix Them

Arc spot welding looks simple: pull the trigger, let a puddle form, move on. That simplicity is exactly why so much of it goes wrong. On a deck or a thin-panel job, the defects that matter are the ones you can barely see, and the fixes are almost always about one variable — how long and how hot you let the arc burn.

Burn-Through

Burn-through gives you a blown hole instead of a puddle. The usual suspects are current set too high, sheet that is too thin for the setting, a missing or undersized weld washer, and arc time that runs long. Thin galvanized deck is unforgiving here. If you are consistently punching holes, drop your amperage, shorten the arc, and confirm the washer is sitting flat before you strike. On very light gauge, a properly sized washer is not optional — it spreads the heat and gives the puddle somewhere to sit.

Incomplete Fusion and Lack of Penetration

A weld can look round and pretty on top and still have no real bond to the base member below. Causes stack up fast: too little heat, insufficient arc time, poor fit-up that leaves a gap, and mill scale or coating trapped between the faying surfaces. Welding through mill scale and coatings is one of the most common but consequential mistakes on the job — the puddle rides on the contamination instead of fusing to steel. Grind or wire-brush the area clean first. This is the same family of problem covered in our guide to the overlap welding defect, where the puddle sits on top rather than melting in.

Undersized Weld Diameter and the “Time Theft” Problem

This is the quiet one. Virginia Tech documented that field crews under schedule pressure often produce arc spot welds in one-third to two-thirds of the laboratory arc time, which cuts shear strength by up to 44%. That is “time theft” — trading a few seconds per weld for a weld that is smaller and weaker than the drawing assumes. Because reduced-time welds are smaller in diameter, the AISI strength equations overestimate their capacity unless you plug in the measured visible diameter rather than the nominal one. Short arc time, low current, and excessive contact-tip-to-work distance all shrink the puddle. Watch your nozzle standoff and keep it consistent.

Porosity

Pinholes and wormholes usually trace back to galvanized coating and moisture. Galvanized (ASTM A653) sheet is the standard deck material in arc spot weld research, and its zinc coating readily contributes to porosity and fume. Damp electrodes, rusty wire, and oily panels add to it. Clean, dry consumables and a short arc go a long way — check our good and bad MIG welds breakdown for reading porosity versus other surface defects.

Spatter, Arc Blow, and Distortion

Long arcs throw spatter; arc blow pushes the puddle off-center. Both get worse on thin panels, where concentrated heat also warps the sheet. Reduce arc length, tighten your work angle, and stagger your weld sequence to let panels cool.

Fit-Up, Clamping, and Technique

Clamp the sheets tight and close the gaps before striking. The recommended technique starts at the center of the weld and moves spirally outward to form a circular weld with fusion around the arc-puddle perimeter. Field studies also recommended keeping the plate-to-sheet thickness ratio at or below 2.5 to maintain quality.

Safety, Fume, and Ventilation for Arc Spot Welding

Arc spot welding may be a brief process, but it is still an open arc. That single fact separates it from resistance spot welding, where OSHA requires only transparent face shields or goggles for splatter protection because no bright arc is produced. With arc spot, you need a proper arc-rated filter lens dark enough for the amperage you’re running, plus a welding helmet, safety glasses underneath, leather gloves, and flame-resistant sleeves or jacket. Sparks and spatter fly in every direction on a deck. Our guide to what protective eyewear welders actually need breaks down shade numbers and lens options.

Fume Hazards: Zinc, Manganese, and Hexavalent Chromium

The fume is the sneaky part. Base metal and consumable decide what you’re breathing: stainless electrodes and wire release hexavalent chromium and nickel, both classified carcinogens; carbon steel consumables carry manganese; and galvanized deck coating vaporizes into zinc oxide fume. Even outdoors, arc spot welding galvanized deck releases zinc fume, a well-recognized cause of metal fume fever, the flu-like chills, fever, and aches that hit a few hours after exposure. Metal fume fever is rarely fatal, but it is nasty and entirely preventable.

Ventilation Under OSHA 29 CFR 1910.252

OSHA 29 CFR 1910.252 requires local exhaust or general ventilation sufficient to keep toxic fumes, gases, and dust below the maximum allowable concentrations listed in 29 CFR 1910.1000. The rule gets specific: mechanical ventilation is required when you weld in a space smaller than 10,000 cubic feet per welder, a room with a ceiling under 16 feet, or a confined or partitioned space. General mechanical ventilation under 1910.252(c)(2) must move at least 2,000 cubic feet (57 m³) per minute per welder unless local exhaust hoods or approved airline respirators are used instead. A low ceiling traps fume right in your breathing zone, so dilution ventilation alone often fails there. On a roof deck, that usually means portable fume extractors with a capture nozzle positioned at the puddle.

Fire, Shock, and Working at Height

Hot work starts fires. Where practicable, combustibles must be relocated at least 35 feet (10.7 m) from welding work under 1910.252, and NFPA 51B governs hot-work permits and fire watches, with a fire watch mandatory wherever combustibles can’t be moved. Keep a charged extinguisher within reach and check for smoldering material after you quit. On steel decking, sparks and slag drop through openings to the floor below, so screen or wet down combustibles and post a fire watch underneath when needed. Electrode holders and cable connections fall under 1910.254; inspect them for cracked insulation or loose lugs before every shift, and never use a wet glove or stand in water. Because arc spot welding on a deck means standing on a roof, fall protection is not optional: guardrails, personal fall-arrest, or a full roof safety plan.

Good safety habits scale. Setting up a safe shop layout with fume extraction, screens, and clear hot-work zones prevents most of these problems before the first arc. If you’re welding during pregnancy, the concerns shift toward fume exposure, ergonomics, and prolonged standing; our guide on welding while pregnant covers the specific risks. Treat every arc spot weld as a real arc, because your lungs and your eyes can’t tell the difference between a two-second puddle and a full structural pass.

Practical Tips to Master Arc Spot Welding (and Key Takeaways)

Everything in this guide converges on a single truth: arc spot welding looks deceptively simple, but it rewards discipline. The welders who lay down row after row of sound puddle welds on steel deck aren’t the ones with the most arc time behind them. They are the ones who make every arc time identical. The checklist below condenses everything covered in this guide into the order you should practice it. Most of these habits take minutes to learn, so start now.

The Mastery Checklist

1. Dial in the machine with the melting-rate method. For a 0.75 in weld with a 5/32 in E6010 electrode, a melting rate of roughly 13 in/min is a practical field starting point. Adjust for electrode size, process, and position, then lock the settings in. Write the winning numbers on the machine with a marker so every welder on the crew starts from the same baseline.
2. Verify with a peel test. This is the fastest field check: separate the sheet from the plate and confirm a nugget of at least 0.45 in diameter remains attached to the plate. No nugget, no weld.
3. Practice arc time with a stopwatch before production. Full penetration and fusion around the arc-puddle perimeter are the acceptance criteria that separate a sound weld from a cold-lap. Cutting arc time to two-thirds drops strength by 11%; cutting it to one-third drops it by 44%. Because a puddle forms in just a few seconds, a half-second drift is a massive percentage error. Consistent arc time is the highest-value quality control lever you own.
4. Use the spiral technique, every time. Make the weld in a single pass, starting at the center and moving spirally outward to form a circular weld. This distributes heat evenly and prevents the center from burning through while the edges stay cold.
5. Set and hold the correct contact tip to work distance. Arc length controls penetration, so a drifting CTWD produces a drifting weld. If your gun geometry wanders, review the fundamentals of contact tip selection and maintenance.
6. Clean or account for coatings and mill scale. Galvanizing, oil, paint, and mill scale all fight fusion. If you cannot remove them, factor them into your settings and accept the inspection consequences.
7. Use weld washers on very thin sheet. They bridge the gap, prevent burn-through, and give the puddle something to bite into.
8. Respect edge, end, and pitch distances. The AISI S100 spacing rules exist to stop tearing at sheet edges and to let the diaphragm develop its full strength. Crowding welds does not make them stronger.
9. Inspect every weld, and test periodically. Visually check each puddle, run destructive peel tests on sacrificial panels, and use ultrasonic testing where the spec demands it. Never let schedule pressure steal arc time.

Key Takeaways

• Arc spot welding is a one-side, filler-added, arc-based alternative to resistance spot welding, used heavily for steel deck and diaphragm construction.
• Execution is governed by AWS D1.3; strength and spacing by AISI S100.
• Arc time is the single biggest quality lever in the whole process.
• Safety is non-negotiable: ventilation, protective eyewear, and a fire watch per OSHA 1910.252 are not optional extras.

Now it is your turn. Start small: one panel, twenty welds, one stopwatch. Burn some practice plates, time your passes, peel a few welds apart, and see where you stand. If you want to build the underlying skills, our complete welding practice guide walks through the drills that transfer directly to puddle welding, and if you are running flux-cored spot welds, the gasless MIG guide covers the wire and settings side of the equation. Master the fundamentals, respect the codes, and those puddles will hold when the inspector pulls out the hammer.

Frequently Asked Questions

Is an arc spot weld the same as a resistance spot weld?

No. A resistance spot weld uses electrode pressure and electrical resistance to form an internal nugget with no filler metal, while an arc spot weld uses an arc process like SMAW, GMAW, FCAW, or GTAW to melt through an upper sheet and fuse it to a lower member with filler metal. Arc spot welds can be made from one side and are visually inspectable, whereas resistance spot welds are usually ‘blind’ and made with electrodes on both sides.

Why is an arc spot weld also called a puddle weld?

Because the process forms a small puddle of molten metal as the arc melts through the top sheet, deck erectors long called it a puddle weld. The technical term ‘arc spot weld’ was adopted to reflect the actual arc welding process and to distinguish it from resistance spot welding.

What is the minimum size for an arc spot weld?

AWS D1.3 specifies arc spot welds by their minimum effective diameter at the faying surface, and the minimum allowable effective diameter is 3/8 in [9 mm]. Edge distance must also be at least 1.5 times the visible weld diameter.

When do I need a weld washer for arc spot welding?

Weld washers are required when arc spot welding sheet steel thinner than 0.028 in [0.7 mm] to prevent burn-back. The washer must be 0.05 in to 0.08 in thick with a minimum 3/8 in prepunched hole.

What current and settings should I use for arc spot welding?

There is no single universal setting because thickness, material, and process vary. As a reference, arc spot welds made with 1/8 in E6010 electrodes have been tested from about 105 to 200 amps depending on sheet thickness. A practical approach is the melting-rate method (roughly 13 in/min for a 0.75 in weld) confirmed with a peel test.

Why do my arc spot welds keep burning through the sheet?

Burn-through is usually caused by too much current or arc time, thin sheet without a weld washer, or a very short arc length on light gauge material. Reduce the current, shorten the arc time, and add a weld washer if the sheet is thinner than 0.028 in.

How do you inspect an arc spot weld?

Visual inspection checks size, shape, and fusion around the perimeter, while a peel or chisel test is the standard field verification for soundness. In higher-volume or critical work, ultrasonic testing can evaluate the weld non-destructively, and destructive tests are used on sample welds.

Is arc spot welding safe to do without ventilation?

No. Arc processes generate hazardous fumes and gases, and OSHA 1910.252 requires adequate ventilation to keep exposures below permissible limits. Welding galvanized deck also releases zinc fume, so local exhaust or general mechanical ventilation at a minimum of 2,000 cubic feet per minute per welder is generally needed unless engineering controls or respirators are used.

Authoritative Sources & Further Reading

• https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.252
• https://www.ecfr.gov/current/title-29/subtitle-B/chapter-XVII/part-1910/subpart-Q/section-1910.252
• https://pubs.aws.org/Download_PDFS/D1.3-D1.3M-2008PV.pdf
• https://pubs.aws.org/Download_PDFS/D1.3-D1.3M-2018-PV.pdf
• https://pubs.aws.org/Download_PDFS/D1.3-D1.3M-2025-PV.pdf
• https://www.cfsei.org/assets/docs/research_report/AISI%20RP16-1%20Resistance%20of%20Arc%20Spot%20Welds%20%E2%80%93%20Update%20to%20Provisions.pdf
• https://cfsei.memberclicks.net/assets/docs/technotes/tn-f140-10.pdf
• https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=1317&context=isccss
• https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=1310&context=isccss
• https://buildusingsteel.org/wp-content/uploads/2020/12/CFSD-Report-RP01-6.pdf
• https://ahssinsights.org/joining/resistance-welding-processes/resistance-spot-welding/
• https://onlinepubs.trb.org/Onlinepubs/hrbbulletin/346/346-001.pdf