Modified Bitumen Roofing: What You Need to Know

Modified Bitumen Roofing: What You Need to Know

Not all roofing materials are created equal. Choosing the right one can depend on the project, the property, and even the characteristics you’d like the roof to have. Even within broad product descriptions — like membrane roofing — there are still specific types of materials to choose from. One of these is modified bitumen.

It’s always important to make informed decisions when choosing your roofing material. So, let’s take a moment to explore modified bitumen roofing a little further. What is it and what do you need to know about it?

What is Modified Bitumen Roofing?

Effectively, it’s a form of asphalt membrane roofing that’s been enhanced by modifying materials. Each different type of material has its own characteristics.

So, why might you want to invest in these types of modified membrane materials? Well, depending on the modification there are various different characteristics your roof could benefit from. 

These might include:

  • Enhanced flexibility: In some instances, traditional asphalt roofing can become brittle over time. Some forms of modified bitumen roofing, however, tend to remain pliable and adaptable. This flexibility allows the roofing system to expand and contract with temperature fluctuations. As a result there is often less risk of cracks and leaks that are more common with traditional materials.
  • Greater overall durability: Modified bitumen tends to have excellent resistance to impact, punctures, and tears. This can make it an excellent choice for various weather conditions and particularly in locations that see a lot of debris due to high winds and storms. This durability not only protects the property, but also contributes to an extended lifespan for the roofing system.
  • Superior waterproofing: The materials used to modify the asphalt help to create a material that — alongside effective adhesives — forms a watertight seal. This means modified bitumen roofing can effectively safeguard the property from heavy rainfall and leaks. Though this is essential for all properties, it makes modified bitumen roofing particularly useful for flat or low-sloped roofs that may usually see water pooling on the surface following rain or snow.

What Are the Types of Modified Bitumen Roofing Systems?

Modified bitumen roofing systems come in a range of formulations. This basically means that there can be different materials blended with the asphalt. When selecting a system, this will usually be referred to as a membrane, which is just a way to describe the roll of material. Getting to know the different types is helpful when making an informed decision about the roofing membrane that can give you the performance you want to achieve.

The main systems tend to be the following.

Atactic Polypropylene (APP)

This refers to asphalt that has been combined with atactic polypropylene (APP) polymers to create the membrane. This means that the asphalt is enhanced by plastics. It’s characteristics can include:

  • Exceptional heat resistance and flexibility. This makes it great for areas that see higher temperatures throughout the year.
  • High tensile strength. This makes APP particularly durable; resistant to weight as well as debris collisions.

Styrene-Butadiene-Styrene (SBS)

This refers to asphalt that has been combined with Styrene-Butadiene-Styrene (SBS) polymers to create the membrane. The result is that the asphalt is enhanced by rubber. It’s characteristics can include:

  • Superior flexibility and elasticity. The SBS modifiers enhance the material’s ability to stretch and recover. As a result, it can be a good choice for areas with extreme temperature fluctuations and frequent freeze-thaw cycles.
  • High tensile strength. SBS modified bitumen roofing is usually applied to a fiberglass carrier material. This tends to make it effective for withstanding debris from the elements and high winds while maintaining its structural integrity.

The Installation Process

Any modified bitumen roofing installation requires careful planning, precision, and adherence to industry best practices in order to be successful. Often, there’s specialist equipment involved as well as specific installation techniques. This is why it’s often recommended to collaborate with an experienced roofing professional to make certain that — firstly — it’s installed correctly and also that the roof is under warranty should anything go wrong. Nevertheless, it’s worth looking at the installation process.

The steps include:

  • Surface preparation: The process usually begins by thoroughly inspecting the roof substrate for any damage, irregularities, or moisture issues. These will need to be addressed or repaired before proceeding. From here, the surface needs to be cleaned to remove debris, dust, and contaminants that could affect the adhesion of the modified bitumen roofing material.
  • Under layer application: In most cases, there will be various layers installed before the first sheet of modified bitumen roofing goes down. This will usually depend on the project, but often includes insulation, a vapor barrier, and cover boards.
  • Base sheet installation: The base sheet is basically the foundation for the modified bitumen roofing system. This is usually mechanically fastened or adhered using hot asphalt or cold adhesive, depending on the specific project requirements.
  • Modified bitumen membrane application: The membrane is rolled out over the previous layers. It is then heated to the recommended temperature (depending on the type of membrane system), which ensures a secure adhesion to the base sheet. In some instances — usually with SBS — a cold adhesive will be used to adhere the modified bitumen to the surface.
  • Flashing installation: If the roof has features, such as piping or vents, it is likely to need flashing. This helps to ensure that the seal around the features remains  watertight, preventing leaks and seepage.

How to Effectively Maintain a Bitumen Roof

A well-executed installation is the first step to a secure modified bitumen roof. However, this doesn’t mean it will be perpetually effective if you just leave it to get on with its job of protecting your property. You’ll get the most longevity and efficacy out of it if you commit to regular and proactive maintenance to preserve the integrity of the roof.

Some of the steps to maintain a modified bitumen roof are:

  • Regular inspections: You’ll need to perform routine inspections at least twice a year, preferably in the spring and fall. However, it’s also vital to inspect your roof following severe weather events. This helps you to identify any potential issues before they escalate. Pay attention to signs of wear, damage, or any changes in the roof’s condition, including blistering, cracking, or loose seams.
  • Addressing repairs promptly: Don’t put off any repairs, whether these are discovered during routine inspections or following severe weather. The last thing you want is for the damage to worsen over time. If you find punctures, tears, or areas with compromised adhesion, you may be able to use the patching materials and techniques recommended by the manufacturer, rather than replacing that part of the roofing material entirely.
  • Clearing debris: Modified bitumen roofing may well be more durable against debris, but this doesn’t mean it’s invulnerable. Be sure to keep the roof surface clear of debris, leaves, and other materials. This prevents the potential for moisture to accumulate and become trapped, which may lead to issues like rot or mold growth. You should also ensure proper drainage by clearing gutters and downspouts to prevent water accumulation.

Some of these elements are relatively simple and you may well be able to perform them personally. Indeed, the durability of modified bitumen roofing often means walking on the material during inspection is less likely to result in breakage than with tiling or shingles. Nevertheless, if you’re not confident in your abilities or are uncertain of what to look for, it’s vital to work with a roofing professional. They can perform thorough inspections a couple of times a year and take care of any issues.


Modified bitumen roofing can be a good choice of membrane system if you’re prioritizing characteristics such as flexibility and durability. It also tends to be relatively tailorable to your specific needs. This can make it the preferred option to traditional materials and even other membrane roofs. While its roll format makes it relatively accessible and convenient, it still requires a certain amount of skill to install correctly. Be sure to collaborate with a professional if you have any concerns or uncertainties regarding your roofing project.

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