Currently the roof assembly in our IBC Type II-B building is the following:
Asphalt Shingles
Plywood
Rigid Insulation
Vapor Barrier
Steel Deck
Does the plywood need to be fire treated?
David,
The answer appears to be a “yes”. Be careful of the number of stories, though (Table 601, footnote c.) - the number of stories depends on which edition you use.
And, does your local state code have any amendments to that section.
Tom
More importantly, your plywood should be preservative treated, but I do not know of a treatment that provides both fire-retardancy and presrevative.
Are you using an undererlayment under shingles.
What method are you using to secure the plywood? Long screws or Z-girts?
Table 503 lists allowable height and building area for Type IIB for differing group use and Table 601 lists the fire-resistance rating for any Type IIB building element.
Does the rigid insulation require a thermal barrier for protection from fire from below?
Wayne
Assuming that you forgot to list underlayment materials beneath the shingles, you do not need preservative treated plywood. Exterior exposure plywood will work just fine (as it has for decades and decades in this application).
You should also know that asphalt shingles over non-vented, insulated roofing decks are subject to a phenomenon called superheating, and that no recourse will be available to your client for this installation in the event that the shingles fail prematurely due to this well-known problem. Warranties from shingle manufacturers are for material only, and do not cover degradation due to superheating cause by a non vented deck.
anon
To modify Tom’s response: Since your building is of Type II construction, footnote c.3.i allows it in Type I and II roof construction in buildings 2 stories and less, and footnote c.3.ii allows the same for only Type II roof construction OVER 2 stories.
So, in a roundabout way, they’ve stated that you can use fire-retardant-treated wood in Type II buildings of ANY height.
Just for everyone’s information, if you have a code question, you can try to post it on the Building Code Discussion Group (it’s very similar to the 4Specs discussion group). The URL is “http://bcodes.infopop.cc”.
I would recommend that you not use treated plywood unless you must.
The chemicals used for treating can contribute to corrosion of the fasteners and framing hardware. In some circumstances Simpson recommends galvanized or stainless steel fasteners.
Fire retarded lumber has its strength reduced by fire-retardant-treated wood. You need to talk to the manufacturer of the treatment process since the reductions varry for each product.
The last I checked, APA-The Engineered Wood Association still did not recommend the use of fire retardant plywood in any application.
Anon,
The insulation beneath the plywood is vented. I specified the Insulfoam Insul-Vent product with 11 inch wide by 1 inch deep channels 16 inches apart.
(http://www.insulfoam.com/insulfoam_rm.cfm?view=ivvri)
Since this thread has included discussion about vented nail-base insulation, I’ll get my question in … I know that there are requirements for shingled roofs to be installed over a vented nail-base. Would there be the same stipulation for a metal roof? It would seem that there shouldn’t be … just curious.
No. The reason for venting an asphalt shingle roofing is to prevent the shingles from overheating and degrading the asphalt. Metal roofs don’t have a problem with overheating.
David,
what do you mean by “metal roofs don’t have a problem with overheating”. Can you explain that?
If they didn’t have a problem with overheating, why do we need to specify a high termperature resistant underlayment? Here in Houston, roofing could easily get up to, don’t quote me, 170-200 degrees F.
The metal roof does not have the problem…the waterproofing underlayment does. Since metal roofs in most locals typically get above 140 degrees, Grace Vycor Ultra (or equivalent) high temperature resistant waterproofing membrane should be specified.
Good catch. Technically, yes, you are right, its not the metal roof that has the problem. It is the metal roof, however, that magnifies the heat which produces higher temperatures under the panel, which does require the venting. I think metal roofing works pretty well as a conductor, subjecting the whole system to broad temperature swings. These temperature variations do need to be considered when specifying the underlayment and decking because they are part of the complete roofing assembly. We require complete system warrantees, so I dont see them as individual materials.
I just read your statement out of context to say metal roofs dont have a problem with overheating.
We specify the Grace Ultra, too. A few months back Polyguard successfully tested their underlayment for a higher temperature rating also. Oops, my bad, I just pulled their literature, its actually rated to 260 degrees. They call it Deckguard Plus. So there are other options.
Anyway, I digress; I know this thread is not about metal roofs.
Yep, that was my understanding regarding the shingled roofs on nail-base. And I didn’t think there should be an issue with the metal over a nail-base, either. But sometimes, you never know …
Thanks.