[Greenbuilding] Polyiso strength on roof

[Alan Abrams]

hmmm, what comes into play is resistance to wind pressure. Of course, the
original structure did not have a plywood roof diaphragm, and held up for
88 years. But if you were building this from scratch, you would have to
account for this, particularly if you're in a high wind zone. It would be
less of a concern if there are meaty masonry gable ends, but if not, I'd
opt for a minimal layer of plywood applied to the rafters. 3-ply 3/8"
should do it.

-AA

Alan Abrams

*certified professional building designer, AIBDcertified passive house
consultant, PHIUS*
*certified passive house builder, PHIUS*
cell     202-437-8583
alan at abramsdesignbuild.com
HELICON WORKS *Achitecture and Education*
<http://www.heliconworks.com/index2.html>

On Sun, Dec 7, 2014 at 10:52 AM, Bill Allen <bill.allen at verizon.net> wrote:

> Hi all
> I am about to reroof my 1926 colonial and am looking to also insulate per
> the BSC "deep energy retrofit" plans consisting of plywood sheathing, 2
> layers of 2" polyiso, another layer of plywood, then roofing (in my case,
> standing seam metal roofing).
> The original roof was wooden shingles that are still in place with 2
> layers of asphalt on top. Also still in place are the nailers for the wood
> shingles...roughly 1x2s, 5.5" on center across the rafters.
>
> My question is: if I leave the 1x2 nailers in place during the demo can i
> exclude the first layer of plywood and install the polyiso directly on the
> nailers? It would then have plywood on top, secured with long screws into
> the rafters. Less material and less cost.
>
> Thanks for any thoughts/comments.
>
>
> Sent from my cell phone
>
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