Asphalt shingle retention?

After 32 years my asphalt shingled roof needs replacing.
What are the advantages/benefits and disadvantages of leaving the original layer of shingles on my roof and just placing a new layer of shingles on top?
My roof requires approximately 75 squares and is walkable, located in the northeast.
Please estimate the $ savings, as well.

Only advantage is it’s cheaper. Most major disadvantages in my opinion are extra weight on your roof, shingles not laying as flat and future cost to remove two layers of roofing.

While they aren’t ideal I will install a second layer of shingles if the circumstances are right. The roof need to have minimal amount of flashing work, pitch of 6/12 or greater and existing shingles must be laying flat. I suspect in such a large roof you will have a lot of flashing work and if so it would rule out the possibility of a second layer from me.

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I feel two layers hold more heat and degrade the shingles quicker than what a single layer does. The shingles not layering as flat may contribute to that as well. But all the multi-layers roofs that we have replaced have been in worse shape than single layers of a similar age.

By not tearing off the old shingles, you also don’t get a chance to inspect the decking for rot/damage. It’s harder to flash correctly because you have to deal with the thickness of the first layer; you may need to trim back siding so it fits.

We rarely do shingle layovers and don’t offer the same warranty on any that we may do. For a roof that large, I would want to tear off the first layer. There’s too many areas for something to go wrong.

You might save 25% with a layover as opposed to complete replacement. It’s not worth it if you can afford to do it right.

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Second layer will ruin 1/2" plywood, especially if it’s on trusses. There will be moisture in the existing roofing that will try to migrate out, shorten the life of the new roof. Not worth it, in my opinion.

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You have a 75 square roof which translates to a big and probably custom home. Why wouldn’t you pay respect to that house and do things properly? Would you overlay a tile floor? I get the little old lady with 15 square and no money. What’s your story?

I refuse to do a single over. Only advantage is save a little money. Numerous disadvantages.

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A Shingle roof over Requires a decent pitch
And little walls or protrusions
Because those need to be treated as new.
All the material needs to be removed in those areas.
Not really confident in most roofers going over the valleys Either.
A 75 sq roof i would imagine has lots of these things that would eliminate the time savings of not removing all of it.
Plus a roof that old should not be considered as a candidate for a roof-over either.
Ughh

And is the roofer responsible later to fix, replace rotten wood that a home inspector finds
Because the roofer couldnt see it and shingled over it?
If you are trying to sell the home or re-finance it is when this is crucial.

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I do a couple nailovers a year. They have many more disadvantages then they have advantages.
It’s not worth it.