I have to fix a flat roof of about 60 sqm. Right now it has an ancient layer of bitumen (at least 35 years old, maybe even over 50, I'm not sure). The bitumen is slightly porous and not really smooth (but not terribly bumpy either, I hope).

As far as I understand, the way to go is to cover it with a layer of EPDM rubber. There is a lot of information available online about how to use the EPDM, even a lot of videos that explain very thoroughly how to install the EPDM layer.

But most videos I found show people putting the EPDM layer onto a wood layer. I found one video that showed adhering the EPDM on bitumen, so I guess that generally it should be okay to put the EPDM on bitumen, right?

But what about a bitumen layer that is not smooth? Is it still okay to put the EPDM directly onto that, or would I have to put another layer onto the bitumen first, to make it smooth?

And also, what kind of glue should I use? To my understanding, there is glue with or without dissolvent, but I couldn't find any information about what's the practical difference between the two. Which one would be advisable to use in my situation?

1 Answer 1


Your new roof needs to “move” independently from the old roof. In order for your new single ply membrane to be separated from the old roof, you need to add a “slip sheet”.

I’d use a geotextile fabric and fasten it down “mechanically”. That is to say, use a special fastener to screw it down. It’s about 45 mil thick and the single ply membrane can be attached to it.

The slip sheet can be extended up a parapet wall, around vents, over curbs, etc., so the new single ply roofing can completely seal the roof.

How the single ply membrane is attached to the slip sheet, depends on the type of single ply roofing you use.

BTW, you can take this opportunity to add rigid insulation down before you install the single ply membrane too. However, make sure vapor barriers, venting (or eliminating it), etc. is proper before choosing this installation.

  • 1
    I add my vote for taking this opportunity to add some rigid insulation. You can also get rigid insulation boards with a slight taper to them to ensure that water runs off in the direction you choose and won't ever pool up.
    – brhans
    Commented Apr 23, 2018 at 13:16
  • Yes, using “tapered” insulation can eliminate low areas, etc.@brhans
    – Lee Sam
    Commented Apr 23, 2018 at 15:18

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