Roof Truss Bracing Methods

Internal

• Basic truss bracing consists of internal members called webs, which connect the bottom chord or joist and the top chords or rafters. These brace the sloping rafters against the compressive or downward weight placed on them from roofing, snow and ice and other external factors. Webs are either vertical or diagonal, or a combination of both, depending on the roof design. Some truss styles, especially longer ones, use multiple webs in varying combinations.

Temporary

• Trusses must be braced temporarily when they are set on the walls of a house. This is typically done on trusses at the ends of the roof by nailing 2-by-4-inch boards to two or three (or more) points on the truss, set with stakes in the ground outside the roof. These braces hold end trusses vertically plumb until they can be secured to walls and braced against other trusses on the roof, often with a ridge board that runs atop the trusses from one end of the roof to the other end. Internal trusses sometimes are braced temporarily with boards nailed diagonally between trusses.

Lateral

• Small structures such as sheds and garages often get lateral bracing -- end to end -- with the addition of roof decking or sheathing, usually oriented strand board (OSB). This ties the trusses together so they do not move laterally. Larger structures need additional lateral bracing; the need increases with the span or width of the truss. Purlins, which are braces that run between the top chords of trusses, provide this support; they are sometimes removed on shorter house roofs before sheathing; other times sheathing is installed over them.

Permanent

• More permanent bracing is required on wider trusses with longer roof lengths. This will vary with the wind and snow loads on the roof, but typically has at a minimum diagonal braces connecting the first three trusses on each end. Most bracing uses three-truss clusters, with braces from the bottom chord of one truss to the top of the third truss, with braces staggered so boards end on different areas of trusses down the roof.

Special

• Some trusses require additional wind bracing, which consists of diagonal braces connecting three-truss clusters the length of the roof, or cross braces nailed in an X pattern across three-truss clusters at the ends of the roof and every 20 feet in between. Diagonal wind braces are fastened between top chords in a triangular pattern; X braces are fastened on bottom chords with the braces crisscrossed over one another in the shape of an X.