The functions of trusses are the same
as rafters, roof joists and ceiling joists.
- supporting the dead loads such as sheathing, roof coverings and roof-mounted equipment
- supporting the live loads, including wind, snow, rain and people on the roof
- supporting ceiling finishes and insulation
- laterally supporting exterior walls
- creating attic space and ventilation space
- supporting soffits and fascia
However, trusses are fundamentally different from assemblies of rafters and joists. Rafters and joists are predominantly designed to resolve roof loads by resisting bending, while trusses resolve roof loads by resisting tension and compression.
The most common materials are wood with metal or plywood gusset plates. Gusset plates are connectors that join chords and webs in trusses.
Chords form the upper and lower edges of the trusses. There are typically two top chords and one bottom chord, although this can vary, depending on the truss. A parallel chord truss, for example, has one top and one bottom chord. A scissor truss, designed to create a cathedral ceiling, has two bottom chords and two top chords.
Roof sheathing is typically fastened to the top chords and ceiling finishes are attached to the bottom chords. These chords are usually made of 2 by 4s, 2 by 5s, or 2 by 6s.
Webs are the internal components of trusses. Webs typically run from the top chord to the bottom chord. They may be vertical or at an angle to the vertical.
Bracing Of Compression Webs
The webs are wood members, often 2 by 3s or 2 by 4s. Some webs are in compression and others are in tension. It’s not easy to visually determine which webs are being squeezed and which are being pulled. Tension webs are stretched and are not inclined to buckle. Compression webs, however, are squeezed from either end and are prone to buckling. As a result, compression webs are often braced with 1 by 4s, running along several trusses. The braces are fastened to the midpoints of all the compression webs to prevent them from buckling. Many truss manufacturers staple paper signs to their trusses with the words “brace here” to help with proper assembly. You can see these signs and if there are no braces, you’ll know something was left out.
Trusses Need Lateral Support
Trusses require lateral support to prevent rotation or sideways movement. Diagonal bracing is often used during construction to keep the trusses in place until the sheathing and ceiling finishes are installed. The drywall below and sheathing on top may provide adequate stiffening and lateral support for most trusses. Be on the lookout for ceiling drywall that may have been removed or never installed, particularly in garages.
It is common to find trusses spaced 24 inches on center. This has some implications for both sheathing and drywall. It’s common to find that the sheathing sags between trusses. This is because:
- the sheathing wasn’t thick enough, or
- the sheathing has been weakened, often by rot resulting from condensation.
Drywall ceilings often sag below trusses. This is because the trusses are 24 inches on center and standard 1/2-inch drywall was used. Special 1/2-inch ceiling board or 5/8-inch drywall helps prevent sagging, even with wet-applied textured finishes. Some strap the undersides of trusses with 1 by 4s, every 16 inches. This allows the drywall to be fastened every 16 inches rather than every 24 inches.
Shapes of Trusses
Several common truss shapes are illustrated below.
Truss Spans and Sizes
We can’t tell in the field whether the spans are acceptable and whether the truss chords and webs are the right size. We have to rely on the designer. However, we can look at the performance of the truss. If there is movement, we can describe that, without knowing whether it’s a design problem or some other difficulty. As in most areas, we identify nonperformance rather than analyze the design.
We won’t discuss all of these conditions, but we’ll talk about cuts, notches and holes. The two most important points regarding trusses are 1: Trusses shouldn’t be damaged, altered or deformed in any way. 2: Any damage, deformation or alterations should only be addressed by an engineer
or truss designer.
Common truss problems include:
1. cuts, notches and holes
2. mechanical damage or splitting
3. damaged or weak connectors
4. buckled webs
6. rotation or lateral movement
7. poor end bearing
8. rot or insect damage
9. truss uplift
10. fire damage
Watch for trusses cut where they meet chimneys, vents or stacks. Field cutting and re-supporting often creates weaknesses and stress concentrations the designer didn’t anticipate. When in doubt, recommend further evaluation.
Notches and holes are not permitted in trusses unless the design allowed for them. This would be unusual. Notches and holes are typically caused by people working in the roof space. Wherever you see holes or notches in truss members (chords or webs), write this up for further evaluation. Simple sistering or other repairs common with joists and rafters may not work with trusses. It’s possible that the truss design allowed for this, but you have no way of knowing.
The bottom chord and the web members have been clearly damaged through cutting – this roof structure requires immediate repair.
A notched top chord will reduce effectiveness of the overall truss.
Where members have been notched or drilled, look for evidence of movement indicating distress. Where you see buckling or sagging, you can be sure that the design did not contemplate these. Trusses that have been cut or damaged should be considered ineffective, and further evaluation by a specialist should be recommended.
We have briefly introduced the topic of trusses and discussed some of the systems, components and installation methods that make them work. We have also outlined some of the conditions found during home inspection. More information on the implications and the strategies for inspection can be found in the ASHI@HOME training program.
ABOUT ASHI @ HOME
This article is from the ASHI@Home education system, a comprehensive distance-learning program developed by Carson Dunlop with ASHI. Individual modules are approved for ASHI CE credits. Choose the printed version or the online learning program. Call 800-268-7070, Ext. 251, to learn more.