Flat Roofing Options: Modified Bitumen vs. Spray Polyurethane Foam

When it comes to flat roofing installation on your commercial building, you have an enormous sea of options to choose from. But since you’ve got far too many decisions to make already, we thought we’d break down the two most popular types of flat roofing systems for you.

These are modified bitumen and spray polyurethane foam (SPF).

Modified Bitumen Flat Roof

Modified bitumen is a type of flat roof that your contractor installs by rolling single-ply sheets of material onto the roofing surface. It’s a “built up” roofing system, meaning you’ll add several layers of modified bitumen to make the roof more durable.

There are two types of modified bitumen roofing materials:

Modified Bitumen

  • SBS modified bitumen. (That stands for “styrene-butadiene-styrene,” but we promise not to quiz you.) This type is rubber-based, and very flexible, even in cold weather. Sometimes the SBS membranes are installed on hot asphalt, and some require cold adhesive.

  • APP modified bitumen. (“Atactic polypropylene.” Memorize that one for parties.) This type is plastic-based. It requires heat installation, so your contractor will use a blowtorch.

(Of course, it’s up to you, but we tend to think that using a blowtorch on your roof could be a fire hazard. If you’re going to choose a modified bitumen roof, please choose the safer, non-fiery option.)


Let’s take a look at the advantages of modified bitumen roofing (MBR). Probably the best one is that it is reflective. That means that it sends most of the sun’s rays back out into the atmosphere rather than absorbing them into your building.

This should cut down on your energy costs.

When you compare modified bitumen roofing to traditional asphalt, the MBR comes out ahead. It has better strength, flexibility, and UV ray resistance. The materials used in MBR are waterproof, and they can withstand extreme weather.

And a modified bitumen roof cost can be pretty low too. With most modified bitumen roofing contractors, a new roofing system will run you $1.50 to $3.00 per square foot.


Unfortunately, it will be a lot more expensive if you need to remove the old roof. When you add in the cost of stripping and disposing of the existing roof, your entire project might come out to $4.00 to $8.00 per square foot.

Another downside? It will only last 10-20 years. It’s susceptible to tears also. So if your roof gets a lot of foot traffic, you’ll have to replace it even more often. And because it’s rolled out in sheets, it will have those dreaded seams we’re always warning you about. This means you could get leaks.

Finally, a modified bitumen roof requires a lot of maintenance. You’ll have to check it frequently for leaks, broken welds, holes, and curling edges.

Spray Polyurethane Foam

We may be a little biased, but we think that SPF roofing is your best option for a flat roofing system.

Modified Bitumen alternative

Advantages of SPF Roofing

First of all, it’s easy to install. In fact, it’s so quick and easy, we can do it without interrupting your daily routine. We can apply SPF roofing over any type of existing roof, so your old roof doesn’t even have to be torn off. You’ll barely notice we’re there.

In addition, SPF is energy efficient. It’s a commercial roofing system and insulation barrier in one. That means lower energy costs.

Perhaps the best advantage of polyurethane over modified bitumen roofing is its waterproof properties. Spray foam roofing is completely seamless. There are no panels. Just one, solid sheet of foam that will not let a drop of water through. It will even fill any gaps or cracks in your substrate or existing roof.

A spray foam roofing system does cost a bit more than modified bitumen for the initial installation. It will set you back about $4.00 to $7.00 per square foot. But the SPF roof will last 50 years or longer. A modified bitumen roof will need to be stripped away and replaced three to five times over that 50 year period. And remember, stripping off the old modified bitumen roof will raise the price even more. In the end, the SPF roof gives you a lot more bang for your buck.

Spray polyurethane foam is low maintenance too. Remember, a modified bitumen roof has to be checked for wear and damage pretty frequently. Not so with spray foam. You should inspect your SPF roof twice a year to ensure it hasn’t been punctured by flying tree branches or animals. But aside from that, it will need a recoating after the first 5 years, and again every 15-20 years. Do that, and your spray foam roof will last 50 years.

And since we know you are concerned about the environment, we want to assure you that SPF roofing is 100 percent green. Not only will we apply your SPF roof with no emissions, the fully formed foam roof is inert and nontoxic. What’s more, you’ll never have to tear the roof off, so it will never get dumped in a landfill

Finally, foam roofing is durable and practically weightless. It can withstand weather, critters, and plenty of foot traffic.


It might sound like SPF roofing is too good to be true. You’re only half right. We happen to think that spray foam is the best option for any flat roofing system. But there are a few disadvantages.

First, you have to have a well-qualified contractor who specializes in spray polyurethane foam. (Hint: We can help you with this!) But please don’t hire any old roofer for this job. Installing a foam roof takes a high degree of technical expertise. If your contractor makes a mistake with the application, that can cause big problems for your building.
SPF roofing installation also requires sunlight and warmth. This means that we can only install between late spring and early fall. There should also be no ice, frost, or moisture on the existing roof during installation.

If you’re looking to install a new flat roofing system, you should be aware of your options. When deciding between spray polyurethane foam and modified bitumen, we think the right choice is clear.

Side-By-Side Comparison