Modified Bitumen Roof – Best Choice for Flat Roofs

Picking out a new roof for your low-slope or flat-roofed home can seem challenging. But, unfortunately, typical roofings like metal and asphalt shingles are out of the picture, leaving you to wonder, what's left? 

Some roofing contractors might recommend BUR (built-up roofing), an old-fashioned method of stacking layers (plies) of material - referred to as tar and gravel. But a safer, easier roof protection is modified bitumen (MB), a single-ply, asphalt-based waterproof membrane.

What is a Modified Bitumen System? 

Modified bitumen roofing started in Europe in the 1960s as an upgraded relative of the BUR roofing. By 1975, this weather-resistant, waterproof roofing membrane started appearing on buildings throughout the US and Canada. 

While initially used for commercial buildings, modified bitumen is also an excellent choice for residential homes with little to no pitch. 

SBS Modified Bitumen

A modified bitumen system consists of a single-ply (layer) membrane (cap sheet) that goes down over the top of roofing underlayment (base sheet) - which does not count as a layer, ergo the single-ply label. 

Most base sheets self-ad adhere to the roof or mechanically attach with disc plating or cap nails, while there are three methods of fixing the cap sheet. 

After the base sheet is down, the mod-bit roofing roll is stretched out and fixed into place. The ease of using cold-adhered or self-adhering applications is one of the most significant selling points over roofings like BUR, which require torching - heat-adhered. 

Self-adhesive (SA) modified bitumen consists of a roll-out sheet of asphalt, tackifiers, and polymers reinforced with fiberglass or polyester (or a combination of both). Some types of mineral granules coat the top for durability and appearance. A plastic layer protects the built-in adhesive, which peels off before installation. 

Modified bitumen can also be torched down, but most people prefer to avoid this method, as it releases harmful VOC (volatile organic chemicals), both during installation and for some time afterward.

What is Modified Bitumen's Framework?

The Modified Bitumen membrane has five layers of construction to give it the same durability and resistance as a BUR multi-ply roof. This system consists of asphalt bitumen, modified polymers, and polyester or fiberglass reinforcements permeated together to create a waterproof barrier that can resist harsh weather, fire, debris, and foot traffic. 

Modified Bitumen Layers

The layers of a single-ply Modified Bitumen roofing system start with the structural deck covered with a vapor barrier layer that adds waterproofing. 

A layer of insulation goes next to provide R-value (thermal regulation), topped with a layer of adhesive. The top layer is the modified bitumen membrane with a pressure-sensitive SecurTape strip for additional hold. 

Additional layers stacked on top of each other made of asphalt and Modified Bitumen membranes can make the roof thicker and more durable. 

Types of Modified Bitumen Sheets

Modified bitumen sheets come in 2 types, giving the material special features that allow for better performance in specific situations. These sheets have an equal thickness and property function throughout the entire piece providing equal protection and coverage throughout the roof.

Atactic Polypropylene

Atactic Polypropylene(APP)  is a type of thermoplastic polymer mixed with asphalt for protection against cold temperatures and UV rays. This type of modified bitumen does require heat welding for proper installation. 


Styrene-Butadiene-Styrene(SBS) is a type of rubber mixed with asphalt to provide flexibility that helps the material resist impacts, winds, and storm debris. If you're using an SBS-modified bitumen, your base layer must have a nail application rather than self-adhesive. 

Pros of Modified Bitumen

A few different benefits make people want to choose a modified bitumen roof over other suitable low-slope or flat roof materials.

Perhaps the most significant benefit is the low maintenance. Once you install an Modified Bitumen roof, there's little upkeep besides annual inspections to check for damages. Repairs for this type of roof are also easy and inexpensive when addressed early enough to prevent significant problems.

Another benefit is that modified bitumen can withstand extreme weather temperatures, whether high or low, without cracking or breaking. 

And bitumen is a waterproof material, making it perfect for roofs in climates with a lot of rain, given that your roofing contractor installs proper drainage. Flat and low pitched roofs need special consideration to ensure water doesn't pool up on the surface, causing additional weight strain.

Cons of Modified Bitumen

A common downside of modified bitumen with granulated surfaces is that although the granules give UV protection, these granules can make it harder to find leaks and cracks early enough to make repairs before the problem gets too large. 

Modified Bitumen VS. TPO Roofing

Another type of roofing material for flat and low-pitched roofs is TPO - Thermoplastic Polyolefin. TPO is a newer product, only hitting the market in the 1990s compared to the 1970s with Modified Bitumen. 

In a side-by-side comparison of modified bitumen vs. TPO, both products work great in cold or hot temperatures, are resistant to UV rays, and significant rainfall. Both roofing types also classify as single-ply. 

Application of TPO can be made using mechanical fasteners, ballasted, or fully adhered. A great feature of using TPO is that you can add it directly over existing roofs (metal or bitumen), saving you costs from removing old materials before installation. 

The thicker material gives TPO roofs more durability against tears, cracks, and damage from falling debris. And they have a smooth surface rather than the rough texture that comes with the granulated modified bitumen roofing.

Common Issues

No material is without problems, and modified bitumen roofs are no exception. However, the majority of issues occur due to failed or improper installation rather than materials. 

Seam Defects

A common issue that can occur with MB roofing is defects or damage to the seams. This is because each section of the 30"-35" wide MB strip adheres to the pieces beside it, with a several-inch overlap. 

Issues that can cause defects with the seams include improper application, thermal contraction and expansion, and stresses on the seams. Reheating the seam or adding adhesives can fix the problem before it becomes severely damaging.


Modified bitumen can get damaged by impacts like falling debris, foot traffic, and other hard impacts. Unfortunately, there's little you can do to prevent puncture holes. Picking an MB with SBS or APP additives reduces potential damages. Patches can repair small holes and tears.


When moisture gets beneath the membrane, your roof can end up with blisters. While you can make repairs to the damaged pieces, it's crucial to seek out and repair whatever is causing the moisture presence. Then, once you've fixed the leak, you can repair or replace the damaged areas.


Q: How long does a modified bitumen roof last?

A: A modified bitumen roof has a life expectancy of twenty years. However, with proper maintenance, timely repairs, and minor damage, your roof may last even longer. 

Q: How much does a modified bitumen roof cost? 

A: Modified bitumen can cost $1.50 to $3 a square foot or $15 to $30 a roofing square (100 square feet). So for a 1,500 square foot home, you'd see costs of $2,250 to $4,500.

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