Flat Roofing 101

There are several types of flat roofing available.  In this article we’ll explore the major options and their pros and cons.


  • Also known as rubber roofing, this black membrane style roof comes in 45 mil or 60 mil thicknesses.  It comes in 10 foot wide sections or 20 foot wide.  Due to it’s weight it is commonly cut to smaller sections and then carried up onto a roof.  The seams are then taped with a special tape than then has sealants applied to the sides.
  • EPDM is glued to a fiber board in most cases.  The fiber board, which is mechanically secured to the roof deck with screws and plates, absorbs some of the glue and provides a surface for EPDM to expand and contract without tearing.
  • Durability is a little dicey with EPDM if there are any trees or anything that can impact the product.  If an area of EPDM is pierced, the fiberboard wicks out toward other areas and then when the heat comes that moisture is trapped underneath it.  The dark color, combined with moisture and heat create a breeding place for mold.
  • Color:  EPDM is black so it absorbs heat.  Some EPDM is now coming in while, but not all of the accessories, such as the tapes for the seams, are readily available in white.  Therefore, a white EPDM roof, with white accessories, could look like a bit of a Frankenstein roof, cobbled together with spare pieces.  Some readers may think there would be a benefit in the winter, but we remind them that all roofs in northern zones like Wisconsin are snow covered white in the winter.


  1. Because it is white, TPO is more reflective and therefore, cooler.  This can save you money on your cooling bill as well as make it so that your cooling unit does not get overworked and prematurely burn out.
  2. Roofers find it a little more cumbersome as it does not bend quite as nicely as EPDM.  While it a PVC based product, some have even dubbed it “Temporary Plastic Overlay”.  In fact, it’s a fairly good product and rather than requiring seam tape as EPDM does, when pieces connect they use a heat gun to effectively melt the two pieces together.
  3. It is durable and energy efficient.

Self adhesive low slope products (Modified Bitumen)

  • There are several self adhesive low slope roofing options.  Among them is Flintlastic which is a two ply product which gives you more protection than a single layer.  The first layer, or base sheet, can be either nailed (Mechanically fastened) or self adhered to the roof decking.  Because it can go right to a plywood base, it doesn’t have the cushion under the fall that can assist trees in the puncturing of your roof.  While this is used primarily in commercial roofing, some residential roofing companies offer Flintlastic for small accent features on a roof.
  • Puncture resistant: Unlike EPDM or TPO roofs, self adhesive products like Flintlasic, which have both a base sheet and cap sheet, are very durable.  When applied directly onto a hard surface decking, even small branches can impact it without tearing it.
  • Modified bitumen in some regions is also known as “torch down” because prior to the self-adhesive technologies, this product needed the use of a torch to “relax” it so it could adhere to the roof.  Today, most insurance companies will not insure companies that use fire on a roof and therefore, the self adhesive technologies were introduced.
  • Repairability is one reason why modified bitumen is a good choice.  Modified bitumen roofs can be repaired with roofing cement.  Optimally granules can be spread on top of the repair to protect the roofing cement, extending the life of a repair.
  • Downside:  The downside of Modified Bitumen is that it is not made for true flat areas as it does not withstand ponding.  However, a workaround would be to coat the area with a silicone based product that does withstand ponding.  Companies like like Gaco or Henry both offer silicone based coatings.

What goes under these 3 products?

There are 3 ways that roofers apply low slope roofing.

  1. Over fiberboard.  The fiberboard absorbs some of the glue that is also put on the back of the EPDM.  They let the glue dry on both sides and when they put it together, it bonds like rubber cement.
  2. Over poly-ISO board.  A closed cell foam, often used to create a slope to a roof that is true flat, is stacked up and built up in pieces that are typically 4′ x 4′.  The taper can go from a full 2 inches on one side, to virtually nothing on the other side.  Poly ISO can give you a slope of 1/4″ per foot up to 1/2″ per foot.  Consider the height though when considering building on a slow foam roof.  With a 30 foot run sloped at 1/2″ per foot, you would have a 15 inch crest at the height.  Therefore, so as to reduce the ridiculous factor, the 1/4″ tapered foam is the most common.
  3. Directly onto the wood.  This method is generally not recommended and the manufacturers frown on it.


Share this Post:
Posted in

Leave a Comment