Built-up Roofing

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Built-up roofs consist of a number of plies of asphaltic roofing felts, typically "mopped" over insulation and covered with a flood coating of asphalt and a gravel surface.

Asphalt is in almost all cases a byproduct of the oil industry; high-quality roofing asphalt is manufactured from oil as the intended purpose. Asphalt deteriorates through photo-oxidation and this process is accelerated by heat. As it degrades, the melting point of the asphalt rises and elasticity decreases. The asphalt shrinks and forms an alligator-skin surface. Water can also break down asphalt over time. It also dissolves quickly when exposed to oils or solvents.

 

There are four types of roofing asphalt, modified by application of heat and oxygen. The melting point of the asphalt rises the longer it is processed. Type I, with a relatively low melting point, is only appropriate for "dead level surfaces" due to its capacity for running. Conversely, Type IV has a high melting point and is recommended for higher slopes. Since the application of heat does degrade the asphalt, the lower-slope varieties may perform better over extended periods of time than their higher-slope counterparts.

 

Asphalt roofs may be maintained and refurbished by repairing visible deficiencies and recoating the roof with a compatible product.

 

One of the less expensive roofing options, asphalt roofs are robust and durable; ideal for roofs that will have high amounts of traffic. On the downside, hot asphalt produces a strong smell during installation, which may disturb building occupants (if the project is an occupied re-roof) or neighbors. Built-up roofs are typically not eligible for LEED points and do not have high solar reflectivity.