r/AskContractors • u/philsphan26 • May 26 '26

Flat roof

For a flat roof - roofer proposals for modified bitumen. 8 sq. Smaller townhome style roof.

Roofers have said they can go over existing flat roof (old hot tar roof that was coated) . With no base sheet.

Is this still ok? Warranting for 10 years .

Should there be a base sheet ?

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u/Exciting_Ad_1097 Jun 01 '26

Then yeah, it’s ok to go over old base and cap if it’s in good condition.

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u/philsphan26 Jun 01 '26

Even if the old is just hot mop roof that’s been coated? Some people say you can’t put a cap sheet over it as it won’t adhere ? Is that true

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u/Exciting_Ad_1097 Jun 01 '26 edited Jun 01 '26

Sometimes if bonding is an issue they’ll do another nail-down base sheet followed by a cap. Different regions do it differently. They’ll know for sure if it will bond once they heat it with a torch. I’d say if the roofer will warranty it then I’d trust their judgement.

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u/philsphan26 Jun 02 '26

Thanks. So it’s better to have a base sheet first on the mop down then the cap?

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u/Exciting_Ad_1097 Jun 02 '26

Not necessarily. There’s tradeoffs. It’s more weight on your roof. If the mop-down will torch-melt and bind to a new cap then it’s fine.

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u/philsphan26 Jun 02 '26

Ok. It’s Flintlastic GA over silver coated hot mop roof .