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Old 05-20-2018, 09:07 PM   #1
Steve in Denver
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Roof framing question

I have a question about roof framing - specifically, how much of what things can I cut out before I compromise the structural integrity.

OK, now that I have your attention...

Let me explain. I am trying to install a whole house fan, and the place where it "needs" to go is blocked by part of the roof framing (the sheathing, really) I would like to cut a hole or two in the sheathing.

The whole house fan is of this type: https://quietcoolsystems.com/products/
(so it needs a lot of clearance above)

The first picture is taken from a cross gable section that intersects with the main part of the roof. (The sheathing is part of the main roof)

There is a support spur (I'm sure there is a real word for it, but that's the best I can come up with) that connects to the cross gable trusses at one end (not pictured) and to the sheathing of the main roof at the other end (pictured) The support spur is continued from the back side of the sheathing and terminates "somewhere down there" (I presume it connects to the bottom part of a truss, but I can't see it)

(It is not clear at all to me what load that support spur carries)

In an ideal world, that spur wouldn't be there, and the sheathing in that area would also not be there. The section of sheathing just "down slope" of the spur is where I would ideally have my fan ducting running.

1. What is the purpose of the sheathing? I was surprised to see an un-shingled area with sheathing installed. I presume there is a structural reason for it, but it's not really obvious to me what purpose it serves (unless it was to hold things together during construction or something)

2. There is a section of sheathing that was cut out, presumably to provide access to the rest of the roof from the attic access hole. Can I get away with cutting a hole in the adjacent sheathing, too, so I can route my duct through it?

I can take more pictures if that would help.

I'd really like to be able to put my fan in that location, but not at the expense of my roof caving in.

Any insights appreciated.

Thanks

-Steve
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Old 05-20-2018, 10:01 PM   #2
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I don't think sheathing under the actual roof would serve any purpose except to stabilize the framing, which could be done with cross-bracing.

But you should let those with a little more experience weigh in on that.
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Old 05-21-2018, 11:18 AM   #3
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Steve, I always hesitate to comment on questions such as this. With only a couple photos there is soooo much to miss, eh?

But I think Kevin has it right. We commonly sheath one section of roofing and frame a dormer or intersecting roof over it for convenience, in which case cutting out a section of sheathing after the fact would make no structural difference at all.

Difficult to guess exactly what that support piece is for without seeing the whole picture (guessing a purlin support?), but I'd certainly recommend you replace it if you cut out that section of sheathing and need to move or relocate the support.

My opinion; worth price charged.
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Old 05-21-2018, 01:44 PM   #4
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I think it might be supporting the ridge of the dormer. When I was a pup I set trusses on tract houses for a year, 3 houses a day. We used to do dormers (or roof offsets) the same way. You don't need to frame in valleys (which you can't really do with trusses anyway).

As CX says, take some more pictures, Steve.
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Old 05-21-2018, 05:57 PM   #5
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Thanks, guys. I will take some more pictures when I get home.

-Steve
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Old 05-21-2018, 08:08 PM   #6
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OK, here are some more pictures.

I'm still not really sure what the brace does. The movement it would control would seem to be well taken care of by the top ridge piece that terminates at the main roof (as in picture 4). Is it possible that it was a temporary brace to hold the trusses while the roof was being built?

I guess it doesn't really matter one way or the other....it's in the way, but I'm not cutting it...

Mostly it's the sheathing that I'd like to cut out...not all of it, by any means.
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Old 05-22-2018, 10:46 AM   #7
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After taking a closer look at the framing (in particular the support piece) I think it could have been a brace to support the trusses until the rest of the (site built) roof was framed.

In the first picture I have drawn a green line indicating the last truss (actually a doubled up truss) of the side gable. The support piece (blue) connects to this last truss. The red line is the ridge board of the site-built part that continues the side gable from the last truss until it terminates at the main roof.

All speculation of course, and as someone with literally 0 experience with roof framing I am just guessing, really...But it does appear that the support piece would have been used to hold the trusses in place while the rest of the roof got framed.

I'd like to take it out, but I'd have to be very confident before doing that...at this point I'm going to work around it, assuming I can cut some of the sheathing out.
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Old 05-22-2018, 03:06 PM   #8
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I think you're correct, Steve. Looks to me like it was there to plumb some portion of the roof framing during construction. From what I see in your photos I don't think I'd hesitate to remove it.

I'd watch to see what moves as I took it out, though.
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Old 05-22-2018, 04:36 PM   #9
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And wear a helmet.
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Old 05-23-2018, 08:12 PM   #10
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Thank's, guys - as always I appreciate your input.

I will proceed with caution.

-Steve
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Old 05-26-2018, 02:11 PM   #11
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What CX said, as usual.

or, one more vote for the offending board being a temp brace used during the building of the roof. Very rarely are those things required to left in place permanently. If you have any doubt, you can always replace the one you're cutting with another board that would do whatever that one does, just relocated to a different position out of your way.
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Old 05-26-2018, 02:26 PM   #12
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Just curious, in new construction these days, will a roof like the OP's always have a stick framed portion connecting the two truss-framed portions? Or have the truss manufacturers come up with a way to handle that area with special trusses?

Cheers, Wayne
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Old 05-26-2018, 02:48 PM   #13
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Truss manufacturer I dealt with when I was building, Wayne, would tell you there is no such thing as a roof that they couldn't build trusses for.
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Old 05-26-2018, 07:15 PM   #14
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Agree. Of course, some "trusses" in the roof package may just be a plain ol 2x4.
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Old 05-26-2018, 11:03 PM   #15
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Project update and follow-up question..

The update is that I got the whole house fan installed today. It was a real son-of-a-bitch to get installed for various reasons, not the least of which was available space in the attic. I'm sure the truss roofs are superior, but when they are framed 24" OC and the fan (and ducting, and...) are 24", it makes things a bit interesting.

WAY more work than I anticipated...per usual.

It does move a lot of air, and it is much much quieter than the whole house fan I grew up with, but it's definitely not silent (low speed is pretty close, though).

So, took the support member out. It was under no tension or compression, and based on how it was nailed, it sure looked to be temporary. I did not replace it and I am not at all concerned about it at this point.

Question: what about the sheathing.

I ended up cutting a good deal more of the sheathing out, primarily in the opening that already existed. I probably removed an additional 30 inches up toward the peak of the roof. Do I need to / ought I add some 2x4 bracing to tie the two adjacent trusses back together?

(I did notice some of the interior chords on the large trusses were tagged saying "a permanent lateral support is required between trusses..." (and there was a lateral support in place at the tagged position) but no such tags were anywhere in the vicinity of where I took out the sheathing....but, the assumption might be that it will be sheathed, so no need to call for a lateral support)

It wouldn't be difficult to add as support (or two), but it would restrict access in the case that I need to get back up there in the future.

Thanks again for the help.

-Steve
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