What is a Flat Roof?

The roof deck is considered “flat” or low slope when it is less than 3/12. This means that the roof drops vertically less than 3″ for every 12″ it runs horizontally.

Low Slope Membranes

TPO: Thermoplastic Olefin
This material is typically bright white and is laid out over insulation board. The seams of the TPO membrane are welded with a hot air welder. Once the seams are welded they are usually as strong as the membrane itself. TPO manufacturers make accessories to waterproof any number of roof penetrations such as pipes, railings, roof hatches and drains. By making most of the critical pieces in the factory the roof installation is faster, cleaner and more durable than having the installer make the accessories in the field. The advantages of this roof are its light weight and ease of maintenance. The bright white color also reflects most of the heat from the sun; this can lower the air conditioning bills during summer heat.


EPDM: Ethylene Propylene Diene Monomer or Rubber:
This material is typically black and is also laid out over insulation board. The seams of the EPDM membrane are bonded with glue and then sealed with a bead of special sealant. When done properly this is a very durable and reliable flat roof, however it can be temperamental. When gluing the seams and flashings the membrane must be cleaned with a solvent cleaner, and then primed with a compatible primer, and then the glue must be applied to both sides of the seam. After the glue has dried the seam may be mated together and bonded using a hand roller. The seam must then be caulked with an edge sealant to make sure that the seam cures. The installer must wait between each step and cleanliness is a must; in some cases installers cut corners as the work is very labor intensive. Unless each step is followed the seams will fail within a very short period of time.

Modified Bitumen:
Sometime referred to as “Torch Down” this material comes in short rolls. First a layer of rigid insulation board is installed over the roof deck. Then a layer of fire resistant base sheet is installed over the insulation. After the base sheet has been fastened the Modified is rolled out over the roof while being heated with a large propane torch. The material is basically being melted into place; the subsequent layers overlap each other and are also melted in. The finished system can be covered with an additional layer of Modified called a cap sheet which has a granular surface or painted with a reflective silver coating. This is a durable flat roof when installed correctly but it does require some maintenance. Since the system requires open flame during installation many municipalities are either banning its use or requiring fire professionals on site during the work.

Built Up or Tar and Gravel:
This flat roof consists of plies or layers of felt/ tar paper between which alternate layers of hot melted bitumen (asphalt) are applied. Typically 3 to 5 plies are used in building the roof membrane, after membrane is built up a flood coat of hot bitumen (asphalt) is applied over the entire roof, before the flood coat cools a layer of crushed rock is imbedded in surface. The crushed rock protects the typically black membrane from the UV rays from the sun and also keeps energy costs down by reflecting the heat during midday sun. This type of roofing is seldom being used in residential applications, between the weight, the noxious fumes from the heated tar kettle, and the danger from hot liquid asphalt this system is becoming less frequently used.

Share this post

Scroll to Top