Flat roof upgrade advice


Hi everyone. I have a small flat roof that does not leak, but is getting old and blistering. It is on a 1925 house in New York City. The current roof is black tar on plywood, about 2 layers deep. The roof of the house is spray foam insulated on the underside, and it is a warm attic. There are various skylights in the roof that have slopped on tar as flashing. The house has a pitched, slate roof in the center, dividing the two flat roofs.

The contractor wants to cut open the blisters, and nail them down. He wants to then nail a thin layer of insulation on the existing roof, and then put “90 grade” (not sure what that is) paper with glue on top of the insulation. He will use metal flashing on the pitched roof and tuck it under the slate tiles. He will then coat it with aluminum paint. The job is $2,500. Pretty cheap. He says that as long as we re-coat it every 8 years, it will last “forever.”

We hope to put some rubber mat on the roof and walk on it.

Is his plan a good one?
We have the budget for higher-end materials, would PVC or another material be superior?
Any advice?




Can’t say much unless I see it myself but from the description it seems your roofer is trying to offer you the best solution with in the budget. If the roofer has excellent credentials, strong word of mouth and is insured then you can go for it. You can also take estimates from other reputed roofers and choose the best.


You intend to use the roof as a walking deck?


Smooth mopped down ( cold process) modified bitumen??
Ive never seen that before.
But if its true and there is no side seams.
(Structure under 33 ft wide)
Than it could be lifetime if you keep it coated with aluminum roof coating yes.
But you do need to coat it more often than every eight years.
Whether it is smooth or granulated, make sure it is actually “modified bitumen”.
A "cap sheet"
Meant to be exposed by the sun warrantied for 12 years without any coating at all.
Keeping it coated makes it last a lifetime.
Recoat after you see cracks in the paint.
I say two coats every three years.
It does take some work on your part.
The sun eats the paint instead of the surface of the material.
Do not be fooled and use more expensive urathane coatings.
Yes, they last twice as long,
But when they fail, they start peeling up
And any further coatings will not work because the underlying layer will still peel away off the substrate.
You want a product that wears away and not peels away.


Maybe if you are unsure of his plan, you could get a different bid from a another roofing professional. Researching things on your own could be another alternative. At Chicago Roofing Company they have some great information on flat roofs. That article blog might help you in some of your decision making. Wishing you the best.


Thanks everyone!

roof_lover, that’s what he is offering. The structure is about 10 feet wide. The flat roof meets a slated peak of the house, and is then flat again on the other side. There is no side seam. I will make sure that it is modified bitumen. He showed me the sample. It looked just like tar paper. I couldn’t tell the difference between the high grade and regular grade.

I don’t mind going up there and re-doing it every few years.

Thanks Amycent. I will read that, too. It seems like all the great contractors who post online are in Chicago!! I’m not sure why that is. But for roofing and masonry.

Axiom, just put some rubber on it for light, private use, not parties!


I will post pictures when I get back. Thanks!!


You still have a lot of reason to be concerned if you really think it looked no different than tar paper.
You need to make sure that is not the case.

Tar paper is so thin. It is paper thin.
Modified bitumen is as thick/thicker than a roofing shingle and you cannot rip it with your hands.

I am bothered that he did not specify the brand and actually say Modified bitumen.
Who does that?
You need to research more before you say yes.


Thanks roof_lover. That is exactly correct, I could have ripped it up with my hands.

In your opinion, what would be the best choice of material for covering a roof with blisters. It sounds like nailing down insulation and then covering it with a high grade bitumen is a good choice? I would like to invest in the roof so that it lasts, but avoid the cost of removing the existing roof if possible.

I spoke with a number of contractors, but I’m really too ignorant to assess which are good. I also have done quite a bit of research on line, but most of the topics on flat roofs cover replacement, including removing the existing materials underneath.

This guy also did not cut a test patch in the roof. He just said that he could tell it was 2 layers.


If you can find a way to post some pics up,
Many of us can really help you.
I dont want to waste time speculating what the top surface really is and how to fix it.
But we will all know when we see it.
Need to see the skylight area also.


Thank you so much!! I will as soon as our upstairs guests leave.


Put some TPO on it . :sunglasses:


Just for discussion sake, when one X air pockets and such (air pockets are caused by what - just the adhesive not adhering or water intrustion between the sheet that has delaminated it). Doe cutting and nailing create a ~new water point~ of intrustion and now the last mile of defense is the new roof layer…whereas formally, it was a single sheet with no protusions down to the base, but just a air pocket


Thank you again so much for your help. Here is a link to a Google Album. I have also uploaded these below.



Hi everyone, any thoughts :slight_smile:


Aluminum coating is used to preserve a membrane. Not to stop leaks. You have some puddling/ponding up there. That would be a cause of concern without putting a tpo or a pvc roof on for me. Is the current roof leaking?


It’s difficult to say because there is closed cell foam sprayed to the underside of the roof. No apparent leaking yet, though there could be water infiltration between the membrane and the foam.



If it was my house I’d be using PVC. Also ask potential roofers if they know how to properly remove and replace the bottom coarses of slate before you hire them. Its an easy thing to do but I see so many slate roofs destroyed by ignorance.


Will do. Do you work in NYC? Or can anyone on the forum recommend someone who does?

I know how to hang slate, but prefer to leave the hooks and drilling to a pro.



No, I don’t have the proper liscenses and insurances to work in NYC. Oh and BTW, if you want a truely long lasting roof you could use flat lock copper, the same companies who install slate usually recommend that on flat roofs. However it will be very pricy.