What is Modified Bitumen Roofing?
Modified Bitumen is made from asphalt and a variety of rubber modifiers and solvents. It is the next evolution of asphalt roofing. In an application process the seams are heated to melt the asphalt together and create a seal. There is also hot-mopped application, similar to how conventional roofs are installed.
How Modified Bitumen Works
Modified bitumen can be installed overtop of an existing tar roof unlike rubber flat roofing, which can be eaten away. It is also very rugged and can sustain a fair amount of foot traffic.
Modified bitumen roofs involve some traditional materials, but use modern fabrication methods, and traditional or more contemporary installation techniques. Modified bitumen roofs are made from prefabricated rolls of modified asphalt or coal tar reinforced with a fiberglass or polyester reinforced mat. Rubber-modified asphalts, such as styrene-butadiene-styrene materials, are granular surfaced and are normally installed in two or more plies using mopping asphalt, cold adhesives, or torch welding. Plastic-modified asphalts such as atactic polypropylene systems are smooth or granular surfaced and can be heat welded or laid in cold adhesive.
Modified bitumen membranes combine the features of a built-up roof with the added strength from its polymer modification. Using a reinforced sheet that is prefabricated in the plant, modified bitumen systems require a less labor-intensive application than other types of roofing and can be applied in both commercial and residential roofs.
A modified bitumen roof is composed primarily of polymer-modified bitumen reinforced with one or more plies of fabric such as polyester, fiberglass or a combination of both. It can also include mineral granules, aluminum or copper. The bitumen determines the membrane’s characteristics and provides primary waterproofing protection, while the reinforcement adds strength, puncture resistance and overall system integrity.
A roofer will ensure modified bitumen membranes undergo strict quality control standards to ensure uniform thickness and consistent physical properties throughout the membrane. The finished roofing is usually two to four layers of modified bitumen membrane and a base sheet, with additional plies for added strength if needed. Usually if more layers are applied, the roof will last longer.
How to Identify Modified Bitumen
The best way to identify a Modified Bitumen roof is to look at the material edges. Modified bitumen roofing is thicker, and its edges are sealed by heating with a torch. You should see a little runout of melted bitumen at the material seams. If there is no runout the roof may have been adhered using some other method, but if it was “torched” it was not heated sufficiently and may be less durable.
Secondly, , it also is better at resisting tearing and breaking. If you find that it is easy to tear into the roof material edge it’s probably roll roofing not mod-bit.
Modified Bitumen Roofing Properties & Installation Methods
Most modified-bitumen roofs are torch-applied, although there are also self-adhesive and cold-process systems. The waterproofing membrane, sometimes called “single-ply modified,” consists of asphalt bitumen reinforced with a polyester or fiberglass fabric and modified with polymers to give it greater strength, flexibility, resistance to UV degradation, and resistance to heat and cold.
A variety of different chemical formulations have been tried over the years. It is best to stick to a product with an established track record. In general, modified-bitumen roofs can be applied to slopes as shallow as 1/4 inch per foot.
There are two main forms of modified bitumen roofing installation: the torch-down installation method, and a peel-and-stick installation.
Installation Methods for Modified Bitumen Roofs
A torch-applied, or torchdown, roof starts with a non-flammable base sheet made of asphalt-saturated felt or fiberglass that is mechanically attached to the roofing deck. In residential construction, the base sheet is usually attached with roofing nails driven through metal caps.
The second layer is the waterproofing membrane. This is heated with a torch as it unrolls, fusing it to the base sheet, to itself at seams, and to penetrations such as skylights. Installers must learn to heat the membrane so it is hot enough to fuse but not so hot as to burn through.
Membranes may be either smooth or have a granular surface like roll roofing. Smooth-faced membranes need a third coating, which has colored or reflective pigments to protect against UV radiation. The smooth type is preferable where foot traffic is expected or where decking is going over the roofing.
Torchdown roofing is self-flashing and uses no adhesives or solvents to seal around openings. The material can be run up parapets and abutting wall, and patches are used to seal around metal skylight curbs and similar openings. A special patching compound is used to seal to PVC stacks. If applied correctly, the torchdown membrane is essentially seamless.
Pros and Cons of Modified Bitumen Roofs
Bitumen roofs are very durable and are one of the longer lasting types of flat roofing materials, easily lasting 20 years or more. They are also easily repaired without solvents or adhesives, making it less expensive to maintain. It is compatible with asphalt shingles and asphalt compounds, although patching with roofing cement is not recommended. The reinforced fabric layer isolates the membrane above from building movement and gives the material enough strength to support occasional foot traffic.
The main drawback of modified bitumen roofing is the risk of fire during installation. While the risk of fire is low in the hands of trained installers, care must be taken when using torchdown on a wood-frame structure. A number of fires have started with sawdust that has accumulated in empty cavities, such as crickets and parapets.
Inspection of the roof for sawdust pockets while it is being framed is advised.
Typical Slopes for Modified Bitumen Roof Systems
Modified bitumen roofing is normally installed on low-slope roofing, up to 3 inches in slope, or depending on the application method, up to six inches of slope per foot may be permitted. Because there are quite a few approved installation methods, manufacturers typically use a alphameric roof application method name that encodes the basics of how the roof covering should be installed.