Modify something means to change it. Chemicals called polymers are added to the bitumen. They change the chemistry of the bitumen.

Modified bitumen sheets are thicker than individual felt plies. Some modified bitumen sheets are made with a surfacing. There are different kinds of surfacing. They are foil, mineral granules or coatings.

These sheets are often used as flashing material for BUR systems.

The two main types of modified bitumen membranes are APP and SBS. APP (atactic polypro-plene) and SBS (styrene buta-diene styrene) refer to the type of polymers that are added to the membrane. APP membranes are usually heat welded. SBS membranes are applied several different ways. Some of the ways are with hot asphalt, cold adhesive or heat welding. There is also a self-adhering type. This type is usually only used on steep-slope roof systems.

APP Membranes

As mentioned, APP membranes are usually heat welded. They are installed using heat from a propane torch or electric heat-welding machine. The goal is to melt the bitumen on the back of a sheet and lay the sheet down. It adheres to a substrate when it cools.

You need special equipment for applying heat-welded systems. One type of equipment is a torch-and-tank assembly. A torch-and-tank assembly is made up of a propane tank, regulator, pressure gauge and high-pressure hose. The other type is an electric heat-welding machine. These machines do not have a flame. They use hot air.

Flashings for modified bitumen systems are usually made up of a field membrane sheet plus another sheet for the base flashing. For APP-modified systems, flashing pieces are torched and then laid into place.

SBS Membranes

SBS-modified bitumen systems are usually mopped in hot asphalt, like BUR plies. First, hot asphalt is mopped in front of the sheet. Then, the sheet is rolled into place.

Sometimes, a base sheet is tem may include three layers. A base sheet will be put down first. A modified bitumen sheet will be mopped to the base sheet. Finally, another sheet will be mopped to the membrane. The final sheet may have a surfacing, or it may be smooth. If it is smooth, a surfacing has to be put on.

The laps are heat-welded or sealed with hot asphalt or cold adhesive. Flashings are usually installed with hot asphalt or cold adhesive. In some cases, they are heat-welded.