Minimum sheathing requirements for composition shingles?

[jimAKAblue]

I've been perusing the IBC 2006 code book looking for a definitive answer to this question.

Can composition shingles be installed over 3/8" plywood?

The reason I'd like to know this is because many of the older roofs (70's era) were built with 3/8? plywood. That is scary stuff when it's been cooking in an under-ventilated TX (or OK) attic. I would love to be able to provide my customers with a re deck roof and fit it into the code compliance benefit of their insurance plan (if they have that rider).

I've only come upon vague answers..see picture.

[BigLou80]

I would say, if the stamp on the ply wood say's it's a go then it's a go

[frenchie]

I don't know the IBC very well, but in the IRC, roof sheathing thickness is in chapter 5, one of the tables... hold on...

http://www.nachi.org/forum/attachmen...hing-table.pdf

Nope.

Your other possibility would've been the manufacturer - but I think you're SOL there as well. I just looked up GAF's code approval report, to get an idea... scroll down to section 4:

http://www.icc-es.org/reports/pdf_fi...S/ESR-1475.pdf

Nope.

I think you're SOL on this one.

[dgbldr]

You can always hire a 350 lb estimator to walk their roof (with a harness of course). Make sure the client watches him fall through.
Nothing like a visual aid to get the point across :)

[J.Buesking]

We just did a 300 Sq. deck replacement that was previously all 3/8". Scary stuff, especially in the rot areas.

[Mbeezo]

Just how much more do the composition shingles weight? Never really thought about that before.
I hate the 3/8 plywood roofs. Only way for a guy my size to work on them is to always be staning on a truss and not in the middle of the span. Hate to hear that cracking sound. And it always surprised me how that thin plywood could hold the roofing nails anyway.
Not sure if it would work but could you just add another sheet of ply on top of the 3/8 and go from there? Or would that be a problem with the strength of the trusses?

[jimAKAblue]

Beez, I'm sure it would be okay to laminate the osb. It would pose some challenges for the installers to make sure that they broke their vertical joints over a framing member.

I'm just searching for some ironclad proof that more than 3/8" is necessary to hold the new roof. If I can find a written code, or some manufacturer language that prohibits me from installing on 3/8", then I can demand that the insurance companies pay the bill for the new decking.

At this point, all I have is the argument that the roof deck is in such bad shape that it won't properly hold the nails, therefore voiding the wind uplift rating. It's a good argument on some of these roofs but I don't know how I can prove it on paper.

I did two questionable roofs like that last year and it killed me to lay over it but the owners were in no financial position to add the extra bucks so....I grinned and bared it.

Maybe I can devise a test that would measure the relative holding power of the seriously cooked 3/8" plywood. Anyone got any ideas on that? It'd be a good invention and you could make some pennies on it. Maybe an Iphone app that tests the holding power of old plywood that is one leg away from dust LOL?

[jimAKAblue]

We just did a 300 Sq. deck replacement that was previously all 3/8". Scary stuff, especially in the rot areas.

Some of it is scary. Maybe I should just stomp a hole through by walking heavy in the middle and then snap a picture of me trying to extract myself LOL.

[jimAKAblue]

I don't know the IBC very well, but in the IRC, roof sheathing thickness is in chapter 5, one of the tables... hold on...

http://www.nachi.org/forum/attachmen...hing-table.pdf

I did

Nope.

Your other possibility would've been the manufacturer - but I think you're SOL there as well. I just looked up GAF's code approval report, to get an idea... scroll down to section 4:

http://www.icc-es.org/reports/pdf_fi...S/ESR-1475.pdf

Nope.

I think you're SOL on this one.

Thanks for the effort Frenchie.

I did save that icc-es document. I'm not sure where it originates from but I like that it has all the shingles specs in one document.

I did notice that the requirements are "3/8 exterior plywood". I've never thought to verify that the existing sheathing is, in fact, exterior plywood but from now on, I will. If I find a house without the exterior stamp....they'll be getting a new re-deck! I would tend to think that most ply would be exterior but I can remember a few houses in my early years in the trades where they shipped "interior" grade. The builder just instructed us to put it on because the shingles were coming the next day and he didn't think it would matter.

[frenchie]

No worries, Jim. It was kind of interesting. The whole idea of 3/8 for a roof deck is completely weird to me. Never seen it.

I guess that's one advantage to living in snow-load country.

[DonMirabito]

Jim,
I'm not sure of the particulars of your local building codes, but it might be of value for you to look over the APA E30 documents for roofing panels. (Also, if you use the IRC for anything in your local area take a look at the table 503.2.1.1 and see if that helps.) The APA specifies how they stamp and rate their panels and the loads they can handle given the rafter spacing, spans, roof pitches, etc. There may not be a specific reference in your local codes about this, but most codes require you to install a manufacturers product in accordance with their specifications else you are in violation of the code by default. Depending on the specific composition shingles you are using they may weigh MORE than the panel loading can support.... Giving you the answer you are looking for...

Don

[jimAKAblue]

Don, that might be a good angle. I do know that the 3/8" ply used to be rated 24/0, so it did meet code. Most of the roofs I see with 3/8 are 3 tab, 20yr shingles so the weight probably fits. However, it might not with a laminated shingle. That will be another angle I'll look for.

Frenchie, I installed a lot of 3/8" plywood in my early years up in MI. The stuff will hold the shingles and a snow load. I think though that after it cooks in the attics here in TX, it gets too brittle. I'm also afraid to tell the adjuster that it's too cooked because he will say it's a maintenance issue and the homeowner will have to pay. Then, if we don't put the new ply on, they could force the issue and tell me that the roof is substandard because it's being laid on an unfit substrate. So, it's a fine line that I'd be walking and I don't want to walk it without a definite reason.

[Roger P]

Note the edge support requirements. http://publicecodes.citation.com/ico...-P-2009-000019

[DonMirabito]

Jim,
I'm from cold country so I have to admit I have NEVER seen 3/8 ply on a roof. However, someplace in my past readings I remember seeing some engineering report someplace about over perforation of lumber and plywood and its effects on the strength, etc. of the wood. Depending on how many times the roof has been re-shingled 3/8 may be so weakened as to be completely unwalkable and/or an unsuitable substrate for the new roofing. Just trying to be helpful...

Don

[jimAKAblue]

Jim,
I'm from cold country so I have to admit I have NEVER seen 3/8 ply on a roof. However, someplace in my past readings I remember seeing some engineering report someplace about over perforation of lumber and plywood and its effects on the strength, etc. of the wood. Depending on how many times the roof has been re-shingled 3/8 may be so weakened as to be completely unwalkable and/or an unsuitable substrate for the new roofing. Just trying to be helpful...

Don

Thats actually an excellent angle too, given that in the hailbelt, they reroof every few years. I'm going to try that angle.