The subject of cutting roof trusses for skylights has come up more than once, and I thought the subject was maybe worthy of a whole new thread. This would also apply to trusses cut for chimneys, attic fans, and attic stairs.
Probably the most common modification in this situation is to cut out the top or bottom chord, and header it off to the adjacent trusses. On the surface this seems like a sound idea – That’s what you do with rafters, typically.
But trusses aren’t like rafters. The top chord is in compression, and the bottom chord is in tension. Once you cut the chords, there’s nothing left to deal with that tension/compression.
Even if it were possible to header off the truss to the adjacent trusses, it still wouldn’t be a good idea. That would basically add 50% to the loading on the adjacent trusses, which they weren’t designed to handle. And you’re adding it as a concentrated load, which may be in the middle of a long chord segment.
So what’s the solution? The easiest would be to plan ahead of time – Before the structure is built. Double trusses can be added around large openings, with the area in-between filled in by hand. Or girder trusses can be added no both sides, with headers and snubbed trusses to fill in.
But what if the building is already framed in? Attic fans can be set on a frame on top of the bottom chords of the trusses. That will save any cutting. Changing from one big skylight to a pair of smaller ones might be an option.
If you just can’t get around cutting the trusses, call the truss company who made them first. They can submit the problem to their engineers, and they can come up with a repair. Trouble is, it will be a difficult and expensive repair. It will likely include adding web members in both the cut truss and the adjacent trusses, and a lot of plywood gussets glued on the sides. Not something to be taken lightly, but it’s possible.
Replies
Thanks Boss,
Sounds like a mess. I'll have to get up there one of these days and see exactly what things look like.
Been too long since I had engineering classes (I'm no engineer though). I was once taught some of this, but just as a quick overview.
More I think of it, I think there are 2 trusses cut, but I've never looked at it real close. Not sure much can be done with it, but I'll see.
Hey, remember people, like me, encouraging you to write an article for FHB? Maybe this would be a good subject, particularly w/ some pictures showing how not to do it, and some showing ways around doing it, and how it has been done right.
Later
Bill
In the thread about certifying truss designers, I seem to remember you writing something to the effect of you weren't sure you had ever heard of a truss failure. I assume you were talking about unmodified trusses in that case.
What have you heard about failures of trusses that have been butchered for skylights, etc? I've never heard of any, but that's not very meaningful because I'm not in the trade. I've been reading FHB for probably 15 years, and seen dozens of admonitions not to cut the trusses, but no examples of failures due to truss cutting.
You write "Trouble is, it will be a difficult and expensive repair," with the intention of persuading people not to cut trusses. The thought that occurred to me is that if people don't really believe truss failures happen, instead of dissuading them from cutting the truss, this will just dissuade them from making the correctly engineered repair.
I should add, you don't have to worry about _me_ cutting trusses. I'm not a builder, and even if I were, I _like_ overengineering.
You make some interesting points.
" I seem to remember you writing something to the effect of you weren't sure you had ever heard of a truss failure. I assume you were talking about unmodified trusses in that case."
I said I'd never heard of a failure of a residential truss. By a failure, I mean an absolute collapse. Trusses that are modified incorrectly are likely to sag considerably, cause drywall cracks, etc. Nothing as spectacular as a collapse, but a problem nonetheless.
"I've been reading FHB for probably 15 years, and seen dozens of admonitions not to cut the trusses, but no examples of failures due to truss cutting. "
If you had a serious screwup on your house, would you want that in the pages of FHB?
"....instead of dissuading them from cutting the truss, this will just dissuade them from making the correctly engineered repair. "
Don't really know how to answer that. People will be doing things incorrectly as long as people are building houses. I'm just trying to persuade them to try to do things correctly, so they don't have problems down the road.
Make it idiot-proof and someone will make a better idiot.
>> ... would you want that in the pages of FHB?
LOL! There's always a way if you think it's important to get the information out. "Why, I remember this one old boy over on the other end of the county, kind of a dough head ..."
Not to me its not with your attitude .
Tim Mooney
>> Not to me its not with your attitude.
If you can tell me what it is you think I've done, I'll see if there isn't something I can do to make it right. I don't see anything in the message you responded to that looks like an attitude.
I re read the entire post over . I apologize .
Tim Mooney
What have you heard about failures of trusses that have been butchered for skylights, etc?
As Boss noted, it depends on how you define "failure."
As a home inspector, I see a fair number of cut trusses, most with no visible evidence of associated problems.
There are a significant number, however, where I do see related problems, and I've seen some with serious safety problems which resulted.
My personal opinion is that anyone who screws around with trusses is asking for trouble, and if you're in the trades, if anyone is seriously hurt, you could be headed for jail. It has happened!
It is sometimes amazing how extensively the various construction rules can be broken without problems arising. I saw a garage once with a gambrel attic truss roof structure where all of the internal chords and braces and whatever you call each of them things had been cut out except on the gable ends!
Of course, it had just been built, and hadn't had any loads (like wind or snow) applied. The next time I drove by, a year or so later, the garage roof had been changed to straight gable, but I don't know if something happened or if they believed my report and advice that it required significant and major immediate repair.
But, keep in mind, I've also seen where "minor" truss modifications have screwed stuff up.
Imagine what defense you'll be forced to use if you modify a truss and someone is injured or killed:
"Mr. Contractor, are you aware that any truss modifications have to be reviewed by an engineer?"
"Yes" [Imagine the result if you say "No. I didn't know that.")
"Did you have your modification reviewed?"
"No, but we ...."
"Never mind the but, sir. The prosecution rests."
Desperately, over the sound of clinking chains echoing down the hallway "But we always do it this way!"
_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
Bob -
Call it morbid curiousity, but I'd be curious to hear more about what you've seen with regards to trusses. What problems were caused, etc.
I've never seen an attic truss with all the webs cut out. But I'm not really surprised. I do know of a job with truss uplift problems where the contractor cut all the webs in half.When you shut the door on a refridgerator, how do you know that little light inside really goes off?
BH,
The most notable one (for me) was where I though I was coming down through the roof (I do roofs before attics.)
It was hard to say exactly what happened, but it looked like a couple of trusses had been cut for a drop down stair, which lead (in a way) to using the space for storing the guy's Chevy short block collection, or maybe his collection of National Geopgraphics, which lead to some cracked and broken truss members and a bunch of separated or separating plates.
Plate problems often seem related to cut trusses.
That gambrel roof was something. It was so massively wrong it took a couple of minutes of close looking to believe it had actually been done!
_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
Hi B.H......You were asking about some of the things we have seen that was unusual about truss's the following is an on site truss modification that left out a web member for a 7 foot cant. and I didn't catch it until they were set "Field repair. For truss with cant. added to the right end ,add 2x4 and plywood gusset as shown and noted. Use a structural adhesive on all surfaces contacted by plywood . Prepare surfaces per manufacturers specifaction and glue full width of all members ,glue squeeze out MUST be evident at all contact edges. Use 1/2 32/16 APA rated sheathing on both sides of truss sized and placed as shown with face grain indicated. Fasten gusset with 8d common nails 3" o.c. -2rows staggered from both sides ." The plywood was 2ft wide by 4ft long the structual engineer was LUMBERMATE in 1994 and the truss's came from down south.
""...the truss's came from down south."
Just exactly how far south did they come from ???
That text sounds very familiar - Typical of what I'd expect a repair drawing to say. And I've seen a ton of 'em. None of 'em were MY mistakes, of course..................(-:
Actually, what I was asking Bob about was what kind of things he'd seen that damaged a truss, done by the homeowner or framer. The one you mentioned was reviewed by an engineer, and should be fine if it was done correctly.I'm not completely worthless. I can be used as a bad example.
"Attic fans can be set on a frame on top of the bottom chords of the trusses."
You can , but if the joist is still intact [or bottom cord] the shutter wont foldup when in use . So you are back to modificaton.
Tim Mooney
I figure something can be done with the shutters, too. They could be mounted on some trim boards that are a couple of inches lower or something like that. If you can't hear me, it's because I'm in parentheses.
Only as a last resort as it doesnt look too good. Of course it depends on where it is at in the house . Normally they are in the hallway. I usually cut them and rely on the walls . I brace them off of the walls.
Tim Mooney
Brace them off WHAT walls? The hallway walls are typically non load bearing partitions.
Even if the hallway walls are load bearing, the truss needs a specific design for a repair. Adding a couple of braces isn't adequate.The more you complain, the longer God makes you live.
I know a lot of builders that have to cut trusses and Ive never heard one of them hire a truss company to come out and desighn the repair But we are here .
Are you saying that your trusses never touch an interior wall? Never rest at all on one ? There are plenty here that do. I realize that they need to be able to move with heat and cold because of the moving stress.
I brace off the cuts on top of the walls and shim if needed . There are two connections that need support after they are cut . They recieve a header above the fan that is resting on the walls , not nailed . Now you may line me out on this method.
Tim Mooney
"Brace them off WHAT walls? The hallway walls are typically non load bearing partitions."
There is typically not a lot of difference between an exterior wall and an interior wall except over door openings. However if the hallway walls held the weight of one truss , it would be less than half of the weight . Four walls holding what two normally hold. I dont see a problem with two 2 bys flat holding this weight in the event that it didnt work out on the stud. However common sense should come to play and the drywall could come off and two studs could be headered between with cripples. That would be a lot easier alternative to doing above with insulation and drywall hung and finished. It brings up good question how big the truss to do it differently. Here they dont worry about the truss hitting a stud when the trusses are small, such as 5/12 up to 32 footers. What does that truss weigh? I guess you add 30 lbs per sg foot to that, but not in the south. No snow load . Of course the building regs dont figgure that very well. I will tell you one thing that has always amazed me here ; They frame walls on 16 inch centers and the roofs on 24. Go figgure . I have never agreed, but thats the norm with single story houses. I did add plywood to the out side of that , but even that is not the norm. Blue board is the norm. So in that respect the interior wall would be stronger because of two layers of drywall on a single wall . Sometimes walls are layed out on 19.1/4 , over lvls but the roof never is laid out over the top to match. But we are in the hot south.
Tim Mooney
"There is typically not a lot of difference between an exterior wall and an interior wall except over door openings."
The difference is that one is load bearing and the other isn't.
I realize that you don't have the snow loads that everyone else does, and that may help you get away with doing things that won't fly in other places.
Even without snow, roofs can be overloaded. I did trusses for a house a couple of years ago where the roof collapsed. The roofers set up a conveyor and put all the shingles in one area near the ridge. The rafters couldn't hold the load, so it collapsed. Live loads on roofs aren't only for snow.You're just jealous because the voices only talk to me.
"The difference is that one is load bearing and the other isn't."
With the exception of doors, what would that be other than stacking trusses over studs ? Somewhere I read that drywall had no load bearing characteristics. I didnt buy it then or now . I believe that the manufactor doesnt want weight on drywall. Actually though a layer on both sides of a wall gives it a lot of rack strength since its tied to all edges of a wall. It helps quite a bit to brace plates under stress. Boss, its the same thing other than stacking studs under the trusses and its not done here . I dont agree with it either , but that is the Romans way where I live. I would prefer everything on 16 inch centers and stacked from floor joices up to trusses. Of course that would mean that a person would have to purchase trusses to cover that and its not going to happen here. We need to have 16 inch centers for siding and 1/2 inch drywall. So the norm becomes in this area 16 centers on walls , 24 on trusses with 5/8s drywall and decking on ceilings and roofs. I would also guess that its probably common throughout the south.[Southern Building Code ]
Stacking a whole load of roofing in one spot on a roof would be a stupid thing to do anywhere and does not repersent truss loads of any description . I dont think you meant that it was , but you mentioned it . I consider that damage by stupidity.
Remember too that an interior wall would be holding less than half in a hallway. I dont see a problem .
Tim Mooney
"With the exception of doors, what would that be other than stacking trusses over studs ? "
The difference is what's under the walls. A load bearing wall has a continuous path to a foundation or beam. A typical hallway wall would more likely just be sitting on top of the floor framing system. Such a hallway wall could be 2" from a beam or 10'.
"Stacking a whole load of roofing in one spot on a roof would be a stupid thing to do anywhere and does not repersent truss loads of any description ."
I would agree that it's not terribly bright. But it does happen. Stack a bunch of shingles on one of your trusses that's cut for an attic fan, and you're asking for trouble. I doubt it would affect a typical roof in good condition.Smile, it's the second best thing you can do with your lips.
"The difference is what's under the walls."
I agree. Concrete floor and your good to go for half the weight of one truss. Normally hallways are center of the floor plan . If the floor was stick built with a beam in the center I could do it in good conscience with perpendicular joices.
Common sense applied to different situations would demand a different approach to lvls for example or floor trusses. Also , if the wall wasn't sitting on anything but flooring . My estimate of the weight is 250 lbs max on the truss I used as an example at less than half of the weight. A much larger truss would change things considerably. So, each application would need to be considered . We don't really have one set problem here to work with in our discussion . So , we are back to common discussion, not relative to any specific problem ..
I didn't mention it but I think that heading over would support the truss any way , so its questionable whether we are talking about half the weight being applied to the interior walls. In the case of a very large truss it would be much more a problem that should be addressed by a truss rep I will fully agree. Again common sense comes in to play. I know I will think about it more now since this discussion . Thank you again Boss .
Tim Mooney
"My estimate of the weight is 250 lbs max on the truss I used as an example at less than half of the weight."
At 250#, you seem to be allowing for only the weight of the truss. But I think you're way underestimating.
If we assume a 32' wide house (Which is pretty normal for this area) and about 10 PSF of typical dead load that's actually on the truss, the reaction of on half of the truss would be only 160#. But you need to allow for various live loads too - Maybe a 200# guy on the roof carrying a bundle of shingles. Or shingles stocked on the roof before being installed. (Maybe the 2nd or 3rd layer)
Then there are situations where there are snow loads - With a 50# total load, that reaction would be 800#. Go up into the northern states and Canada, and you're probably way over 1,000#. (I don't really know the loading requirements up that far)
Assuming you can come up with bearing at the hallways, the trusses could be repaired to bear there. But the repair would have to be designed by an engineer.
"I didn't mention it but I think that heading over would support the truss any way..."
Don't know why you would think that. I explained in the first post that this assumption isn't correct.A bartender is just a pharmacist with a limited inventory.
"Don't know why you would think that. I explained in the first post that this assumption isn't correct."
Why does a cathedral truss work if thats not true ?
"My estimate of the weight is 250 lbs max on the truss I used as an example at less than half of the weight."
At 250#, you seem to be allowing for only the weight of the truss. But I think you're way underestimating.
Ok, that was off my head . Lets do it exact.
Lets assume that a 32 ft truss weighs 250 lbs. [I was guessing 200]
250 divided by 4 = 62.5 lbs per power point on each wall.
Approximately 36 ft of top cord [with out looking at charts] at 30 lbs per sq ft of load = 2160 on 2 ft centers divided by 4 = 540 lbs at each power point added to 62.5 lbs truss weight = 602.5 per power point .
I stand corrected .
Tim Mooney
"Why does a cathedral truss work if thats not true ?"
A truss with a sloped bottom chord works just like a flat bottomed truss - The top chord is in compression, and the bottom chord is in tension. The only difference is that a truss with a sloped bottom chord will have a lot more tension or compression in the chords.Veni, vidi, velcro: I came, I saw, I stuck around.
Would it be better to pitch the splice off the hall walls then in the case it was necesary in some form of an A?
Tim Mooney
Not really sure what you mean by that.............I was going 70 miles an hour and got stopped by a cop who said, "Do you know the speed limit is 55 miles per hour?" "Yes, officer, but I wasn't going to be out that long..."
Meaning cathedral over the attic fan opening and tie in to truss .
I think that's vague at best . Your recommendation was to hire a truss company . Good enough . You severely warn against cutting trusses with out professional help in design. I think that's good advice . I've never done it , but I guess if I wasn't comfortable I should . It depends on when you catch it . I could stick build it from scratch if I was given the opportunity. A home that was occupied probably wouldn't be worth it if you didn't use the hall walls so it would not be a question because of expense. Moot point really . As you said , It would be expensive.
Tim Mooney
If you mean buying a cathedral truss that passes above the fan location when the house was built, I guess that would work. But it would be expensive to buy just one truss of a different type for that purpose.
If you mean modifying them in the field to do that, it would probably also work. But the amount of clearance required would make it difficult in some situations. And you'd still need an engineer to design the repair. Q: Why don't women blink during foreplay?A: They don't have time
Boss man, I borrowed a digital camera and took a couple of pictures, actually took 4 but the disk ate two. Good thing I did too, found out I have some water leaking around it. Not much yet, but gonna have to get someone to fix it. Wasn't any last time I was up there. Still hasn't shown on the drywall below it.
My chimney actually cuts one truss. It is tied to the two adjacent trusses, top and bottom. Both the top and bottom chords are cut, and one web removed.
Is there a good way to deal with this? Or, is it one of those things where it hasn't sagged, doesn't seem to be causing any problems, so you ignore it? I'd prefer to fix it.
I use the chimney, so it stays.
Thanks for any info.
Billy
Whether or not you want to fix it is entirely up to you.
If you do, it will be difficult and time consuming.
If you don't, it may sag eventually. No way to tell.
Tough choice.I have a nice body, and its in my trunk.
Boss,
You've really peaked my curiosity. Just how could you fix this? Obviously it isn't done right in my house, but I have found nothing on how to do it right anywhere else either.
I understand transfering the load to adjacent trusses isn't ideal, though it would at least attempt to provide some support.
Do you know of anyplace that discusses how to do this right. Or any pictures of how it's been done? It would seem if it's such a common problem there must be reams of info out there somewhere.
Thanks
Bill
You won't find anything about repairing this kind of a situation on the internet - Every repair depends on the exact situation - The loading for that area, building codes, where the truss is cut, if there are interior bearings that can be picked up, etc.
The only real way to do it "right" is to have the truss company's engineers design a repair that's specific for your house and trusses. This typically involved repairing the cut truss with new webs and plywood gussets, which is then headered off to the adjacent trusses.
The adjacent trusses must be re-designed and repaired to handle the added load too. This typically involves adding webs that are fastened in with plywood gussets. And many of the joints will also be overstressed, so they will have to be reinforced with plywood gussets or scabs.
All of this must be accomplished in the attic, in very limited quarters. That's why I said it would be difficult and time consuming.Money won't buy happiness, but it will pay the salary of a large research staff to study the problem.
That's kind of what I figured you'd say.
I'd already thought of adding bigger plywood gussets, nailed and glued.
And, readding in the web that is missing, but offset to the side of the chimney. Strengthening the headers by doubling them and fastening them to the adjacent trusses, mostly w/ a top plywood gusset that is nailed & glued to both the header and the adjacent truss.
Just hoped there was some actual info out there to look at. Hard for me to make it in to the local truss company. There hours and mine don't agree much. Perhaps next time I have a day off.
Just for your info, I doubt there are bearing walls that could be picked up. The truss on the right of the picture is above the house/garage wall, but whether it does any bearing I don't know. The cut truss and left truss are a 30' free span over the garage.
Knew it'd be difficult and time consuming. I've already spent a huge amount of time fixing things in the attic. Added a walkway down the center so I didn't have as much risk of going through the ceiling. Just about done, then I can beef up the insullation from the current R15-19. Need to double it, but haven't wanted to as long as there's a lot of work that needs done up there.
Really need to get the leaking fixed before it gets serious though. Prefer to wait till the roof isn't a sheet of ice. Any ideas on good sites for flashing chimneys? I'm gonna search, but need to find a fireplace guy to come out anyway. I have a used insert stove I want to put downstairs, using the unused 1/2 of the chimney shown. Need it inspected, it looks scary in the attic, as you see from the picture.
I think I'll have the pro do any new flashing, but would like some info to know if it's done right.
Thanks for the help
Bill