Backing Strip — What It Is and Why It Matters
A backing strip (also called a backing bar) is a strip of material placed behind a weld joint to support the root pass, contain the weld pool, and enable complete joint penetration from one side. The backing prevents the molten metal from dropping through the root opening, making it easier to achieve full penetration without burn-through.
Steel backing strips that remain in place (permanent backing) are the most common type in structural steel fabrication. They are typically 1/4" thick and several inches wide, tack welded to the backside of the joint. Removable backing (ceramic, copper, or fiberglass tape) is used when permanent steel backing would be undesirable — the backing is removed after welding and the root may be back-gouged and welded from the second side.
Using backing simplifies the root pass considerably because the welder can push through with high heat without worrying about root drop-through. However, the junction between the backing strip and the base metal can create a notch or stress concentrator, which is why some fatigue-critical applications prohibit permanent backing or require it to be removed and the root completed from the back side.
Frequently Asked Questions
When should I use a backing strip?
Backing strips are commonly used on single-sided groove welds in structural steel, especially when the root opening is large or when the joint is difficult to access from the back side. They simplify the root pass by providing a surface for the weld pool to rest on. Use permanent steel backing for structural work or removable ceramic backing for joints requiring clean back surfaces.
Why is permanent backing sometimes prohibited?
The gap between the backing strip and the base metal can create a crevice or notch that concentrates stress, especially under fatigue (cyclic) loading. Some codes (AWS D1.1 for cyclically loaded structures, for example) require backing to be removed and the root welded from the second side, or specify specific conditions under which permanent backing is acceptable.