A wood roof truss is a marvel of modern engineering. It enables home builders to use less lumber to build a roof in less time with less labor cost, and frame a complicated roofline without puzzling over rafter-cut calculations. 

    But the important thing to know about a truss is that the loads it carries zig-zag through the webs, alternating between tension and compression, as they head towards the bearing points at each end of the truss. Even a single missing or damaged component can disrupt the load path and significantly reduce the structural strength of the truss.

    Since a truss is an engineered product, any repair to a damaged chord (outer piece) or web (inner piece) must also be designed by an engineer. Sistering a 2x4 on either side of a fractured web, for example, is not adequate. Also, using the metal mending plates you find at big-box home improvement stores—even the ones that look like the plates already on the truss—will not work. The plates installed at the truss factory are pressed into the lumber under hundreds of pounds of pressure, and sized and positioned exactly for the loads being transferred.

    If you are considering buying a house that has had roof truss repair, here’s what to look for to tell if it was done correctly: 

• Nails or wood screws are uniformly spaced typically 2 to 4 inches apart.
• Plywood or Oriented Strand Board (OSB) used for gusset plates.
• Gusset plate typically oriented in the strong direction on the broken member.
• Engineered (sealed) plans should be attached or available for inspection.
• (Optional) Structural glue such as LiquidNails may have been required as part of the design.

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Here’s links to some of our other articles about trusses:

• Are roof trusses better than roof rafters (stick framing)? 

• What are the most common problems with wood roof trusses?

• Are wood roof trusses with plywood gusset plates alright for house construction? 

• What are the rules for cutting, notching, or boring holes in an engineered wood truss?

• What is truss uplift? 

• Can I remove the interior walls under a roof truss? 

• How much of a roof truss can I cut out to make a storage platform in the attic? 

• What is the average life expectancy of wood roof trusses? 

— Thanks for Randy Mayo, PE, of RLM & Associates, Rolla, MO, and the International Association of Certified Home Inspectors (InterNachi), for assistance with this article.