0 Helpful?

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 12:47 PM ET
Edited Jul 29, 2014 1:08 PM ET

Tags:

3 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.
1.

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 1:42 PM ET

2.

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 4:39 PM ET
Edited Jul 29, 2014 5:43 PM ET.

3.

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 7:28 PM ET

Other Questions in Green building techniques

Atlas ThermalStar X-Grade Reviews?

In Green products and materials | Asked by michael Iacona | Jun 24, 16

Upgrading a crappy HVAC setup

In Energy efficiency and durability | Asked by Matt Culik | Jun 24, 16

I am thinking of an unvented cathedral ceiling

In Energy efficiency and durability | Asked by Mike Tomlinson | Jun 24, 16

I'm chemically sensitive and the neighbor uses lots of pesticides that make me ill

In Interior design | Asked by Brenda Koehlert | Jun 24, 16

Footing to foundation capillary break

In Green building techniques | Asked by Steve Wilson | Jun 24, 16
Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!