Cut a Truss? Never, Almost - 4/12/2004 - Home Foundation Structure Framing

Cut a Truss? Never, Almost

I’m currently doing an addition to my house. Because I’m acting as my own general contractor, all the innumerable odds and ends my subs won’t claim are falling back to me. I’m too cheap to hire this miscellaneous stuff out, and so have been strapping on my trusty old tool belt each weekend.

One of the most devilish details has proven to be the new doorway from old to new. The new floor level is two feet higher than the old, which means there is an existing ceiling that would clock you in the forehead every time you walked through. Raising the old ceiling at the doorway became a requirement.

This required cutting a truss.

The phrase, "cut a truss," should throw all builders into a cold sweat. You can get away with drilling and notching beams, rafters and joists if you know where it’s safe. (You can learn where it’s safe and why by reading my book, “Basic Structural Concepts for the Non-Engineer," available on line at www.constructioncalc.com.) It is never okay, however, to snip or heavily notch a truss. Well, almost never.

A little technical background. Wood (and steel for that matter) trusses are among the most efficient structural devises there are. This is due to their shape and construction; and to the extreme competitiveness of the truss industry. There is virtually no extra meat in a pre-engineered roof or floor truss. If, for example, a truss manufacturer used 2 x 6’s where 2 x 4’s would work, he’d lose a lot of bids.

Some observant person is likely to seize this opportunity to remind me that I’m forgetting about the factor of safety… that everything built to code includes a factor of safety of approximately 2.5 (i.e. is 2.5 times stronger than necessary to avoid failure). Why then would cutting just one piece of one truss cause a problem?

That factor only applies to complete, whole trusses. It covers things like an extra large knot in a member, or a gang-nail plate not placed exactly in the right spot or perhaps even a small hole drilled through a member for a piece of wiring.

It definitely doesn’t cover the four-foot hunk of bottom chord I had to hack from one of mine. Even though I analyzed the thing and added two new supports, plus lots of structural straps in strategic places, it was with a thumping heart and sweating brow that I guided my sawsall through that bottom chord. Nothing budged, of course, but cutting a truss is always a nerve-wracking experience.

Not all builders and tradesmen share this opinion. I’ve seen many trusses cut, cored and heavily notched. When this occurs, there are usually ripple effects such as cracking drywall, overstressed adjacent members, caved-in ceilings and other collateral damage. Add to this an outrageous engineer’s bill to design the fix and you have a very expensive repair indeed. Not to mention the outright danger associated with a violated truss.

So, next time your vent pipe or remodel requires the cutting of even one truss member, try to think of a way around it. If there is no other way, get an engineer involved before you cut. His outrageous bill will be less than if you call afterward.

Tim K. Garrison, P.E., M.S.C.E., of ConstructionCalc.com™ has authored a book and several short courses, and lectures on topics relevant to builders. Reach Tim at timg@constructioncalc.com.

The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.