Components of Built Up Roofs

Many people use the analogy of a layer cake to describe BUR to customers. The base sheet and felts are the cake, and the hot asphalt is the icing that gets spread between the layers and over the top. The aggregate is like the sprinkles that get scattered on top on the layers. The asphalt fuses the felts together, forming a waterproof membrane. The felts reinforce the asphalt and prevent it from cracking. The aggregate protects the surface of the membrane from fire and mechanical damage — and, even more important, it prevents the asphalt from being degraded by the sun’s ultraviolet rays.

Base Sheet

After the cant strips are installed, the sheathing is covered with a layer of rosin paper, which functions as a slip sheet for the base sheet to follow. The base sheet is an asphalt-coated “fabric” mat.. The mats are made with synthetic materials like fiberglass. The base sheet runs to the top of the cant strip and is cut or notched to fit around plumbing vents and other penetrations.


There are different types of standard asphalt. Some are designed to produce fewer fumes when melted, but the main difference between types has to do with softening points. Hhigh outdoor temperatures can soften asphalt, and if the pitch is steep enough it may begin to slide down the roof. To prevent that from happening, we choose a type of asphalt based on our local weather conditions and the slope of the roof.

Mopping in the Felts

The roof is laid up from low to high, a section at a time. We apply the asphalt with a large fabric mop. The first coat goes on the base sheet and is immediately followed by a layer of felt, which must go on while the asphalt is hot. We then lay successive layers of felt onto freshly applied asphalt. The roofing stiffens and solidifies as the asphalt cools. A roof with three layers should last about 15 years. Each additional layer adds another five years.

Maintenance and Repair

Built-up roofs last longer if they are maintained. We recommend inspecting the roof once every five years. Periodic maintenance involves removing piled-up leaf debris, which can break down to form acids, and using roof cement to reseal around vent pipes and scuppers.