Roof Trusses

 

 

Roof trusses perform the same function as rafters, collar ties, knee walls, and ceiling joists. The roof truss holds up the roof sheathing and shingles, transferring the roof loads to the outside or bearing walls. The bottom of the truss also supports the ceiling finish, upon which the insulation rests.

Most trusses used in residential construction are made up of wood components. The top and bottom members of the truss are referred to as "chords". The interior members of a truss are referred to as "webs". Individual wood members of the trusses are secured together with "gusset plates". These may be made of plywood or steel.

Different configurations of trusses have different strengths, and engineers can use the shape and component size that best suits them. Trusses are typically preengineered systems. Trusses are normally spaced twenty-four inches apart, but this can vary, again depending on the spans and depth of truss desired.

There are two common truss types used residentially. The Fink or "W" has web members that form a "W". The Howe truss can be identified by vertical web members, including a vertical web running up to the peak. The Howe trusses are somewhat stronger, although as long as spans are within the capabilities of the truss, either will perform well. There are many variations of these found in residential construction.

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Figure 35. Roof Trusses

 

In either truss type, the webs should be at least two by fours, unless special engineering consideration has been given. Where the compression webs are longer than six feet, they are susceptible to buckling under heavy loads. Braces, such as I x 4s should be fastened to the midpoints of these webs.

Where the bottom chord has a long span between support points, it may not be strong enough to carry the ceiling load. If the span is more than ten feet between support points, the bottom chord should be at least a 2 x 5. If the span is more than twelve feet, the bottom chord should be a 2 x 6. Again, special engineering consideration can result in deviations from these rules of thumb.

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Figure 36. Roof Trusses

 

Like any wooden member, trusses are subject to rot, tennite damage, mechanical damage and fire.

Individual chords or webs which are cut or damaged can be a serious problem. Cutting a truss in one spot may seriously compromise the entire truss structure. Where trusses have to be cut to accommodate chimneys or other interruptions in the roof line, special engineering consideration should be given.

Trusses must, of course, be well secured to ensure their performance.

Overspanning of trusses can lead to deflection under load and, in worst cases, roof collapse. Reasons for overspanning, other than a simple error made on original construction, would include re-roofing with a heavier roof covering, or greater snow loads than expected, due to unusual conditions or drifting. Overspanning cannot typically be identified during a home inspection, but evidence of deflection or failure would be noted.

Reinforcing trusses which are overspanned is more difficult than strengthening a rafter roof system. An engineer specializing in this area should be consulted to determine the most cost-effective approach.

Trusses with compression web members longer than six feet may be subject to buckling. Braces should be added to the midpoints of the webs. One brace attached to each web with two nails should connect several webs in adjacent trusses. The braces should be at least 1 x 4?s.

Undersized bottom chords should be stiffened to prevent ceiling sag and cracking of ceiling finishes. Adding a second member to the bottom chord would normally be satisfactory.

A phenomenon known as "truss uplift" is relatively common in new houses. This is not well understood, but does involve the bottom member (chord) of the truss deflecting upward during winter weather. It is argued that the temperature and humidity changes in the attic during the winter months affect the sections of the truss above the insulation level, differently than the bottom chord buried in the insulation. This results in an upward bowing of the bottom chord.

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Figure 37. Truss Uplift

 

The result of truss uplift is that the center section of the bottom chord moves upward, and gaps as large as 1-1/2 inches appear at the top of the interior walls, where they join the ceiling. The ceiling is picked up by the truss. It is less common but also possible, that the entire wall below will be lifted up, and separation will occur between the bottom of the wall and the floor.

At present, a good solution is not known for this problem, although common corrective action is to secure a molding to the ceiling (but not to the wall). As the ceiling moves up and down, the molding will slide up and down the wall although no gap will appear. Another solution is to disconnect the ceiling drywall from the truss. Alternate ceiling support is generally necessary.

Research is being conducted into control of this phenomenon. Truss uplift is not a serious structural problem.