How to Repair Roof Trusses

Prefabricated wood roof trusses have been used in the US for more than 30 years in house construction. Builders often use trusses to reduce costs compared to site-framed roof framing. After the house is built, roof trusses are often modified when home owners and contractors make alterations. Many such modifications are not made correctly. Although damage often does not occur, for reasons described below, the potential for major problems remains.

In many cases, improper truss modifications are found by a municipal inspector, or home inspector for a prospective buyer, after a house is put on the market for sale. Remedial work is then usually necessary to allow the sale to proceed. Concepts for the most common modifications are discussed below. A qualified professional engineer should be consulted for all truss modifications.

BASIC PRINCIPLES
Roof trusses are an assembly of wood pieces (most often 2×4s) connected together with steel plates to form a unified structural member. Within the industry, a truss is referred to as a “component”. A truss serves the same purpose as any beam, which is to support loads across open space. This basic principle applies to roof trusses, even though the shape is obviously different than a “straight” member (such as a 2×12) that is more recognizable as a beam. The truss does behave quite differently than a solid, straight beam. Each member (piece) of the truss must resist force in tension or compression, parallel to the length of the member.

The key principle to understand, relative to modifications, is that every member of a truss is essential for adequate performance. If a truss member is cut or removed, the entire truss becomes defective, unless remedial work is performed to properly redistribute loads around the modified part of the truss. Cutting or removing a truss member has essentially the same effect as making a cut through the entire depth of a solid beam, such as a 2×12 floor joist. Roof trusses typically require lateral bracing, perpendicular to the plane (length) of the truss. This bracing should not be removed, especially for trusses with relatively long “web” members.

TERMINOLOGY
To understand discussion of roof trusses, it is necessary to understand standard terminology. Reference to a truss diagram is of course useful. The Wood Truss Council of America (WTCA) provides basic information, including diagrams, on their web site; http://www.sbcindustry.com.

Understanding the following terms is necessary to clearly discuss residential roof trusses;
Joint Location – where truss members intersect.

Truss member – Individual piece (segment) between joints.

Long chord – members may consist of two or more segments joined by light-gauge steel splice plates.

Bottom chord – One or more members that form the bottom of the truss. Bottom chord members are most often horizontal. However, for some applications, such as a “cathedral-type” ceiling, bottom chord members may be sloped.

Top chord – One or more members that form the top of the truss, which also forms the roof surface. Top chord members are most often sloped. For the simple gable (”A”) roof, there will be two lines of top chord members; one for each roof slope.

Web Truss – member between bottom chord and top chord. A web member is almost always vertical or sloped.

Connection plate – Light-gauge steel plate used to connect the various wood members. Plates are installed at the factory. It is not practically feasible to install the standard connection plates at the site.

TYPICAL TRUSS MODIFICATIONS
Roof trusses are most often modified to install an opening in the ceiling under the roof. The ceiling is attached to the underside of the truss bottom chords, which also support any attic floor that may have been installed. One typical reason for making an opening is to install folding stairs to provide access to the attic space. Almost all residential roof trusses are spaced at 24-inches. Clear space between bottom chords of adjacent trusses is then 22-1/2 inches. If the length of the opening (for folding stairs) is parallel to the trusses, the frame for the stairs if often too wide to fit within the available space between trusses. Therefore the bottom chord of one truss must be cut.

In some cases, the only way that the stairs can be useful is to install the length of the stair opening perpendicular to the trusses. For this orientation, the bottom chord of several trusses may have to be cut. As noted above, when a truss member is cut or removed, the truss is no longer capable of supporting load across the full span of the truss, between ends.

EVALUATION
Before cutting any member of a truss, an evaluation should be made to determine how the cut truss will be supported and reinforced after the modifications. Of course in many cases, this evaluation must be performed after the truss has already been cut. First step is to determine if there may be interior walls that can be used as a bearing wall, to support the modified truss. In many cases interior bearing walls are available, especially when new folding stairs are installed above a hallway. A bearing wall must have adequate support under the wall. A continuous “load path” must then be available so that all loads supported by the bearing wall can be safely transferred through the structure, all the way down to foundation elements.

CUTTING BOTTOM CHORD – WITH INTERIOR BEARING WALLS
If a bearing wall is available for each segment of the bottom chord (that was cut), each segment can usually be supported on a wall. However, some reinforcement of the truss may be required, such as installing a new 2×4 or 2×6 against the bottom chord. New web members may also be required, with adequate connections made by plywood “gussets” and either nails or wood screws.

CUTTING BOTTOM CHORD – WITHOUT INTERIOR BEARING WALLS
In the event that interior bearing walls are not available (for both segments of the cut bottom chord), the following work is required;
1. Install a header across each end of the truss bottom chord that has been cut. This will require two headers, which fit between adjacent full-length trusses that have not been cut.
2. Install light-gage steel joist hangers or other hardware to connect cut end of the truss bottom chord to the header.
3. Reinforce the full-length adjacent trusses as necessary to support new “point” load from the headers. Structural analysis by a qualified professional engineer is required to determine the extent of reinforcing necessary. If possible, analysis should be performed by the truss manufacturer.
4. Install joist hangers to support each end of each header, at the reinforced full-length trusses.

Installation of headers is practical when only one or two trusses have been cut. However when several trusses have been cut, it may not be practical to reinforce the full-length adjacent trusses due to the relatively large point loads imposed by the long headers.

CUTTING TOP CHORD
When a top chord must be cut, perhaps for a new skylight, repair requirements are similar for the case of cutting the bottom chord. However, installation of new web members is more likely to be necessary.

CUTTING WEB MEMBERS
When a web member must be cut, the truss can often be modified by installing new web members near the location of the cut web. However, when this is not feasible, much more extensive reinforcement of the truss will be required.

REPAIR OF DAMAGED TRUSS MEMBERS
In general, it is always possible to repair a damaged truss member using relatively simply methods. Damage can occur for many reasons, including impact during construction. Many trusses that are damaged by construction workers are not repaired by the builder. Essentially, another wood member must be installed alongside, and securely connected to, the damaged member. More difficult is repair of damaged steel connection plates. Much larger plywood gusset plates often must be installed to provide the same connection capacity.

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John F Mann, PE
Structural Support
1212 Main Street, Suite 2
Belmar NJ 07719
732-556-6080
jmann77@optonline.net

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