Holes & Notches in Trusses
Holes & Notches in Trusses
Engineered to achieve optimum efficiency, metal plate connected wood trusses are manufactured to handle specific stresses. If everything goes according to plan, a truss should perform exactly as it was designed. Not every job goes as planned though. Shipping, handling, storage and other trades may unintentionally alter a truss, leaving holes or notches, which can affect a truss’s performance.
If there are holes or notches in a truss, is there an easy way to tell whether a repair will be needed?
Whenever material from a truss is removed, analysis by a licensed engineer is required.
For small holes or notches, such as for electrical wiring, it may be possible to avoid a repair in visual graded lumber. A repair may not be needed if, after adding the holes, the lumber still meets an adequate grade according to the appropriate grading criteria. If the original lumber is relatively free of knots and holes, an engineer may determine that no repair is required. (See Figure 1.)
For example, No.2 lumber allows for one hole up to 1-¼" per lineal foot. If the existing lumber does not have any holes or knots within 1 lineal foot, and the hole added does not exceed 1-¼", the lumber may be acceptable without a repair. Altered lumber is also subject to edge distance requirements (among others), which are not listed in the summary in Figure 1 above. See sidebar below for more information on lumber grading rules publishing agencies.
If the lumber in question is not visually graded, or does not meet the visual grading criteria by inspection, an engineer will look at the Combined Stress Index (CSI) of the lumber to determine what extent of a repair is needed. The CSI is defined as the summation of axial and bending stresses divided by their respective allowable stresses for a specific truss member. This ratio, or index, represents the structural “efficiency” of the member. The CSI shall not exceed 1.00.
When material has been removed from a board, a greater percentage of the board’s capacity must withstand the same stresses. Therefore, the CSI will increase. If the original CSI was low, removing small amounts from certain locations on a stick of lumber may be acceptable, if doing so does not result in the CSI exceeding 1.00. The new CSI of the board can be determined by hand or spreadsheet calculations, or by using one of many software programs available. However, the CSI of the repair material plus the CSI of the remaining lumber from the original board must not exceed 1.00. This is why members with very high grades of lumber may require a similarly high grade of lumber on the repair, instead of more commonly available lower grades of lumber.
It is also important to note that, depending on the shape and location of the hole or notch, stresses may occur during handling that create further damage to the truss. In cases like this, a repair would be wise to prevent further damage.
Ultimately, the best way to deal with holes or notches is to avoid them altogether. By coordinating with trades before construction (for example locating plumbing drops and needed chase returns on a truss layout), holes in trusses, and the costly repairs associated with them, can be avoided.
Lumber Grading Rules Publishing Agencies
For more information on lumber grading requirements, consult a lumber grade handbook from one of the following organizations:
- Northeast Lumber Manufacturers Association (NeLMA): nelma.org
- Northern Softwood Lumber Bureau (NSLB): nelma.org
- Redwood Inspection Service (RIS): calredwood.org
- Southern Pine Inspection Bureau (SPIB): spib.org
- West Coast Lumber Inspection Bureau (WCLIB): wclib.org
- Western Wood Products Association (WWPA): www2.wwpa.org
- National Lumber Grades Authority (NLGA): nlga.org