Modified Bitumen Roofing

The Middle Ground in Flat Roofing

Modified bitumen is a flat roofing membrane that combines traditional asphalt technology with modern polymer modifications to create a product that is more flexible, durable, and weather-resistant than standard built-up roofing. Available in rolls that are torch-applied, cold-adhered, or self-adhered, modified bitumen offers easier installation than traditional BUR while providing multi-layer protection when installed in a two-ply system. It is widely used on low-slope residential and commercial roofs.

Modified bitumen claims fall between shingle claims and single-ply membrane claims in terms of damage assessment and estimating complexity.

Types and Installation Methods

Modified bitumen comes in two main formulations. APP (atactic polypropylene) modified bitumen is typically torch-applied, meaning the installer uses an open flame to heat the underside of the membrane and bond it to the substrate. SBS (styrene-butadiene-styrene) modified bitumen is more flexible, particularly in cold climates, and can be installed with cold adhesive, hot mopping, or torch application. Some products are self-adhering with a peel-and-stick backing.

The installation method affects Xactimate line item selection. Torch-applied, cold-adhered, and self-adhered modified bitumen have different unit prices reflecting the different labor and equipment requirements.

Estimating Modified Bitumen

Xactimate includes specific line items for modified bitumen under the RFG category. The estimate should specify the modification type (APP or SBS), the installation method, and whether the system is single-ply or two-ply. Cap sheet (the top layer with granulated surface) and base sheet (the bottom layer) are separate line items. Modified bitumen flashing at walls, curbs, and penetrations is also a separate line item. Accurate specification prevents underpayment on a material that costs more than standard BUR.