# Plywood over existing roof deck or replace existing?



## eliotssc (Jan 14, 2020)

I need to replace my roof shingles, however one of the roofing contractors I talked to (who I trust) said that the original roof decking on my 1950s ranch house has many gaps over 1/4" between the boards that will cause a problem. Otherwise, the decking is in good shape. He recommended that they add 1/2" plywood over the existing roof decking for the entire roof before adding underlayment/asphalt shingles/etc. He mentioned that the shingle manufacturers won't honor the warranty unless I address the gaps I have, but I am more concerned that the gaps in the existing decking won't meet code when I re-roof unless they are addressed. My other concern with doing what is recommended is the additional weight on my roof. My roof was made with 2x6 rafters and having two layers of decking seems like a lot of weight. I had a structural engineer stop by for a quick look and he said that I could add the 1/2" plywood decking on top of the existing and it would support 1 layer of asphalt shingles, but no more. I don't think the shingle manufacturer's warranty is worth the paper it is written on, so that is not a concern. I am not sure what I should do because I am relucant to add that much weight to the roof even if the engineer said it could take it... Is adding plywood really even necessary? If I do end up needing to put on new roof decking, would it be more feasible to remove the existing decking and just put new 1/2" plywood on? I have no idea what it would cost to remove the existing decking, so don't know how feasible that option would be yet. Has anyone had a similar situation or have any other advice to offer? Any feedback is appreciated. Thanks...


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## PatChap (Nov 17, 2012)

We do this all the time
The added weight isn't enough to cause any problems unless your house is extremely poorly built.
No need to go down to the rafters unless the boards are completely rotted. 
Going over with new plywood works great.

If your boards are in good shape and have very small gaps,it may be not be necessary
A photo or 2 wouldn't hurt.


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## eliotssc (Jan 14, 2020)

Thanks for the feedback. Here are some photos of my attic where you can see some of the roof decking. You can't see it here but there are some boards with gaps greater than 1/4" where you can see the underside of the underlayment. However, what you see here is probably average or the majority. 

There are a couple of damaged rafters that I am going to have a general contractor sister before I install the roof (which the structural engineer said I should do before adding the plywood decking). The GC also recommended that I add collar ties to the roof, however the structural engineer said that it wasn't necessary.


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## PatChap (Nov 17, 2012)

You don't need to sheath over those,but you probably would have problems if you ever need to use the shingle warranty.

If it was my home and it wasn't crazy covid prices,I would slap a layer of 7/16 osb on it. This year.. I don't know,maybe not.


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## carpdad (Oct 11, 2010)

Ask for hand nailing entire roof, or one hand nail each shingle to verify, or mark the gaps on the weather barrier. Little more time to make sure. I disagree on plywood solution. A roofer with experience should be able to tell if nails are hitting voids or solid wood. I'm no roofer and I could feel the difference with a pneumatic nailer. Could be cheaper than installing the plywood. If hit the gaps then adjust little as long as the nails aren't exposed. Do some math and figure the likelyhood of nails hitting the gaps. Your boards are x widths. Shingles are nailed at x distance from the edge and overlap with x inches. Draw the things and see. This is for your peace of mind too.
Vents, plumbing vents, chimney flashing should make no difference. They should all be replaced or completely removed before shingling.


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## Colbyt (Jan 27, 2014)

Unless there is a major problem (unlikely) in the mfg. process it is highly unlikely that the warranty is worth the paper it is printed upon.Read closely they are pretty much worthless after 10 years. Your insurance company will be on the hook for storm damage if you carry replacement cost coverage or the depreciated value if you don't.

I just replaced shingles on a house with 1x6 decking and the problem was never mentioned. We did cover a small addition with new OSB because we were installing EPDM over that.

Bottom line; I would roof the house with the best shingles I could afford and sleep well at night.


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## eliotssc (Jan 14, 2020)

Thanks everyone for the feedback. I am more concerned about a code compliance issue because I believe the local code essentially says to install per the manufacturer's instructions, and the manufacturer(s) of shingles I want to use don't want more than a 1/4" gap between the decking boards. Most of my existing roof decking doesn't have gaps that large, but there are definitely some that do (that may not be visible in the photos). My local building dept also has a reputation for being pretty strict on code requirements and I am not sure they will allow new shingles to be installed without addressing the 1/4"+ size gaps in the decking. The roofer who proposed adding plywood decking said he was surprised that decking wasn't added when the current roof was installed 9 years earlier (by another roofer).


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## Colbyt (Jan 27, 2014)

Is this storm damage? Why would you be repacing 10 year old shingles?


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## HotRodx10 (Aug 24, 2017)

eliotssc said:


> I am more concerned about a code compliance issue because I believe the local code essentially says to install per the manufacturer's instructions, and the manufacturer(s) of shingles I want to use don't want more than a 1/4" gap between the decking boards.


My understanding is that the gap limitation would not be considered an installation instruction, but a warranty limitation. I'm not an expert on codes, so I could be wrong.


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## PatChap (Nov 17, 2012)

eliotssc said:


> Thanks everyone for the feedback. I am more concerned about a code compliance issue because I believe the local code essentially says to install per the manufacturer's instructions, and the manufacturer(s) of shingles I want to use don't want more than a 1/4" gap between the decking boards. Most of my existing roof decking doesn't have gaps that large, but there are definitely some that do (that may not be visible in the photos). My local building dept also has a reputation for being pretty strict on code requirements and I am not sure they will allow new shingles to be installed without addressing the 1/4"+ size gaps in the decking. The roofer who proposed adding plywood decking said he was surprised that decking wasn't added when the current roof was installed 9 years earlier (by another roofer).


Should be a pretty simple question for your building department then. Get whatever they say in writing if you can.


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## eliotssc (Jan 14, 2020)

Colbyt said:


> Is this storm damage? Why would you be repacing 10 year old shingles?


Good question and long story. The short story is I bought this house 2 years ago and was recently told by a roofer doing a small roof repair job that the condition of my shingles didn't look anywhere near their age and he suspected hail damage (and explained and showed me why). I had two other roofers do an independent inspection of my roof without giving them history or other info beforehand and they provided a similar assessment. I called my insurance company and told them the story and their adjuster came out and inspected and verified the roof shingles as having been hail damaged. They paid my claim less deductible and now I have to get the roof replaced.


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## eliotssc (Jan 14, 2020)

HotRodx10 said:


> My understanding is that the gap limitation would not be considered an installation instruction, but a warranty limitation. I'm not an expert on codes, so I could be wrong.


I am looking at installing Owens Corning shingles and I came across technical bulletin TB-10022499-BW regarding roof deck requirements for installation of asphalt shingles and that seems to "require" no more than 1/4" spacing between boards.... however I am not an expert either, so I could be wrong, too.


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## carpdad (Oct 11, 2010)

It's a good thing to have solid nailing base. Good for you, good for the roofer. But it is better for the roofer than for you since it is roofer's job to make sure the shingles are nailed and not into the joints. A roofer who says shingles may not be nailed properly because the sheathing is boards, look for other roofers. Your attic is exposed. Along the job, after the job, inspect and look for nails sticking out of the joints. Prepare a contract where wrong nails will be repaired. At least 1/4 of the total cost to be paid after the inspection. Get 3 estimates under the same conditions about the nail inspection.
BTW, check the board joints. Stick a flat stick into the joint. They are usually tongue and groove boards and there is some wood in the joints, though that does not excuse poor nailing.


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