Modified Bitumen Roof Systems
A modified bitumen roof system is a hybrid, asphalt
built-up roof (BUR). It has the benefits of the built-in redundancy of the BUR,
along with the added strength, flexibility, and UV resistance of a modified
Modified bitumen membranes consist of an asphalt and polymer blend that
allows the asphalt to take on characteristics of the polymer. There are
basically two “types” of modified bitumen; APP modified bitumen or SBS Modified
Bitumen. There are several surfacing options for this system which include a
factory applied mineral surface, a gravel surface laid in bitumen -- or a liquid
applied coating that is typically reflective in nature.
- APP (Atactic Polypropylene) is a thermoplastic polymer that forms a uniform
matrix within the asphalt. This enhances the bitumen’s performance by increasing
its UV resistance and increasing its flexibility at low temperatures.
APP-modified bitumen sheets are generally applied using a propane-fueled torch.
Applicators use the heat to soften the modified bitumen on the underside of the
sheet. The sheet's bottom surface becomes a molten adhesive that flows upon the
substrate and then cools to form a waterproof adhesive bond.
- SBS (Styrene-Butadiene-Styrene) modifies the asphalt by forming a polymer
network within the bitumen. SBS gives the bitumen rubber-like characteristics
and improved resistance to aging and weathering. Most SBS-modified bitumen
sheets are either set in hot mopping asphalt, torch-applied, or adhered with
cold-process adhesives. SBS-modified bitumen sheets that do not have factory
applied granule or foil surfacing need some form of field-applied ultraviolet
- While modified asphalt provides the primary waterproofing characteristics of
these membranes, a reinforcing ply adds strength and puncture resistance. Glass
fiber and polyester plies are the most commonly used reinforcing fabrics. Each
has distinctive properties. Polyester has excellent elongation, tensile strength
and recovery. It provides good puncture resistance and stands up well to foot
traffic. Glass fiber resists flame penetration and provides excellent tensile
strength and dimensional stability.
Advantages of modified bitumen roofing are:
- Proven history
- Tough, durable roof
- High resistance to roof-top traffic
- High puncture and tear resistance – available in different thicknesses
- Excellent elongation allowing ability to accommodate building movement
- Versatile application options
- Easily maintained