The asphalt used in built-up systems provides an excellent water barrier, but as with other materials, asphalt is highly subject to degradation from exposure, just as other materials are. Asphalt hardens progressively when exposed to roof temperatures. This occurs with all asphalt-based components, including cap sheets, saturated felts, and asphalt moppings.
Temperature changes cause the underlying asphalt to expand. Since the asphalt skin cannot stretch to accommodate the movement, random crack patterns appear to relieve the stress. Fresh asphalt is exposed below the broken skin.
Over time, the fresh asphalt within the cracks forms a new skin. The new skin layer will then eventually crack in the same line again, due to temperature changes, leaving a slightly deeper depression. This process repeats itself until the crack lines grow very deep and into the felts, allowing water infiltration.
A vital component of a successful built-up roof system is surfacing to protect the asphalt waterproofing component from the effects of exposure. The two most common methods of surfacing built-up roof membranes are a flood coat of aggregate embedded in asphalt or a mineral-surfaced cap sheet.
A cover of aggregate or minerals over the asphalt can minimize surface deterioration, but it has little effect on the hardening of the underlying asphalt. These types of surfaces simply do not keep the roof temperature from rising. In some cases, depending on the type of aggregate or color of minerals, they might absorb heat, further elevating temperatures.
So why are these two types of surfacing used most often? Because they provide a durable surface to withstand foot-traffic abuse, another important factor in roof longevity.
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