I read that an edpm rubber roof requires a minimum fall of 1 in 80 in order to be able to shed water. I calculate that to be 0.7 degrees. My flat roof is 1.7 degrees. I was thinking to also install a sheet of polycarbonate on the roof (or even better perspex - glass like window onto the sky). But it is often stated that polycarbonate requires a minimum 5 degrees fall! Am I missing something - polycarbonate requires more than 5 times the fall than edpm rubber to shed water? Would using perspex make any difference?

  • Are you attempting to design and build and install your own skylight on a flat roof? Give serious consideration to this before you start cutting holes! Until 2000 (or so) professionally engineered and commercially produced skylights were fairly notorious for leaking. Before you let water into your building give it some serious thought...
    – FreeMan
    Jun 29, 2020 at 12:26
  • @FreeMan Well, a skylight of sorts but not a constructed one. The idea was to have edpm from the top of the flat roof down to say a meter from the gutter and then have a section of perspex for the last meter down to the drip to the gutter. The edpm would overlap the top of the perspex by 100/150mm? and stuck down with contact adhesive. I fear that horizontal winds might lift the edpm but I was hoping that if special attention was made to bonding the the edpm to the perspex it would be ok. what do you think. Jun 29, 2020 at 13:09
  • @FreeMan It's a garage. All the timber members are in place - just have to put ply or ply and perspex the whole flat area. Jun 29, 2020 at 13:15
  • It doesn't matter if it's a house, garage, shed or yurt, you're still making a skylight. I would contact the manufacturers of all the materials you're using and ask them what they recommend for adhering their product to the other one. You may get the same answer, you may get different ones, then have to reconcile them. If there are concerns about wind lifting the material off the glue, then you may need to use something on top of the EDPM to hold it down. Mentally prepare yourself for water ingress and a repair job. If it doesn't happen, you can be thrilled and pat yourself on the back!!
    – FreeMan
    Jun 29, 2020 at 13:20
  • Consider heat loss (if the garage is heated). Consider snow loads. Impact resistance, especially if there's a tree close to the garage. A strip of natural light coming in would be really nice during the day, but there are lots of things to think about in terms of longevity. There's probably a reason commercially available skylights are pricey, and why others haven't done what you're thinking of. I'm not saying "don't do it", just "think seriously about it".
    – FreeMan
    Jun 29, 2020 at 13:23

2 Answers 2


No, you have to follow manufacturer’s recommendations for fall. And by the way, fall is discussed as a ratio of rise/run. Nobody uses degrees for that; it’s an unwieldy unit that you have to keep converting in/out of. Build some roofs; you’ll get it.

The reason for the fall is simply that an EPDM roof is a membrane. It is glued and sealed, and it could nearly be used as pool liner.

Whereas, “tin roof”, perspex etc. rely entirely on lap seams to keep rain out. If the roof is too shallow, water will do an “S-curve” and run backward up the lap seam and get into the roof.


Just match the existing fall, it is way too difficult to put a window in with a different slope. Possible but what would be the benefits in this situation.

  • Thanks for the reply. I wasn't suggesting changing the fall - it's too late for that. So are you saying that a 1.7 degree fall is OK for polycarbonate and or perspex even though it's often specified as needing a minimum of 5 degrees for the poly? Jun 29, 2020 at 8:20

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