Wood truss failures can vary and identifying the cause requires visual inspection as well as a working knowledge of the structural loads and building codes. These truss systems must transfer the gravity and lateral loads to the foundations. Consequently, the framing system and the foundation provide strength and stability for a structure. The most common type of wood-framed construction uses roof trusses, exterior and interior load-bearing walls, beams, girders, posts, and floor framing to resist the gravity and vertical loads. This type of wood-framed construction engages a system of horizontal diaphragms (roof and floors) and shear walls (vertical exterior sheathed walls) to resist the lateral loads.
Roof trusses are made up of individual wood framing members connected with special galvanized metal nail plates. This truss system utilizes pre-engineered press-plated wood trusses uniformly spaced across the structure. These trusses are typically constructed with nominal 2x dimensional lumber for the top and bottom members, called chords, and the diagonal members, called webs. The members are typically held together with metal gusset plates at each joint. The plates are secured to the wood members by rolling the entire assembly through a hydraulic press. Trusses with this type of framing system typically span from load bearing wall to load bearing wall with no intermediate support.
Storm events, including heavy snowfall, wind, or rain, are a common cause of minor structural problems in the roof system. These are problems which may be present even if no major damage is visible from the outside. The other main cause is the normal aging and degradation of the wood structural members and the fasteners associated with them. When moisture from roof leaks above, or ceiling holes below, is introduced into the attic, this degradation process is accelerated. As such, knowing what to look for is important. The galvanized metal nailing plates that hold the truss members together are another area of concern and should be closely monitored. The increased stress they are put under during a wind or snow event can cause them to pull loose from the truss.
Modification of the truss is another factor that can contribute to failures. The building codes provide guidance regarding modifications that can be made to roof framing members. The 2015 ICC International Residential Code lists the following:
- R802.7 Cutting, drilling and notching. Structural roof members shall not be cut, bored, or notched in excess of the limitations specified in this section.
- R802.7.1 Sawn lumber. Cuts, notches, and holes in solid lumber joists, rafters, blocking and beams shall comply with the provisions of R502.8.1 except that cantilevered portions of rafters shall be permitted in accordance with Section R802.7.1.1.
- R802.7.1.1 Cantilevered portions of rafters. Notches on cantilevered portions of rafters are permitted provided the dimensions of the remaining portion of the rafter is not less than 3-1/2 inches (89 mm) and the length of the cantilever does not exceed 24 inches (610 mm) in accordance with Figure R802.7.1.1.
- R802.7.1.2 Ceiling joist taper cut. Taper cuts at the ends of the ceiling joist shall not exceed one-fourth the depth of the member in accordance with Figure R802.7.1.2.
- R802.7.2 Engineered wood products. Cuts, notches, and holes bored in trusses, structural composite lumber, structural glue-laminated members, cross-laminated timber members or I-joists are prohibited except where permitted by the manufacture’s recommendations or where the effects of such alterations are specifically considered in the design of the member by a registered design professional.
- R802.10.4 Alterations to trusses. Truss members shall not be cut, notched, drilled, spliced, or otherwise altered in any way without the approval of a registered design professional. Alterations resulting in the addition of load (e.g., HVAC equipment, water heater) that exceeds the design load for the truss shall not be permitted without verification that the truss is capable of supporting such additional loading.
Most truss failures are often attributed to one of the following:
- Improper or lack of temporary/permanent bracing
- Incorrect loading or overloading during construction
- High winds during erection
- Utilizing weak members or bad joint connections
- Damaged, broken or improperly repaired trusses
- Installing unacceptable or unauthorized design changes in the field
- Withdrawal of galvanized metal nail plates
- Heavy snowfall
A site visit and visual inspection are crucial in determining the root cause of roof truss failures. A working knowledge of truss construction and installation as well as the proper application of the codes stated above can help prevent truss failures and subsequent property damage.
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