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Roof Lumber sheathing

Uncle Bob

Uncle Bob

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Joined
Oct 17, 2009
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1,409
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Texas
Asphalt shingles decking requirements. I'm trying to find spacing requirements for lumber (example: 1 X 8 boards) for roof decking and I'm lost and can't be found. I remember when I was a carpenter we would put an eight penny nail between boards and/or sheathing; but, since the great hail storm I'm getting requests for code requirements regarding spacing of roof decking lumber and/or sheathing and can't find it in the 2009 IRC. Code section or table please. Both would be nice. Not spans, just spacing between boards and/or sheathing. Is there a maximum spacing (not more than)?

Help,

Uncle Bob
 
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Not going to find it UB, not there, at least I haven't foundit. The code just says roofs shall be solidly sheathed. What does that mean? Guess it is up to you. I've told folks that we would allow 1/4" average, no more than 3/8" anywhere. I and my staff are comfortable with that.
 
Many roofs are sheathed with plywood or OSB and you will find no gap.

If 1x boards are used and they shrink as the wood dries out the gaps will be larger. So could the 3/8" end up as 5/8" if wet lumber was used? Could you have a situation where wet boards were installed with 1/4" GAP but then were allowed to sit for a couple of months in hot weather before being inspected and rejected because the gap was too large.

I would install them with no gap. I believe that this would be supported by the requirement that they be solidly sheathed.
 
We don't see any new roof sheathing that is sawn lumber. The ones I was referring to are 80-100 year old roofs, pretty seasoned lumber.
 
the gap is more for swelling than shrinking, that's what the sheathing clips are for.. proper spacing. It's about 1/8" inch all around.
 
Thanks Ya'll, We had a lot of hail damage lately and insurance companies and roofers are in spitting contests over re-decking (whether too much spacing between the lumber or sheathing requires re-decking); and of course they call me. I couldn't find it anywhere. I have noticed that osb installations always had spacers and that makes sense because of expansion if it rains before they cover it. I have been telling the insurance folks that it's not my call; it's theirs whether they will pay for new deck. :)
 
Bob 8d nail gap is what we told em at the yard! No verification from any known source.

OSB sheeting panels are 3'11-7/8"x7'11-7/8" allowing for an 1/8". On a roof with a long run of sheeting you can get off center missing the rafter if the sheets are pushed to close together. Another roof sheeting issue is the ridge sheeting comes up being pieced with scrap, OSB needs to be a minimum of 24" wide at the ridge.

pc1
 
Manufacturer recommendation maybe? All sheathing I have sen says either "sized for spacing" Or provide "spacing".

In the '80's I don't ever remember seeing the notation, and was further along in my career when an 8d nail space became normal. I will fight tooth and nail to keep from using clips. It has to be on the plans or I don't do it.

Which brings up a story... I once got into with an inspector that checked nailing after a rain, and the spacing was gone. I told him it WAS spaced, and it performed as designed by expanding and not buckling. I waited 2 days and a couple of 90 degree days and it was back to normal. He said I kerfed it. I said nope, I was just patient. As for clips, I'm not sure of the logic because you install tight to the clip, which stays forever, and I have seen that buckle because it still cant move.

Simpson strikes again.

Brent.
 
Thanks,

The information ya'll have provided has helped re-educate this old man.

Uncle Bob
 
The clip is there to prevent the sheets from buckling in different directions, it holds alignment. I've seen ugly seams telegraphing thru when they aren't used so I always use them, no big deal, and they maintain the gap. If the panel needs to move it will crush the area around the clip without buckling much easier than if the entire edge were in contact. But a clip is rarely required so its all good.

If solid sawn is used I apply tight and it will gap. At KD19% the wood is going to end up around 8-12% and will show a gap. If green lumber is used the gaps will end up 1/4" often. Under metal I've skip sheathed many times.

If you read the specs on "sized for spacing" it actually allows a tolerance in the panel sizing. It was a terminology play to allow dimensional variance in the manufacture of the panel.
 
There is a technical article from the APA about sheathing buckling at this link:

Technical Papers | APA - The Engineered Wood Association

Regarding clips, I have always understood that narrow strips of sheathing at the ridge or elsewhere require edge support. Memory tells me sheathing less than 12 inches wide. I honestly can't find where that's from right now, I think an APA publication. Anyone, anyone, Bueller?

If you've ever stepped on an 8" strip of sheathing at the ridge, you know why support is necessary.

Clips are also included in the sheathing table in Chapter five R503.2.1.1(1) as a means of edge support for various rafter spacing and sheathing thickness options.
 
From memory I believe 16" wide is the published minimum width from APA although we've all pushed that one. Clips or edge support doesn't make that go away. You can rip the previous row to get an adequate width on the top strip. I normally use 5/8 roof sheathing, if the needed top strip is small enough to be board width a 1x board for that top strip works well. There is enough going on with the ridge up top that the 1/8" step is unnoticeable. Most of us have planers nowadays as well and board sheathing can normally be 5/8 up to 24" rafter spacing.

There is a "sized for spacing" tech topic on the APA downloads page as well.
 
My gut feeling is that a 1/2" to 1" gap should be OK, because hail of that size shouldn't be heavy enough to dent the shingles. I didn't find anything on the ARMA website, so you might have to check with the single manufacturers.

A lot of old roofs, especially metal or cedar shingle, were installed over spaced sheathing with gaps of a few inches between boards. These gaps should be filled with new boards, or overlaid with plywood (or OSB).

I always specify 5/8" plywood, because 1/2" buckles too much. I also specify plyclips just to be safe.
 
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