Cut a truss? Never. Almost

I recently did an addition to my house. Because I was my own general contractor, all the innumerable odds and ends my subs wouldn't claim fell back to me. I’m too cheap to hire this stuff out, so I spent a lot of time strapped into my crusty old toolbelt on weekends.

One of the most devilish details was 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. (I write and teach on this subject if you'd like to learn more.) 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 2x6’s where 2x4’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 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.

In the photo below the cut truss bottom chord is right next to my smiling forehead.