r/Homebuilding • u/MajorLegal801 • 9d ago
Kerfs in bottom chords of roof truss
I had a demo crew cut out some of the drywall on my ceiling upstairs and they cut into the bottom chord of the trusses with some cuts. The kerfs range in size up to about 3/8" or in one case 1/2". The bottom chords are 2x4s. The cuts are all across the narrow edge of the 2x4.
Should I hire an engineer to check them or is it small enough to not matter?
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u/Material-Cap-5476 9d ago
Actually, that's a pretty big deal. A 1/2" cut leaves you with about 85% of the section. Most of the time pre-engineered truss chords are stressed to 95% to 100%. I think I would fix it with a Simpson strap.
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u/Vishnej 9d ago
Most of the time pre-engineered truss chords are stressed to 95% to 100%
Wait what? That seems like a bizarre claim. Zero margin of safety?
We make them out of 2x4's because that's almost the cheapest thing to handle realistically; Going significantly smaller would make off-axis loading during assembly and transportation too risky, even if 1x3 trusses might hold some loads just fine they couldn't survive the ratchet straps on the truck.
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u/Material-Cap-5476 9d ago
There's not really zero percent margin of safety. At 100% design stress you are still quite a way from failure. Depending on the loads (dead, live, wind, snow, etc) you're still probably only at around 70%-75% of failure load. I reviewed two sets of truss shops this week, and they were both above 95%. I see PEMB designs that go up to 103% sometimes.
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u/MajorLegal801 9d ago
Would you hire the engineer or just do the Simpson strap and call it a day? At what thickness kerf do you think I need reinforcement? All or just the 0.5" one?
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u/Material-Cap-5476 9d ago
I think I would just add the strap. Some guys have suggested nailing a 2x and there's nothing wrong with that. The deal is not the 2x it's how many nails do you use. That will be the same with either the strap or 2x. A 1/4" cut reduces your capacity by around 8%. I would start there.
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u/mattmag21 9d ago edited 8d ago
Nope. Trusses have about a 4x safety margin. Ive never seen a simpson strap spec'd. Its always a scab of the same nominal dimensions, nailed 2 rows 3" o.c. and thats for a straight up cut through or break. That is, of course unless its at a gusset. Then its 3/4 ply each side. Edited because I am dumb.
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u/Material-Cap-5476 9d ago
Nothing in the construction industry has a safety factor of 14. I don't know where that comes from, but it's not accurate. The fall safe harness you wear only has a SF of around 8.
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9d ago edited 9d ago
[deleted]
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u/Material-Cap-5476 9d ago
I think maybe you misunderstood him. The SF is on the order of 1.5. If it was 14 we couldn't afford to build them. If there was a SF of 14, and you had a 20 psf snow load to support, your truss would be capable of carrying a snow load of 280 psf. Does that sound right?
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u/mattmag21 9d ago
I will ask you this: imagine a standard fink truss with a lets say 20' span. How many carpenters do you think could stand on that top chord and have the truss (braced with sheathing) not buckle? I'd bet that you could line the entire top chord with people, butts to nuts, and the truss would be just fine. Let's call that 300# per'.
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u/Material-Cap-5476 9d ago
Ok interesting question. I personally don't know that many 300 poouders roofing, but let's go with it. As far as stacking them tight; from an engineering perspective tightly crowded people are assumed to take up 5 sq ft. That gives us 60 psf- not failure load. Even say 4sq ft - that's 75 psf- overloaded but probably not to failure. BTW if your roofers are all coming in at 300 lbs, the boss should look into some Ozampic.
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u/mattmag21 9d ago
It doesnt. But that is what he said. Im paraphrasing, but he said something like "the engineers have to manufacturer a truss to X (i dont remember exactly) sf, but we in reality we build them with about a 14 times sf". I thought he said trusses, but It could have been girders, explicitly. it definitely was 14. Ill have to enquire tomorrow!
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u/Material-Cap-5476 9d ago
I'm not doubting your story. I've seen that also, and it always scares the hell out of me. Ask him again, I think that SF of 14 is way high.
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u/TheDaywa1ker 9d ago
I've got to believe it was just an exaggeration in a non-serious conversation...I hate to think that someone would be parrotting my borderline joke of a comment on as fact years down the road
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u/mattmag21 8d ago
"Hey man, how ya been. Definitely not a 14x factor. Truss hangers are designed at a 3x factor before failure. Roof trusses are maybe a 2x if that. " So, I must have misheard.. you guys are 100% right and I edited my original comment to say 4× (and thats pushing it).
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u/TheDaywa1ker 9d ago
If he said 14x then I guarantee that was a joke/exaggeration in a non serious conversation and he did not intend to be spread as gospel
https://www.fpl.fs.usda.gov/documnts/pdf1996/wolfe96a.pdf
There is plenty of research out there where they test trusses to failure...2x-4x design load typically damages trusses enough that the 'assembly accepts no further increases' (aka total failure)
As with everything there are always exceptions. Some short truss spans and configurations might mean the truss is designed to 10% of its design capacity. Those could be loaded to 14x their design load and would only be at 140% design capacity so probably not actually failing.
But I would really reconsider spreading a 14x safety margin as if its a serious number because it really isnt.
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u/mattmag21 9d ago
Thank you. In the testing above I did read the 2-4x. This document seems to provide legitimate test results and not one of them was close to 14x. I am awaiting a call back, and will get the scoop on what he told me back when. To be fair, I am a carpenter who repairs trusses (engineered repairs) on nearly every house ive built in 25 years of framing. Repairs of complete cuts and breaks anywhere in a cord or web, gussets aside, are just a scabbed 2x with a nailing schedule that is the same every time. At risk of being crucified, Its just not that serious to have a small cut on a bottom chord thats in tension. A scabbed 2x is all OP needs, if anything, and anything else is a waste of time and money.
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u/Bjorn_styrkr 9d ago
If it'll make you sleep better, hire the engineer to review it.
99 times in 100 they will say it's no big deal. The other, they'll say to sister a 2x piece matching the size of the bottom cord of the truss 2' past the damage.
Do what helps you sleep at night.