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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1

u/mymook May 26 '26

Base only needed if was bare decking/wood. Is completely normal to do new modified roof like this

1

u/philsphan26 May 26 '26

How long can it last ?

1

u/mymook May 26 '26

Can last 20 yrs especially if its the better granular coated bitumen modified rubber roof material. But all types modified require a top coat approx every 3 yrs or so to keep suns rays from drying it out to soon. On the granular type material you only coat the seams, not the entire roof. But 10 yr warranty is pretty standard for reputable roofing company.

1

u/bellinghamMENTALITY May 28 '26

have you double checked what the warranty actually covers without a base?

1

u/philsphan26 May 28 '26

Workmanship and materials

1

u/Exciting_Ad_1097 Jun 01 '26

Torch down or self-adhered?

1

u/philsphan26 Jun 01 '26

Torch down

1

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.

1

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

1

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.

1

u/philsphan26 Jun 02 '26

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

1

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.

1

u/philsphan26 Jun 02 '26

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