To strengthen a truss member or web, add a scab to reinforce it. For example, if you have a point load in the middle of a panel, one or two scabs may be used instead of increasing the grade or size of the member or using a multi-ply truss.
Scabs reinforce overhangs by enhancing bending and shear for fully covered panels; the scab must cover the full length of the overhang, as well as the adjacent panel. Scabs also enhance deflection performance in chords.
Scabs provide increased strength on a truss, as shown in the comparison below. [For images, See PDF or View in Full Issue.]
- Scabs can be used as bearing blocks to enhance bearing capacity. When a scab is placed over a bearing, the Truss Design Drawing indicates that the scab has been used for bearing enhancement; a cluster of fasteners will be indicated in the Scab Fastening Schedule.
- Scabs trim to the truss profile rather than the member profile. When a scab is applied to a bottom chord at a heel joint, it will automatically trim across the top chord to the truss profile, so that it can be used for bearing capacity checks. A scab can be manually trimmed, if needed.
- Scabs are the same lumber grade and size as the web to which they are applied.
- To delete a scab, select the scab and press Delete on your keyboard.
- Previously, when addressing overhangs and tail-bearing trusses, scabs reinforced members only in terms of bending and shear, but not deflection. Now, deflection is also accounted for when using scabs.
- Scab fastening information appears on the Truss Design Drawing.
Additional benefits of scabs include:
- Increased web strength, especially on parapet and drop leg designs
- Increased strength for deflection purposes
- Scabs strengthen your existing lumber grades/sizes, allowing you to use lumber already in stock
- Eliminate the need for larger lumber sizes or the addition of strut braces to reinforce parapets and drop legs.
Simpson Strong-Tie’s Component Solutions® CS Truss Studio™ software has the ability to apply scabs to webs and parapets. Contact your Simpson Strong-Tie Representative or email CSHelp@strongtie.com for more information.
The article, “Truss Highlight Blog – Scabs on Webs & Parapets,” also will appear on Simpson Strong-Tie’s website. To receive post notifications from the Simpon Strong-Tie Engineering Blog, go to seblog.strongtie.com/subscribe. Copyright © 2025 Simpson Strong-Tie Company Inc. All Rights Reserved