FS Roof System Blog

What are the Layers in a Flat Roof?

We get asked a lot to explain the different flat roof layers. The answer that we give is usually fairly generic: It consists of multiple layers from the ceiling to insulation to vapour control layer to insulation to the final top membrane. In this article, we will try to break it down to a more detailed version for you. Remember though that each flat roofing system has different layers. That means an EPDM flat roof will have a slightly different layering methodology than a tar and gravel one.

From the Top

1)     On the very top you have the waterproof membrane. This is the EPDM, TPO, Mod Bit, PVC, Tar and Gravel or whatever layer you choose on. It is the primary waterproofing layer and is the only layer you should see on your roof.

2)     Next, we have the insulation which is much like the insulation you find in between the walls of the rest of the building. Depending on the space we are working with we may suggest different kinds to maximize heat efficiency.

3)     Then there is a structural deck or roof deck that is that structural base for the top 2 layers. It’s what makes you able to walk on the roof.

4)     The vapour control layer that comes next does exactly as its name implies: it helps control condensation so that no mold starts growing. Just so you aren’t confused if you’ve read different; we’ve seen instances where the vapour control layer comes before the structural deck layer.

5)     Next, we have what’s sometimes called ceiling joists. They are the structure that transmit the weight of the roof to the walls of the building. So that the roof doesn’t cave in on itself.

6)     Finally, we have the drywall or whatever ceiling you have chosen. This is the ceiling that you see in your building.

We’ve constructed this list in what we call a “general order”- that is to say, you’ll see different orders in different roofing systems. This list is really just a list of typical components. For instance, in EPDM roofing the vapour control layer often goes before the structural deck and on green roofs the system goes:

  • Vegetation
  • Soil
  • Drainage
  • Water retention layer
  • Insulation
  • Roof barrier
  • Waterproof layer
  • Concrete

As you can see, there are a different number of layers, order and the layers themselves are different.  Another example is tar and gravel flat roofs. These roofs have alternating top membrane layers followed by a roof board and then insulation.

Okay point made, so how do you know what layers make up your current system? And what layers will your new roof have? It’s simple: ask us when we come to inspect your roof. During our inspection and estimate we lay out all of the options for you and will be happy to discuss with the you the exact layers we would be installing on your roof.