Built-Up Roofing (BUR) consists of multiple layers of bitumen alternated with ply sheets (felts) applied over the roof deck, vapor retarder, and (most often) insulation. BUR roofing systems are particularly advantageous for flat or low-sloped applications.
The heart of the system is the membrane which includes the layers of bitumen, which is applied hot, and the reinforcing plies of roofing felt.
The bitumen is the primary adhesion/waterproofing part of the system. It arrives at the jobsite in solid blocks and is heated to liquid form very close to the site of application and mopped onto the surface.
The multiple reinforcing plies laid down over the bitumen layers and which, in turn, become the base upon which another layer of hot bitumen is mopped, are asphalt-coated roofing sheets or felts. They serve to stabilize the entire BUR membrane system. These multiple reinforcing layers help make the system pliable, water resistant, and fire retardant.
Built-Up Roofing Surface BUR roofing membranes can be protected from solar radiation by embedding gravel in the bitumen, applying a surface coating or applying a granular-surfaced "cap" sheet. Light-colored surfacing materials can be used to reflect heat from the building. In addition, surfacing agents can provide additional fire protection.
Advantages of BUR
Built-up roofing systems have had a long-standing popularity, due in large part to the success and proven reliability of BUR. The stock of 20, 30 and 40-year-old BUR roofs still in excellent condition attests to this fact. Specifically, BUR roofs offer:
It's critical to get secure bonding of the roofing felts (plies) using bitumen. To achieve this bond the roofing contractor applies thin, uniform moppings of bitumen. This waterproofs the system and ensures proper adhesion for fusing the membrane system together. The temperature of the bitumen is critical. By heating it to the proper temperature the roofing contractor gets the right viscosity for proper mopping. The contractor heats the bitumen to its EVT or Equiviscous Temperature, the temperature at which it can be most effectively mopped into uniform layers. Each batch of bitumen should be labeled by the supplier with its EVT.
Once felts are rolled into place on the heated bitumen applicators pull brooms or squeegees over the felt or use some other method to make sure that its embedded in the bitumen.
The strength of the membrane depends on the type of felt used, the number of plies, overall ply construction, and the lapping of the overlaying felts. Typically, membrane ply construction is defined by headlap, endlap, and sidelap. Headlap is the distance of the overlap that exists between the lowermost and the uppermost plies of the shingled portion of the roof membrane when measured perpendicular to the long dimension of the membrane. Endlap is the overlap distance that is measured from where one roll of felt ends to where another begins. Sidelap is the overlap distance along the length of the felt where one roll of felt overlaps the adjacent overlying felt.
The application of Built-Up Roofing systems is detailed work, but the professional who pays particular attention to those details such as curbs, walls, flashings or other projections that interrupt the membrane, achieves a quality, efficient, long-lasting product for the building owner.
ARMA offers many sources of information to enable the roofing professional to do just that.