This paper describes a test method for measuring the relative porosity of asphalt coated glass fiber felts. For this paper, porosity is defined as flow through felt.

I tested the effect of the relative porosity of the felt using a computer model to calculate the flow of coal-tar pitch through the felts in an ideal membrane, exposed to normal Boston, Mass, and Miami, Fla. temperatures.

These data show the relative porosity of the glass fiber felts is highly variable. The mass of the built-up roofing aggregate presses the glass fiber felts to the bottom of the membrane.

I recommend, based on my field observations, our laboratory tests and the calculations reported in this paper, that only steep asphalt, ASTM D 312, Types 3 or 4, be used between the plies of asphalt coated glass fiber felts in the construction of built-up roofing membranes. Coal-tar pitch and dead level asphalt should not be used with glass fiber felts until dependable test methods and standards for the relative porosity of the glass fiber felts are established.