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Old 06-06-2018, 09:34 PM   #1
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OSB over existing roof deck or replace


We bought a 1984 3 bed ranch as an investment. We need to redo the roof. It's leaking in 3 spots, has one bad truss, and 2 layers of shingles. We need to replace the roof of course and are wondering a couple things.

We see 4 bad pieces of sheathing already, viewable from inside the attic basically where the leak spots are coming. The total roof size is a little under 1700 sq ft. I would like to replace the entire roof deck. The people I bought the house with want to only replace existing pieces that are bad.

We need to remove the existing layer of shingles before putting on the new roof. Their argument is only replace what's necessary, and my argument is replace everything. I did the math and found the cost is about $850 for lumber, which is something they don't want to spend. They suggested to perhaps put all new OSB on, but put it on as a second layer of sheathing over the existing sheathing.

Sounds like a lot of added weight to the house that might be unnecessary. And I'm worried about putting a second layer of OSB over 30 year old plywood.

So, looking for advice here on what to do...

1. Should I rip off all the existing plywood and put in a whole new roof deck? I was going to put on 7/16 OSB
2. Don't rip it off. Replace only the sheathing that's bad?
3. Don't rip it off. Replace any sheathing that's bad. And then put the second layer of OSB on?

As part of my argument for option one of replacing all sheathing, since we're looking to sell the house after it's all renovated, we can say that the brand new roof also includes brand new roof deck, not just new shingles. Their argument is to save time by not having to rip off the existing deck as well as having to get rid of all the plywood we'd be tearing off.

Been watching a lot of videos and DIY websites on roofing, but still a newbie.

Any help's appreciated.
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Old 06-06-2018, 10:04 PM   #2
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Re: OSB over existing roof deck or replace


I'd just patch what's bad. Unless there is something wrong with the whole roof, like bellies between the rafters, I don't see the gain. If you're selling this house, I doubt you'll get your money out of it if you pull the old sheathing out and re sheath it. I think you would find that removing the sheathing from an entire house and resheathing it is a lot more work than you expected. Not to mention disposal costs.
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Old 06-06-2018, 10:30 PM   #3
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Re: OSB over existing roof deck or replace


Replace anything with mold any more than that is just a huge waste of money. And of coarse kill any mold or rot on the trusses.
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Old 06-07-2018, 05:19 AM   #4
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Re: OSB over existing roof deck or replace


As a rule of thumb.
If it's not broke, don't fix it.

Who's paying to have this done.?
Who's being paid the money.?
The answer to these questions usually seals the deal.
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Old 06-07-2018, 10:41 AM   #5
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Re: OSB over existing roof deck or replace


I agree with the others to only replace whats bad.
The only caveat I would toss in is if close to half of it is bad then rip it all up and upgrade from 3/8 or 7/16 to a real "1/2" (15/32 often)
No way I would go over the existing with more.
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Old 06-07-2018, 02:08 PM   #6
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Re: OSB over existing roof deck or replace


If I was your seller, Id say that I am replacing what needs replacing.

If you want the entire sheathing replaced, Ill credit you my bid amount (roofing plus 4 sheets of plywood) and you can replace the whatever you want to after escrow closes.
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