Can I start fill passes before the CWI inspects the root pass?

That depends on your quality plan. If root pass is designated a hold point, welding stops until the inspector signs off. If it is a witness point, welding may continue at the fabricator's risk but the inspector must be notified. For critical connections or demand-critical welds, root pass should be a hard hold point in the quality plan — not a witness point.

How much undercut is acceptable in a root pass before fill welding starts?

AWS D1.1:2025 visual acceptance criteria for undercut apply throughout the weld, including the root pass. In a tension member, undercut is limited to 1/32 in [1 mm] for any single pass at the root. In compression members the limit is higher, but sharp undercut notches in the root of a CJP weld are always a concern for fatigue. Repair before fill passes begin is far less costly than excavating after the joint is full.

Does every CJP groove weld require back-gouging?

Not necessarily. A joint welded from one side with a backing bar does not require back-gouging. A joint welded from one side with a qualified one-sided welding procedure (where the WPS is tested and qualified for complete penetration without back-gouging) also does not require it. Back-gouging is required when the joint is designed for welding from both sides, the root must be clean before the second side is deposited.

What visible conditions in a root pass tell me I have incomplete penetration?

The most reliable visual indicators are a flat, unfused root face visible inside the groove (you can see the original edge of the root land still present), and a convex or rounded root profile that did not fully melt through. On open-root joints, look at the back side: a properly fused root shows uniform root bead penetration. If the back side shows no bead reinforcement or shows the original fitup gap without fusion, the root did not penetrate.

CJP Root Pass Inspection Under AWS D1.1: What to Check

Root pass is the critical inspection hold point in CJP groove welding. What CWIs check before fill passes start, and when back-gouging applies under AWS D1.1.

The root pass is where CJP groove welds succeed or fail. Everything deposited after it — fill passes, cap passes — depends on a sound root. Catching a root defect before fill passes begin costs one repair pass. Missing it costs excavation of an entire joint after the weld is complete and may cost more in NDE and re-inspection time than the original weld.

Understanding what to inspect at root pass, when to hold welding, and when back-gouging applies keeps defects at the cheapest possible point in the fabrication sequence.

Why Root Pass Is the Critical Hold Point

A root pass in a CJP groove weld has three jobs: fuse to both base metal joint faces, produce complete penetration through the root opening, and create a profile that the next pass can fully fuse to without entrapping slag or creating a lack-of-fusion condition.

Incomplete root fusion is one of the most common CJP defects in structural fabrication. It is also one of the hardest to detect after the joint is filled — ultrasonic testing (UT) can find it, but angular reflectors at the root are notoriously difficult to characterize, and UT acceptance criteria still require the indication to be evaluated, increasing inspection time and cost.

Catching the defect visually before fill passes is free. That is why root pass inspection — whether from the front (groove) side on open-root joints, or from the back (root) side before back-gouging on backing-removed joints — belongs in every quality plan as a defined hold or witness point.

For an overview of how to structure hold points in a quality plan, see weld inspection hold points for CWI and QC managers.

Open-Root Joints: Inspecting Before Fill

An open-root joint — no backing bar — is the most common configuration for structural T-joints, corner joints, and butt joints qualified for complete penetration from both sides. After the root pass is deposited, stop.

What to check from the groove side:

Root pass profile. The root pass should be slightly convex or flat, never deeply concave or sunken. A concave root bead creates a sharp notch that the next pass cannot fully fuse to.
Tie-in to joint faces. Look at the toes of the root bead on each side. The bead should tie smoothly into both faces of the groove without undercut or unfused edges.
Cracks. Root passes in restrained joints are susceptible to longitudinal shrinkage cracking, especially in low-hydrogen processes at low preheat. Any cracking in the root pass must be excavated and repaired before fill.
Porosity. Surface porosity in the root pass must meet AWS D1.1:2025 visual acceptance criteria. Subsurface porosity in the root will be covered by subsequent passes and cannot be visually re-assessed — if porosity is visible, assume it extends below the surface and grind until it is gone before filling.

What to check from the root (back) side:

On open-root T-joints and butt joints welded in position, you may be able to see the root bead profile from the back side. For complete penetration, the root bead should show uniform reinforcement along the entire joint length. Spots where the bead did not push through, or where only partial fusion occurred, will appear as flat, unreinforced areas.

Back-Gouging: When Required and How to Evaluate It

Back-gouging removes the root of the first-side weld before the second side is deposited. It is required whenever:

The joint configuration requires welding from both sides for CJP
The WPS does not qualify one-sided complete penetration without backing
Visual or NDE of the first side raises questions about root integrity

Back-gouging is done by air carbon arc gouging (CAC-A), grinding, or mechanical methods. The depth must reach clean, sound weld metal or base metal — no slag inclusions, porosity, cracks, or unfused areas.

Evaluating the back-gouge before second-side welding:

The back-gouged groove should be:

Clean metal with no visible inclusions, porosity, or cracks
Contoured into a smooth U-groove or V-groove shape that allows full fusion from the second side
Free of sharp corners or undercut that would trap slag in the second-side root
Inspected by magnetic particle testing (MT) or liquid penetrant testing (PT) if specified by the quality plan or contract for demand-critical welds

Do not allow second-side welding to begin until the back-gouge is inspected and accepted. On demand-critical welds or fatigue-sensitive connections, MT of the back-gouge groove is standard practice even when not explicitly required by the contract.

See back-gouging for CJP groove welds under AWS D1.1 for the process-specific WPS requirements for CAC-A gouging.

Joints with Steel Backing: What the Root Pass Looks Like

When a steel backing bar is used, the root pass welds into the joint from one side with the backing in place. There is no open root to view from the back side during welding. The inspection focus shifts.

With the backing in place:

Confirm the root opening and backing fit-up before arc-on. The backing bar must contact both joint members without gaps that could trap the root bead.
After the root pass, inspect the groove side for profile, tie-in, and cracks.
If the backing is to remain (permanent backing), visually verify the backing-to-base-metal fillet weld or fusion line. If the backing is to be removed, back-gouging or grinding removes it after full joint completion.

For specific requirements on backing bar removal and the weld repairs required after removal, see steel backing removal for CJP welds.

Common Root Pass Defects and Corrective Action

Incomplete penetration. The joint faces were not melted through. Excavate back to base metal and re-deposit. Do not fill over incomplete penetration — it will show on UT and require a far more invasive repair.

Root pass underfill. The root bead is below the base metal surface, leaving a concave throat. Deposit another root pass to build up the profile before filling. AWS D1.1:2025 prohibits underfill in the final weld, and a concave root pass is pre-condition for underfill in the finished joint.

Longitudinal cracking. Most common in high-restraint joints (box columns, heavy plate) or when preheat was inadequate. Completely excavate the cracked area. Do not re-weld without increasing preheat and reviewing the WPS for minimum interpass temperature. A crack in the root of a restrained CJP weld is a hydrogen cracking indicator — review the entire procedure.

Slag entrapment. Root passes in SMAW and FCAW can trap slag at the joint face toes, especially in tight root openings. Grind or chip the slag completely out before continuing. Inter-pass cleaning is an absolute requirement on all covered-electrode and flux-based processes.

Putting Root Pass Inspection in Your Quality Plan

Root pass inspection should be a formal line item in the fabrication quality plan, not an informal practice. Define it specifically:

Connection type / weld type (e.g., CJP butt joint, all tension connections on main frames)
Hold point or witness point designation
Inspector signature required before fill passes begin
NDE at back-gouge if applicable (demand-critical connections)

A quality plan that lists these as hold points gives the CWI the authority to stop welding and gives the fabricator a documented reason why welding stopped. Both sides are protected.

Tracking root pass inspection sign-offs alongside WPS traceability in the same audit packet simplifies final documentation. If your current system makes this difficult, see how WPS Welding structures audit packets and inspection records.

Rule library based on AWS D1.1:2025; verify against your governing edition. Visual acceptance criteria and back-gouging requirements may differ in AWS D1.1:2020 and earlier editions.