Inspecting Wood Trusses . Much of this information also applies to open- web floor trusses, although floor trusses have other unique issues not discussed here.
Wood roof trusses connected with metal plates have revolutionized the way we build houses. The earliest trusses in my area were built on- site in the 1. Figure 1). The members were joined together with plywood gusset plates held in place with nails. Fabrication shifted into factories during the 1.
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Figure 1: Site- built truss from 1. With modern trusses, we. Trusses allow us to build larger structures with more complicated shapes for lower cost, using less lumber and at the same time usually eliminating the need for interior bearing walls. Unfortunately, manufacturing and installation problems do occur. Damaged or improperly installed trusses are weak and may not support extreme snow or wind loads. Repairing a damaged truss is usually a straightforward carpentry job, but who should design it? Section 8. 02. 1.
IRC says: . Manufacturing Problems. The factory setting allows for a high degree of quality control.
Incorrectly manufactured trusses are quite rare, but not unheard of. For simple joints, the plate should be centered, although this isn. Misaligned plates are not as strong as the designer intended (see Figure 2). Figure 2: This metal connector plate was misaligned at the factory. You should always see a metal connector plate on both sides of a joint. Missing plates are almost always due to improper handling during construction (see Damage), but if there are no marks in the wood, then the plate was never installed. Since wood is a natural product, some knots are inevitable.
But large knots weaken the member (see Figure 3), particularly if a metal connector plate is pressed into one. Acceptable knot size for metal connector plates should be listed in the manufacturer. If in doubt, refer the issue to a licensed structural engineer. Figure 3: Knotholes weaken web members.
Damage. Trusses are incredibly strong once they. Plates can be torn off quite easily. Long trusses should be lifted with a spreader bar. If lifted improperly, trusses will bend sideways like a piece of spaghetti, resulting in plates that are torn out, plates that are buckled and broken web members. Perhaps the crews in my area have just gotten sloppy, but I find some truss damage in about one- forth of newly constructed houses, whereas damaged trusses are fairly rare in older houses (before about 1. Damaged trusses are often patched hastily by construction crews. If I see prior repairs, I suggest that my clients ask the seller or builder to provide documentation that the repairs were designed by a licensed structural engineer; if not, then the repair should be evaluated by an engineer to determine if it.
I also suggest that they keep a copy of the engineering documents for when they sell the house. Damaged metal connector plates are one of the most common forms of damage. Obviously, a plate that. The Truss Plate Institute (TPI) publication QST- 8.
Quality Standard For Metal Plate Connected Wood Trusses, says that the gap under a plate should not exceed 1. It also says that such a gap should not exceed 1/3 of the plate contact area on each member in the joint. Repair is not as simple as hammering the plate back in because the fingers on the plate are designed to be pressed one time into undisturbed wood.
Figure 4: Metal connector plates loose strength even when pulled only part way out. The normal repair is to install a plywood or OSB gusset plate, nailed in place.
These are usually much larger than the metal plate being replaced. The size and nailing pattern will be specified by the engineer.
Sometimes an engineer will specify a metal replacement plate with nails. As a truss is bent sideways, the metal plates on the outside of the curve are stretched or pulled out (as described above), but the plates on the inside of the curve are compressed and can buckle.
This damage is not as obvious (see Figure 5). If you find signs of damage in the accessible parts of the truss, you should also pull aside insulation to look for more damage to the lower chord. Figure 5: Compression buckling of connector plate on truss that was bent during installation (left). Broken lumber in trusses can range from missing members to cracks that are almost invisible. Several times I have passed by cracked lumber without noticing it, only to find it on my way back out of the attic because it moved when I put my hand on it to steady myself. Take your time and look closely.
Metal banding used to bundle trusses for shipping can also cut deeply into the lumber. Damaged lumber is usually repaired by splicing new lumber along the side (sistering), but may also involve gusset plate repairs. If you see a short piece of lumber added to one side of a top chord member with no visible damage on the other side, it. Inspectors sometimes mistake this for truss damage. Bracing. Buckling is what happens when you push on the end of a yardstick and it bows sideways. Internal web members of trusses are just like big yardsticks, and long web members that are under compression can buckle. To prevent this, designers call for bracing on some web members.
Sometimes the need for permanent bracing is indicated by a stamp or tag placed directly on the web member (see Figure 6). Other times, you have to look at the truss drawings to know if permanent bracing is needed (see Figure 7). In an older house, you won. Figure 6: Ink stamp reads ? According to the IRC, truss design is governed by ASNI/TPI 1.
Along with engineering guidance for designing a truss, that document also states that designing permanent truss bracing is the responsibility of the architect or engineer who designs the structure. The truss designer will indicate on the drawings which web members need to be braced. Sometimes the truss drawings will also specify how to install the bracing, but often they will not. Instead, most building designers simply rely on footnotes on the truss drawings, which refer to industry standards for installation. Truss installation used to be governed by HIB9. TPI. This document was superseded by BSCI 1- 0. Guide To Good Practice For Handling, Installation & Bracing Of Metal Plate Connected Wood Trusses (effective 1/1/0.
TPI and the Wood Truss Council (www. They also publish a series of Summary Sheets . BCSI 1- 0. 3 discusses many things, including both temporary and permanent bracing. Temporary bracing is needed to keep the trusses from falling over like dominoes during construction. Bracing can be either continuous lateral restraint (CLR) or individual web member reinforcement such as T- reinforcement. In older publications, CLR was called continuous lateral bracing (CLB).
Less obvious issues are discussed below. Note that some of these criteria are more stringent in BCSI 1- 0. HIB9. 1; the older the building, the less likely it will conform to modern standards.
Bracing lumber should be 2. CLR must be connected to a fixed point in the building (such as a shear wall or roof plane) or it must be diagonally braced. This includes the top chords of the lower set of trusses in a piggyback configuration. CLR is not effective when the web pattern changes from one truss to the next (see Figure 8). T- reinforcement should be used instead. T- reinforcement should be 9. CLR should be roughly centered in the span (see Figure 9).
Note that some web members may require more than one CLR. CLR should be installed so that it does not block the access hatch. T- reinforcement must be continuous; it. Since the top chord of a roof truss is in compression, it. Normally, the sheathing restrains the top chord against buckling, but the sheathing can be missing where intersecting ridge lines are created by one set of trusses resting on top of a lower set at right angles (also called a . In this configuration, the lower set of trusses should still have sheathing, usually with a hole to provide access between the two parts of the attic.
Installation Problems. Trusses must be properly integrated with the rest of the building. A common problem is nails missing in metal hanger brackets supporting trusses. Structural connectors to hold trusses down to the wall structure should conform to IRC section 8. Figure 1. 0: All nail holes should be filled in metal hanger brackets.
Heavy loads are carried by . A girder truss could be a single truss, but more commonly it. In order to act together as a single structural component, the individual members of a multi- ply girder must be properly fastened together. This can be accomplished with nails and/or special structural screws or bolts, sometimes in combination with each other. Specific fastener type, size and spacing should be spelled out on the truss drawings.
In these cases, the trusses are shipped in two sets, a lower set with a flat top chord and a smaller triangular set that rests on top of the lower trusses (commonly called piggyback trusses). Pay close attention where the upper trusses rest on the lower ones. Just like other trusses, piggyback trusses are designed with a specific bearing point, which is almost always at the end (see Figure 1. In high wind zones, the top section of piggyback trusses may need to be strapped down to the lower set; toenailing may not be sufficient.
Figure 1. 1: These piggyback trusses are not supported at the bearing point. Conventional framing is often mixed with trusses, particularly to create intersecting gables (often called overframing).
At the resulting valley, you. The conventional rafters should rest on a bearing plate to distribute the load to the adjacent trusses.
If this plate is missing, the rafters will be supported only by the roof sheathing, and will likely sag (see Figure 1. A similar situation can develop when a framed chimney chase rests on top of the roof. It should be fastened to the framing, not just to the sheathing. Trusses are designed for a specific spacing, usually 2. Increasing that spacing, even locally, can overload the trusses.
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