Manufacturers typically consider moisture that remains on the roof surface OK as long as it evaporates within 48 hours under conditions conducive to drying. Be aware that, for this reason, asphalt roofing manufacturers require a minimum of 1/4" slope per foot to prevent any possibility of ponding.
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One must be aware of the critical difference in the oil solvent composition of asphalt, in that these solvents can leach out of the asphalt in ponding conditions, evaporate off, and leave the asphalt membrane dried and cracked just where the ponding is most prevalent. Also, coal tar built-up membrane systems should be detailed in accordance with manufacturer requirements especially at drains, scuppers, and roof edges.Īsphalt continues to be the more common built-up roofing material compared to coal tar pitch. Designers should specify coal tar for use in coal tar built-up roof membranes comply with ASTM Standard D450, Type I. Type I Coal Tar Pitch has good adhesive and self-healing properties and is used with aggregate surfacing on roof slopes up to 1/8":12. This is why coal tar pitch can be applied on a dead level roof surface ponding water has no negative effect on it. Coal tar pitch is not composed of solvents like asphalt, and so will not dissolve and evaporate the solvent oils out of the roofing compound like asphalt in a pond situation. In fact, in the earlier part of the century, coal tar roofs were often used to cool buildings by allowing the intentional ponding of water on the coal tar surface of the roof for evaporation and cooling effect. Prior to the mid-to-late 1970s, almost all low-slope roofs were asphalt or coal tar built-up roofs.
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It is intended to provide a "Best Practice" and shall not be construed in any manner to establish the legal standard of care required from licensed professionals.
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It is the intent to provide recommendations beyond the content of those standards, especially as they relate to integrating the roofing assembly into a total building enclosure and mechanical system design. This document relies on many other industry standards, which should also be consulted. The purpose of this section is to provide design guidance for designing high-performance low- and steep-slope roof assemblies. To select, detail, and specify the most appropriate roof system for a project past experience with several of the available material options and an understanding of roof assembly materials and system options, and an understanding of roof design considerations is recommended.