A backing (strip) is a piece of metal that is placed on the backside of a weld joint to prevent the molten metal from dripping through the open root (burn through). It helps to ensure that 100% of the base metal’s thickness is fused by the weld (full penetration).

The backing may be either left on the finished (cofferdam or void space closing welds without both-sides accessibility) weld or removed following welding (Ceramic backing, Copper strips). If the backing is to be removed, the letter R is placed in the backing symbol. 

The backing is often removed from a finished weld because it can be a source of stress concentration and a crevice to promote rusting.

Why would you want to remove a backing strip from a weld?

There are several reasons to remove a backing strip from a weld. The strip can collect moisture and cause the part to rust. A backing strip can keep a weldment from bending evenly under a heavy load, and if it cannot bend uniformly, it will break.

Why Copper Strip is a removable backing?

The high heat conductivity of a copper backing bar or strip, for example, will prevent it from sticking to the weld metal, while its chill-mold effect will assure a clean, smooth weld metal surface. Electrolytic copper has proven to be the most satisfactory material for backing up a weld.

Can we use GTAW for root instead of using Backing strip?

Depend on WPS and Code being used, example: As per AWS D1.1 The WPS qualified with CJP means that the Backing/ Back gouging works must be carried out. The others do not require it. In this case, The GTAW root is considered as welding without backing. 

See Backing vs Retainer in Welding for further information.