How Flat Roofing is Different From Traditional Roofing

Flat roofing

At Renovations Roofing & Remodeling, Inc., we pride ourselves on staying up to date with all expert knowledge about both types of roofing. However, the terminology couldn’t be more different.

You may be familiar with the traditional roofing types and materials or even with the process itself. Flat roofing is a whole other story.

Why is flat roofing so different from traditional roofing?

All traditional roofing relies on the pitch of the roof as a key component in shedding water. If you put asphalt shingles on a flat roof, the layers of material would immediately allow let water in and rot the substrate, leading to leaks and immediate damage to the interior as well.

The flashings and penetrations of a pitched roof are the most vulnerable points where leaks can occur. In a flat roof, the same is true. However, flat or low slope roofing is completely different in that the membrane becomes essentially sealed. All components are either heat welded or glued together with special adhesives and sealants creating a virtually impenetrable barrier.  

The technology is always developing and getting better which is why we strive to stay on top of it. Basically, you have to put down a physical barrier or apply a coating to make something waterproof. There are several ways to do this.

Flat Roofing Comparisons

EPDM Rubber

The attractive thing about a rubber roof is its price and very user-friendly “in the field” modifications. That makes it a popular first choice for flat roofing. Rubber roofing comes in different size rolls and is ordered according to the job needs.

All flat roofing requires a substrate of some type of suitable insulation or re-cover board to be applied first and the membrane is then installed They are applied using specialty adhesives, solvents and sealants to the roofing substrate. It takes skilled tradesman to apply EPDM with the greatest attention to detail.

Typically the EPDM membrane will last 20-30 years and most manufacturers offer enhanced full system warranties of up to 20 years. As with all roofs inspections and maintenance need to be planned. With flat roofing, this is even more important.

Seams and flashing details are typically glued and bonded with a specialty “seam tape” in the seams and together become essentially a single sheet once the job is complete. Rubber roofs are a great choice for colder climates such as Michigan. While much has been touted about “green” roofs…IE white PVC or TPO… the current scientific based research is starting to reveal unforeseen problems and consequences with white roofs.

PVC Membrane

This is still a popular choice of flat roofing material, especially in warm climate states. It’s very strong, very durable, and will last a long time when installed properly. It surpasses the industry recommended standard for strength as its minimum breaking point is 300 PPI (lbs per inch) whereas the industry standard is 200 PPI.

As with EPDM a substrate of some type of suitable insulation or re-cover board must be applied first and the membrane is then installed in rows which are hot-air welded at the seams. However, the seams are strong – even stronger than the material itself. The edges are often mechanically fastened to prevent the membrane from lifting.

PVC membrane can also be glued to the roofing substrate. PVC roofing warranties will vary depending on application and manufacturer. All manufacturers offer enhanced warranties depending on the specific needs of the customer.


TPO is like a PVC or rubber roof in that it is also a single-ply membrane. These are more available in lighter colors – making them energy efficient – and have the added bonus of welded, as opposed to glued, seams. They are closer in price to a rubber roof.

A TPO roof is installed over insulation boards, it comes in rolls, and can be mechanically fastened or applied with a self-adhesive.

TPO roofing has been gaining in popularity since the 1990’s and touted as being the best of both the PVC and EPDM worlds. However, as mentioned before the latest research is showing unintended issues…especially in cold weather states.

Spray-On Roof

The “spray on roof” is rarely used in Michigan and frankly has too many drawbacks to even consider other than on very small projects. Essentially, they are closed cell polyurethane sprayed on and then a reflective coating and granules are applied to protect it from the elements.

The real issue becomes the difficulty in avoiding puddles…which then attract birds and organic debris. Further, the thermal expansion and contraction does not lend well to staying secured to critical flashing points. 

Modified Bitumen

This is where we get into multi-ply roofing. This type of roofing is being used less and less as owners and specifiers are opting to use single ply and improved drainage through modifying the roof deck and drainage components. In fact, new construction code now mandates that “flat roof” have positive drainage.  

A base layer is mechanically attached to the roof deck with plates or bars and then a ply overlap is adhered or torched down. Then a granule top surface may be applied overall to make it look nice and be more energy efficient. There are both smooth and granular Mod-Bit roofing systems.   

These types of roofs can be very high in labor intensity. It takes torching at every ¼ inch turn of the roll of material unless you are using a cold-roll technology. Cold roll technology still uses quite a bit of roofing tar.

Any roofer will tell you that a flat roof is actually harder on the back to install than a pitched roof. Modified bitumen roofs should last 15-20 years with proper maintenance.

Built Up Roof

As with Mod-Bit roofing, we are seeing less and less demand for Built Up roofing.

Built up roofs are also called tar & gravel roofs. This is another multi-ply roof that is installed using multiple layers of ply sheets bonded together with hot asphalt.

A top layer of reflective coating or gravel goes over the top. This type of flat roof has been in use for a long time as it can withstand a lot of foot traffic and collisions better than a membrane roof. Built up roofs should last 20-30 years with proper maintenance.


Different flat roofs will have different needs depending on where the building is and what the roof will be used for. Consulting with your knowledgeable roofing contractor will help you decide which application is best for your home or building.

If you are located in Southeast Michigan, give us a call. We would be honored to help.

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