Modified Bitumen

Modified Bitumen built up roofs are hybrid conventional built-up roofs in that they both consist of alternating layers of roofing felt and hot asphalt to “build up” the waterproofing layer.

A couple of differences are that the traditional 4-ply asphalt roof is “built-up” in the field and subject to all the variables such as weather, asphalt temperatures, applicator skill and so on. Modified bitumen is produced in a factory controlled environment with the same finished thickness and redundancy as the conventional built-up. Another key difference is found in the material’s physical makeup– a polymer modified asphalt instead of a “straight-run” oxidized asphalt. This gives the sheet tremendous strength and durability while also maintaining a unique flexibility. There are two forms of modified bitumen; APP and SBS.

APP & SBS Modified Bitumen

APP (atactic polypropylene) and SBS (styrene-butadiene-styrene) are the two polymer additives that create the “modified bitumen”. Each has its own unique features, but both serve the same purpose; to dramatically enhance the weathering capabilities of the waterproofing asphalt.

The modified asphalts are combined with polyester or glass fiber to reinforce the membrane, providing high tensile strength and flexibility, maintained even in the coldest winter months. A great advantage to the use of APP Modified Bitumen is capability of direct heat-welding application. Sometimes referred to as “torch-down” or “roll roofing”, the rolls of material (36” x 33’) are applied using propane torches. This use is common in the installation of a two-layer modified bitumen system. The system involves a base layer of smooth, APP modified with a granular surfaced cap sheet heat welded to the base layer, providing a membrane mass of 315 mils. You can reasonably expect to get 25-30 years from this type of multi-layered modified bitumen replacement system.

Seams


The seams of a modified system are very crucial items in regard to quality of workmanship. Given that each sheet is only 3’ wide, this can provide for almost 3.5 times the number of seams found in certain single-ply systems. We perform all of our seams in a separate operation using torches and heavy steel rollers. Our skilled modified bitumen specialists carefully inspect each seam for consistent bleed out and proper adhesion.

Flashings / Projections


The flashings, or penetrations, on the roof will 9 times out of 10 be the first items to come under the scope for a roof leak. Roof flashings can make or break any new roofing system and are completely dependent on the workmanship and dedication of the contractor in the details.Modified Bitumen Roofing Systems - Wall Flashing There is no guarantee from any manufacturer that will warranty their material or the workmanship of the contractor should the roof leak due to improper installation. We have been installing these roofing systems for the past 19 years and have been told by leading manufacturers that our workmanship is among the best in the country. Of the hundreds of modified bitumen systems that we have installed over the years, we are yet to have a single guarantee claim. In fact, we haven’t had a single punch list item from the manufacturer’s inspection on a modified system in over a decade.

Summary


There is a reason that this roofing system has been the clear choice for Chicago roofing for the past 40 years. It is tougher and stronger, pound for pound, than any other roofing system on the market. While it does, however, come at an initial cost premium when compared to other thermoplastic single-ply systems, over the complete life cycle of the roof, modified bitumen will likely have the lowest life-cycle costs of any roofing system.

Visit Our Social Media Outlets