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Civil / Structural PE
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Discussion Starter · #1 ·
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.
 

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I appreciate the thoughts etc of the post however I think we all know that the real issue at hand has nothing to do with a persons ability to crunch numbers and design trusses or loads and meet adopted code requirements. I have built trusses that are still functioning after 20 years.


I feel sure that I could modify a truss to perform to my expectations...but that's irrelevent given the audience of this forum. This isn't a DIY forum. This is a contractor forum. As soon as you provide a service for some one else...the subject changes completely.

The issue with truss modification has little to do with one's technical ability but rather his liability. It dosn't matter if a truss design or subsequent modification is adiquate or not. It only matters that you have the financial resources to back up your commercial endevors.

You can have all the degrees you want but if you don't have the insurance to back it up...they are meaningless. And it doesnt matter if you live in a state where in no license is a requirement or no insurance is a requirement. That willl not keep some one from sueing you.

But what do I know.
 

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Registered
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I am comfortable when I have to modify a truss, but I have a strong background in working with loads, and engineering. I also know when to call a PE. That said, the less experienced contractors are the ones likely to get into a bind, without realizing it....of course, I have seen some old guys who fly by the seat of their pants do stuff they should not.
 

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Civil / Structural PE
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88 Posts
Discussion Starter · #6 ·
Truss cutting

What a lot of guys don't know about trusses is that under full load (snow or temporary construction loads) forces in puny 2x members can be in the thousands of pounds. But it takes that "full load" scenario to cause a problem. So trouble with violated or under-deisgned trusses may not show up for years, if ever. Engineers always think in terms of "worst-case" which is why I'm so flinchy about modifying a truss. Heck, for that matter, modifying any structural element.

Thanks for your input.

I appreciate the thoughts etc of the post however I think we all know that the real issue at hand has nothing to do with a persons ability to crunch numbers and design trusses or loads and meet adopted code requirements. I have built trusses that are still functioning after 20 years.


I feel sure that I could modify a truss to perform to my expectations...but that's irrelevent given the audience of this forum. This isn't a DIY forum. This is a contractor forum. As soon as you provide a service for some one else...the subject changes completely.

The issue with truss modification has little to do with one's technical ability but rather his liability. It dosn't matter if a truss design or subsequent modification is adiquate or not. It only matters that you have the financial resources to back up your commercial endevors.

You can have all the degrees you want but if you don't have the insurance to back it up...they are meaningless. And it doesnt matter if you live in a state where in no license is a requirement or no insurance is a requirement. That willl not keep some one from sueing you.

But what do I know.
 

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It seems a little foresight could have had the truss engineered the way it needed to be instead of altering it later. The added room didn't just drop 2 feet without warning................ I hope!
 

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The Duke
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What a lot of guys don't know about trusses is that under full load (snow or temporary construction loads) forces in puny 2x members can be in the thousands of pounds. But it takes that "full load" scenario to cause a problem. So trouble with violated or under-deisgned trusses may not show up for years, if ever. Engineers always think in terms of "worst-case" which is why I'm so flinchy about modifying a truss. Heck, for that matter, modifying any structural element.

Thanks for your input.
I have stopped by a few palaces where the building had improper bracing on piggy back trusses. Those top chords on the lower part have an immense amount of pressure on them and improperly braced is asking for a catastrophic collapse.
 

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Curmudgeon
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11,706 Posts
Personally, I still don't know what you
did to reconfigure the loads, if anything.
I think the take away for the DIY
lurkers here is, one can cut the
bottom chord of a truss as long as
one hold his breath while one does it. :blink:
I guess this was a factory truss?


Add:
I didn't mean to sound harsh, I guess I'm
just missing the point....again.
 

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Capra Aegagrus
Remodeler
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That said, the less experienced contractors are the ones likely to get into a bind, without realizing it....of course, I have seen some old guys who fly by the seat of their pants do stuff they should not.
I've been around a little while, but not enough to take many chances. Or is that long enough not to take many chances? :laughing:

OTOH, I have a [older] friend who is practically an idiot savant with loads and stresses. He couldn't begin to give you the math; he just "knows". And some of the stuff he did 10-20 or more years ago, still standing, testifies to that.

Drives me nuts.
 

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Capra Aegagrus
Remodeler
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25,216 Posts
I didn't mean to sound harsh, I guess I'm
just missing the point....again.
Tsk, tsk, Neo. Are you so old that you don't remember "Do what I say, not what I do!"? :laughing:

Actually, I had the same reaction on first read.

Tim, I don't suppose you'd care to share before & after drawings of that truss?
 

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Registered
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132 Posts
The engineers designed that truss to function as one unit.

The truss builder built it to function as one unit.

You altered it.

It now has new performance characteristics.

Do you know what they are?
 

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DavidC
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2,550 Posts
The engineers designed that truss to function as one unit.

The truss builder built it to function as one unit.

You altered it.

It now has new performance characteristics.

Do you know what they are?
Tim introduced himself as an engineer so I'm thinking he does.

We have a job coming up that will require removing part of a truss. The client wanted us to just do it. I insisted on an engineers input and got it.

As it turned out, the drawings are right in line with what I thought would be right based on my experience. If it was my project at home I feel comfortable enough to do it, but never for a customer.

Good Luck
Dave
 

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Design Build
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I only use trusses when there is no other option. They have their place...but I prefer to install ceiling joists and a stick framed roof system.

In a typical roof with no loaded beams, I am only required to meet or exceed the appropriate governing code book, ie IRC/IBC 2006 or whatever is law in the county.

I have had trusses break or get damaged and the manufacturer always sent out a stamped "fix". When I stick frame, problems like those you mention are an easy workaround.



 

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I'm with you Wallmax, I love a good complicated roof framing job. I have framed complicated roofs and when completed felt proud of how well it came out but I can't say I have ever been proud of a roof truss install.

Dave
 

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Designer/Contractor
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In this case the contractor got away with cutting on the truss. If one did that where a permit had been pulled and inspections were taking place a competent inspector would want calcs and a stamp from an engineer or architect (but only an architect who can do calcs please). Sometimes what seems to be okay intuitively is not okay in reality so it's good to analyze what is going to happen when one starts moving the loads around.

Speaking generally on trusses it's not too surprising how many people, including too many contractors, don't understand that the truss is acting, as pointed out above, as a single unit and has different characteristics than a conventional roof (in the case of a roof truss). Just last night I was talking with an associate who picked up a small job cutting in an attic access pull down door and placing a bunch of sheets of plywood on the 'ceiling joists'. I asked him if any of the 'joists' were actually part of a truss and the answer was "oh yeah".

I pointed out that many roof trusses are two point trusses that are designed to transfer the loads through the walls that they bear on only (that's why we use truss clips for the interior walls under such a truss, to make sure that the truss isn't bearing and transferring part of the load where it's not designed to do so) and that the bottom chord is not designed to take a bunch of weight placed on top of it. The conversation ended with "Well, hopefully he'll just put the boxes with Christmas ornaments in that area". I was going to tell him that what he should also be hoping for is we don't have a big earthquake that shakes everything up and possibly precipitates an incident but he already looked nervous enough.
 

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The Duke
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I'm with you Wallmax, I love a good complicated roof framing job. I have framed complicated roofs and when completed felt proud of how well it came out but I can't say I have ever been proud of a roof truss install.

Dave
I agree with you to a point. Some of the largest, most complicated homes around need to have a hybrid system of trusses and rafters. I love stick framing a roof, but I also took the time to really study trussed roofs. There are good and bad.

I've never seen a truss come out "fix" before though Mike. I'd send them right back. No way would I take responsibility for that. I've seen them come out with some portable gantry press before.
 

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Design Build
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Hey Duke -

They (truss company) would make use nail plywood on both sides of the damaged or cut area at 4' lengths (or whatever it took to span to another member)
 

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hurtlocker
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555 Posts
If they are engineered trusses
you should be able to call any truss company
and get a repair for little to no cost
I almost always use engineered roof and floor trusses for this reason
If anything comes up I dont have to guess
I call with problem they send me a fix That Can I can show the inspector. No gray area
Now if you are an engineer you do not need any of this you can sign off your own repair no worries(i hate you)
 
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