Roof Decking

[laffertym]

I am repairing my roof that was damaged in the Katrina Hurricane. (A tree went through a specific area) My questions is - If I re-roof the entire house (present decking is too thin - but does not leak), can I re-deck on top of the old decking and then re-shingle, or does the old decking need to come down? I specifically wonder about the area between t he two sheets of decking having some sort of moisture build up.

Thank you

MIchael

[paul burrell]

I specifically wonder about the area between the two sheets of decking having some sort of moisture build up.

Thank you

MIchael

Micheal,

I think you have answered your own question. Trapped moisture between the old and new decking could cause a problem including delamination, warping and decay. However if the decking is allowed to completely dry after the old roofing has been removed you may be OK. In any event it is your home you can do what you please within reason including local codes.

Just my 2 cents worth.

Good luck and am sorry about the beating Katrina did to you all. We have several thousand victims that came here to Metro Atlanta. A great many say they are going to stay here and not go back.

Regards,

Paul Burrell

[hausdok]

Hi,

I don't see an issue with it if you use the same type of material. Plywood over plywood will have the same perm as a single sheet and as long as the attic has proper ventilation it shouldn't be a problem. Just don't try mixing and matching different decking materials. If you're really nervous about it, run 1/4 inch thick strapping on top of the old decking directly over the rafters leaving room for air to flow between the two layers, install ribbed ridge vents and ensure you stand the drip off far enough at the eaves to allow air to flow up under the top layer to the ridge. Another alternative might be to use vented nailbase insulation over the old deck.

If you'd like to read a lot of discussions on this exact topic, go to my Building Science Forum at JLC Online (http://www.JLConline.com) and search that forum for anything related to 'roof ventilation,' 'vented nailbase,' or 'roof decking' and I bet you'll find at least two dozen threads over there that will discuss these and other options.

Michael, are you a home inspector? If so, how badly was your business impacted and how many other home inspectors that you know were also impacted by those storms and what's their present state of affairs?

ONE TEAM - ONE FIGHT!!!

Mike

[Jim Katen]

Originally posted by laffertym

I am repairing my roof that was damaged in the Katrina Hurricane. (A tree went through a specific area) My questions is - If I re-roof the entire house (present decking is too thin - but does not leak), can I re-deck on top of the old decking and then re-shingle, or does the old decking need to come down? I specifically wonder about the area between t he two sheets of decking having some sort of moisture build up.

Thank you

MIchael

How thin is "too thin"?

Don't worry about moisture build up between layers of plywood. Make sure that the existing deck is dry and nail the new one tightly against the old.

I disagree with Mike's suggestion of using spacers. In addition to providing a nailing surface for the shingles, the deck provides stiffness to the roof plane and prevents racking of the framing. If you use spacers, you'll lose a surprising amount of additional strength in that regard.

- Jim Katen, Oregon

[Scottpat]

I have to agree with Jim and this is how they are repairing the roofs in my area after Katrina, including my roof. I had many shingles blown off and two sheets of decking damaged by a 4x4 fence post that came from the neighbors fence. I'm scheduled to have my roof replaced around Christmas! In fact my son and I just replaced our sections "Blue" roof today, they last about 90 days in the sun and you have to replace them.

[chrisprickett]

I'd go so far as glueing the two layers together w/ construction adhesive. Can't hurt if another hurricane hits. Make sure you stagger your joints when you go over the old stuff. When you re-nail, make sure the nails are long enough for a good bite into the rafters(min. 8d) and you might even want to go with ring shanked for extra bite.

[kurt]

I'd be squirting some adhesive in there too; it would make for one heck of a stiff box.

Maybe call it a "double ply" roof, heh heh....

[Bill Kibbel]

You guys are brilliant.

Multi-laminated, Staggered Joint, Hurricane Resistant Roof Decking. Pat. Pend.

[Erol]

Hi Guys,

Why isn't moisture damaged sheathing a concern when applying another layer over it?

Thanks

Erol Kartal

ProInspect

[Jim Katen]

Originally posted by Erol

Hi Guys,

Why isn't moisture damaged sheathing a concern when applying another layer over it?

Thanks

Erol Kartal

ProInspect

He never said that it was moisture damaged.

What's "moisture damaged" mean anyway?

- Jim Katen, Oregon

[Erol]

Your right Jim, I read it too quickly. Big difference between being wet and assuming it's damaged - as in mold.

[laffertym]

Since I started this question, I have yet to complete the project. I am stil considering the possibilities and want it done right. I intend to have the roof in place for a long time. Hopefully this hurricane season will be merciful! I am now considering the addition of a layer of felt over the old decking. then I would add the new decking. Any comments on the repurcussions this would bring about?

By the way - Still no leaks in sight.

Would appreciate any suggestions.

thanks for all of your feedback.

[hausdok]

Your wallet would be flatter. Otherwise not a whole lot. There's a plywood product known as 'dryply' used to deck the roofs of mobile homes which is essentially just that.

ONE TEAM - ONE FIGHT!!!

Mike

[mgbinspect]

Just wondering if anyone is concerned about the affect two layers of sheathing will have on attic temp and ventilation?

Could it cook shingles at quickly as two layers of shingles can?

[hausdok]

I don't think it will affect temperature and ventilation at all. I suppose a thicker deck might slow down solar gain during an initial temperature rise, say just after the sun comes up and the sun has to heat a cooled-down roof, but only as long as it takes to warm the sheathing at which point temperature would be the same as it would with a thinner roof.

ONE TEAM - ONE FIGHT!!!

Mike

[mgbinspect]

I guess mostly I was concerned it might keep an attic hotter for a longer period of timeduring "the dog days"

[Paul MacLean]

I can't see any difference between, say, two layers of 3/8" OSB of plywood and one layer of 3/4". In fact the R valve of the 3/4" decking will be better than the 3/8" as Mike pointed out. If I remember correctly from my APA days, 3/4" of plywood provides the same thermal insulation as about 10" of brick. Go for it.

[Steve Boozer]

Not sure if it's right or wrong and guessing everyone may have a different opinion. We've done it on several really old houses with plank decking. Replacing the decking would be best of course, but if you take the plank decking off, then you open up a can of worms. By the time you fix all the rafters and put the new decking down it would cost the homeowner an arm and a leg. So we have been putting down a 1/4 sheet of Luan plywood. Using an automated screw gun to fasten it with. I tell the guys to put a small gap where the sheets join so it can expand. Using 1/4 inch longer than normal roofing nails might be an idea to.

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