The single-V groove weld is the most common groove weld preparation in structural and pressure vessel fabrication. Standard dimensions are 60-degree included angle (30 degrees per plate), 1/16 to 1/8 inch root opening, and 1/16 to 1/8 inch root face. Getting these dimensions right and running the multi-pass sequence correctly produces a sound CJP (complete joint penetration) weld every time.
The V-groove works on plate from 3/16 inch up to about 3/4 inch. Above 3/4 inch, a double-V saves roughly half the filler metal by beveling from both sides. The procedure below covers the single-V in detail, from prep through cap pass.
Joint Preparation Dimensions
Every V-groove has three critical dimensions: bevel angle, root opening (gap), and root face (land). These dimensions control arc access, penetration, and weld volume.
| Dimension | Standard Value | Range (AWS D1.1) | Purpose |
|---|---|---|---|
| Included angle | 60° | 45° - 60° | Arc access to root, weld volume |
| Bevel per plate | 30° | 22.5° - 30° | Half the included angle |
| Root opening | 1/8" | 0 - 3/16" | Penetration access at root |
| Root face (land) | 1/16" - 1/8" | 0 - 1/8" | Prevents burn-through, supports root pass |
Bevel Angle Details
A 30-degree bevel (60-degree included) is the sweet spot for SMAW (stick) and GMAW (MIG). It gives the electrode room to reach the root without requiring excessive filler metal to fill the groove. Narrower angles (22.5 degrees, giving 45 degrees included) reduce weld volume by about 25% but make root access tight, especially with stick electrodes. Pipe welding bumps the standard to 37.5 degrees per side (75 degrees included) because positional welding needs the extra access.
For GTAW (TIG) root passes with SMAW fill, the wider 37.5-degree bevel is common on plate as well. The TIG torch needs clearance, and the wider groove lets you see the root puddle clearly.
Root Opening (Gap)
The root opening provides space for the arc to penetrate through to the back side. Too small and you get incomplete penetration. Too large and you burn through or need excessive root pass manipulation.
With a backing bar, root opening typically runs 1/4 inch. Without backing (open root), 1/16 to 1/8 inch is standard. Consistency matters more than exact dimension. A root opening that varies by 1/16 inch around the joint produces inconsistent penetration and makes the root pass fight you.
Root Face (Land)
The root face is the flat surface at the bottom of the bevel. It prevents the molten weld metal from melting through immediately. A land of 1/16 to 1/8 inch is standard. Zero land (knife edge) makes burn-through almost guaranteed on the root pass. Too thick a land (over 1/8 inch) blocks penetration and causes lack-of-fusion at the root.
Beveling Methods
Grinder with a flap disc works for field joints and short runs. Mark the angle with a protractor or bevel gauge, then grind to the line. Accuracy is operator-dependent.
Oxy-fuel torch handles long bevels quickly on carbon steel. The cut surface needs grinding to remove the oxide layer and any gouges. J-bevels and U-grooves can’t be torch-cut.
Plasma cutting produces a cleaner bevel than oxy-fuel with less cleanup. Modern CNC plasma tables can cut accurate bevels automatically.
Machining gives the most precise bevel for pipe and critical joints. Portable pipe beveling machines clamp onto the pipe end and cut a consistent angle and land around the full circumference.
Backing Bar vs. Open Root
Backing Bar (Steel Backing)
A backing bar is a steel strip, typically 1/4 inch thick by 1 inch wide, tack-welded behind the joint. It catches the root pass and prevents burn-through. The welder can run hotter and faster because the backing supports the molten metal.
Advantages:
- Easier root pass, more forgiving of gap variations
- Higher deposition rate on the root
- Less skill required
Disadvantages:
- Creates a crevice that can trap moisture and cause corrosion
- Not allowed on some piping codes (ASME B31.3 for corrosive service)
- Adds labor to fit and tack the backing strip
- Stays in place permanently (removal requires back-gouging)
AWS D1.1 prequalified joints with backing use a 1/4 inch root opening and 0 to 1/8 inch root face.
Open Root
An open root joint has no backing. The welder controls penetration to form root reinforcement (1/32 to 1/8 inch) on the back side. This produces a clean interior surface with no crevice.
The root pass on an open root is the critical pass. With GTAW, maintain a keyhole at the leading edge of the puddle. The keyhole guarantees penetration. With SMAW using E6010/E6011, use a whip-and-pause technique: advance the electrode into the root, hold briefly to fill, then whip forward to extend the keyhole.
Open root joints use a tighter root opening (1/16 to 1/8 inch) and a more precise land (1/16 inch typical). Fit-up tolerance is tighter than with a backing bar. See the fit-up tolerance guide for specific AWS D1.1 limits.
Multi-Pass Welding Sequence
Root Pass
The root pass fuses both root faces together and establishes penetration through to the back side. It’s the most important pass in the joint.
With backing bar: Run a stringer bead with E7018 (SMAW), 0.035 inch wire (GMAW), or TIG. The backing supports the puddle, so focus on tie-in to both bevel faces. Run hot enough to get fusion into the backing strip.
Open root (GTAW): Use a keyhole technique. Set amperage to produce a keyhole about 1/16 inch ahead of the puddle. Feed filler rod into the leading edge of the puddle. If the keyhole grows, add more filler. If it shrinks, increase amperage slightly.
Open root (SMAW, E6010): Whip and pause. Strike the arc in the root, let the keyhole form, advance 1/8 inch, pause to fill, advance again. The whip keeps the arc ahead of the puddle to maintain the keyhole.
Hot Pass
The hot pass immediately follows the root pass while the root is still warm. It burns out any slag, porosity, or contaminants trapped at the root toes and builds the first substantial layer.
Run the hot pass at slightly higher amperage than the root. Use a slight weave (1/8 inch side to side) to wash into both toes of the root pass. The hot pass should fill the groove to about 1/3 of the total depth.
On SMAW, switch from E6010 to E7018 for the hot pass and all subsequent passes. E7018’s low-hydrogen deposit provides better mechanical properties for the fill and cap.
Fill Passes
Fill passes build up the groove to within 1/16 to 1/8 inch of the plate surface. Run stringer beads, each overlapping the previous pass by about 50%. Keep each bead flat to slightly convex.
| Pass Position | Technique | Key Focus |
|---|---|---|
| Root toes | Stringer, angled into bevel face | Fusion into base metal |
| Center fills | Stringer, flat | Consistent buildup, no valleys |
| Last fill before cap | Stringer, crown slightly below surface | Leave room for cap without excess buildup |
Clean between every pass. On stick and flux-core, chip and wire-brush all slag. On MIG and TIG, wire-brush to remove any oxidation or silica islands. Slag inclusions between passes are one of the most common NDE rejection causes.
Cap Pass
The cap is the final layer and the first thing an inspector sees. It should extend 1/16 to 1/8 inch beyond each bevel edge (reinforcement) and sit slightly convex with a uniform ripple pattern.
Run the cap with a slight weave, pausing at each toe to fill and prevent undercut. AWS D1.1 limits cap reinforcement to 1/8 inch above the plate surface. More than that is excessive and constitutes a defect.
Typical Pass Counts by Thickness
| Plate Thickness | Total Passes (SMAW, 1/8" rod) | Total Passes (GMAW, 0.035" wire) |
|---|---|---|
| 1/4" | 2-3 | 2 |
| 3/8" | 3-4 | 3 |
| 1/2" | 5-6 | 4-5 |
| 3/4" | 8-10 | 6-8 |
| 1" | 10-14 | 8-10 |
Common Mistakes
Inconsistent root opening. If your gap varies from 1/16 to 3/16 inch around the joint, the root pass burns through in spots and lacks penetration in others. Take time to fit up uniformly. Tack welds spaced every 3-4 inches on plate (or every 2-3 inches on pipe) hold the gap consistent.
Too thick a root face. A 3/16 inch land blocks root penetration on most SMAW and GMAW procedures. Stick with 1/16 to 1/8 inch. If the land is too thick, grind it down before welding.
Skipping the hot pass. Going straight from root to fill passes traps contamination at the root toes. The hot pass is there for a reason: it re-melts the root toes and cleans the foundation for fill passes.
Fat fill passes. Large weave passes look productive but they overheat the HAZ, produce more distortion, and can trap slag at the toes. Keep fill beads no wider than 2.5 times the electrode diameter.
Excessive cap reinforcement. A tall, rounded cap wastes filler metal and creates stress concentration at the toes. AWS D1.1 limits reinforcement to 1/8 inch. Grind flush if the drawing specifies it.
For more on joint types and selection, see the butt joint vs. lap joint vs. tee joint comparison. For groove weld dimensions on pipe, check the root pass, hot pass, fill, and cap sequence guide. Return to the joint design category for additional guides.