How to Strengthen a Scissor Truss

Home trusses can be strengthened by adding additional custom cut webbing into the truss.

Wood trusses are prefabricated and installed on top of a home's load-bearing walls to create a roof-deck system. The prefabricated trusses are stronger than roof rafters, and install much faster than individual rafters. When a regular or scissor truss must be strengthened, the procedure is easy to perform, and can be done on site, even after the trusses are installed.

Things You'll Need

• Installed scissor trusses
• 2-by-4-by-8 studs
• 7-1/4 inch builder's saw
• air compressor and framing nailer
• -or-
• hammer and 16D framing nails
• pocket bevel or framing quick square
• measuring tape and pencil

Instructions

1. Determine the points at which the truss must be strengthened. The webbing installed between the upper- and lower-truss cord provide the vertical strength in the truss. These pieces prevent the truss from compressing under load

2. Measure the distance from the top cord to the bottom cord at the point which will receive additional webbing. Measure the longest distance between the points on the cords.

3. Measure the angles at which the new webbing is installed with the pocket bevel, or the framing quick square. If possible, place the webbing at a right angle to the upper cord, butted against a piece of existing webbing. This placement enables the webbing to meet the lower cord at a custom angle.

4. Cut the piece of webbing at the appropriate length, with the correct angle on each end. When the webbing is installed in the truss, each end must be flush to its respective cord.

5. Insert the webbing into the truss and nail it in place. Nail the webbing so that the new piece of 2-by-4 is flush with both sides of the truss. It must become an integral part of the truss, in the same plane as the upper and lower cords, to strengthen the truss.

Tips & Warnings

• Nailing webbing onto the sides of the truss will not strengthen the truss satisfactorily. The strength of the truss is based on the non-compressible nature of the webbing between the upper and lower cord. A piece of webbing nailed onto the side of the cord is only as strong as the nails, which will likely loosen over time.

References

• Backwoods home Magazine: Trusses
• Ask The Builder: Scissor Truss Design
• Dajon Associates: Reinforcing Existing Structures

Resources

• Timber Home Living; Parts Of A Frame; M Morris