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Irregular rafter spacing

My 1907 house has 2" x 4" rafters on 34" centers. After stripping to roof to the original 1 x 4 skip sheathing, I am planning to add 6" of polyiso foam and then a layer of plywood sheathing. The scrap rate is high if I have to cut every sheet to 48 x 70". Is there a precedent for using tongue and groove roof sheathing (assuming I can find it locally) without the seams landing on the rafters in this sort of application?

Asked by Keith Richtman
Posted Jul 29, 2014 11:47 AM ET
Edited Jul 29, 2014 12:08 PM ET

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3 Answers

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1.
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Keith,
First of all, you need to have an engineer evaluate your roof. Your 2x4 rafters, 34 inches o.c., won't support much. You will probably need to beef up your roof structure -- which may make your question moot.

Answered by Martin Holladay, GBA Advisor
Posted Jul 29, 2014 12:42 PM ET

2.
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Martin,
I appreciate the sentiment, but the proposed loading is the same as has been present for the last ~30 years. Wood shingle roofing, battens, stone-coated steel shingle roofing is being replaced by rigid foam, plywood, stone-coated steel shingle roofing. There is no snow load. The foam adds <0.7 lb/ft^2 dead load. Let's assume for the moment that the existing rafters are acceptable for the load.

Answered by Keith Richtman
Posted Jul 29, 2014 3:39 PM ET
Edited Jul 29, 2014 4:43 PM ET.

3.
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Keith,
Assuming that we were (hypothetically) talking about a roof with adequately sized rafters -- say, for the sake of argument, 2x10 rafters -- spaced 34 inches on center, the 1x4 skip sheathing would be inadequate.

I would want to start with new sheathing before I installed any rigid foam above. If I had to sheathe a roof with rafters that were 34 inches on center, I would probably choose 1-inch plywood or 2x6 rough sheathing. I would certainly consult an engineer before choosing my sheathing.

After installing a decent layer of sheathing that was thick enough to prevent sagging, I would install my rigid foam. If I wanted another layer of sheathing on top of the foam, I would go ahead and install a layer of tongue-and-groove plywood, without worrying whether the joints fell on the rafters.

But you can't do that over 1x4 skip sheathing.

Answered by Martin Holladay, GBA Advisor
Posted Jul 29, 2014 6:28 PM ET

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