Open and Closed Roots in Weld Joints

Depending upon the end-use or service conditions of the weld joint the welding engineer or designer may choose between an open or closed root. Closed roots occur when the adjacent members touch each other with no gap or space between the two. On the other hand, the open root has a predefined gap or space. We call that space between the two joints the root opening.

The open root is usually used to join complete penetration joints between the welded members. When we think of penetration we usually think of how much the welding melted in or depth of bond between the two adjacent surfaces. The open root may also allow the backside of a joint to be welded from the front side. An example of this might be pipe that is joined to handle high-pressure, or high strength joints.

If adjacent members don't need to be completely welded through the full thickness or what we might call partial penetration, it may be acceptable to use a closed root opening. We can also use the closed root on full penetration welds as well. When using closed roots for full penetration welds we either need to chip, grind or gouge out the unwelded section from the opposite side prior to welding.

There are a number of factors that can also guide the decision whether to use an open or closed root joint when welding. Most important is usually the thickness of the base metal that we are trying to join. Thin materials usually will not tolerate a large root opening for most welding processes Thicker metals may use open root joints to aid with increasing penetration of the parts being welded.

Position will also affect the type of root opening we use. If we are making the weld the flat position the liquid from the puddle may drop through and causes problem. If we need to make a pipe weld in the 5G position where the pipe is running in the fixed horizontal position then an open root may be required. In almost all situations where a back up bar is used an open root is used with the backup bar spanning the back of the weld joint. Many times this back up bar is left in place after welding of the open root joint.

Depending upon the type of material may also influence whether or not we use an open or closed root joint. Some materials don't work well with open root joints on full penetration welds. Materials that have what we call hot shortness or lose strength at high temperatures are good examples of open root joints and materials that don't work well together. Aluminum is a good example of a material that works well as a full penetration weld using a closed root opening.

MIG welding of open root joints using short circuit transfer is a good example of a great combination of process and open root welding. Many contractors prefer to join pipes together made from carbon steel using the short-circuiting transfer method of gas metal arc welding the root pass. In some locations and uses, this is replacing the use of Gas Tungsten Arc Welding for root pass applications.

Check out the Longevity website ( or YouTube channel ( for more details and information about equipment for different welding and cutting processes. Longevity has the right machine for your exact application, so take a look and choose what is the best fit for your materials, product and needs.