glass felt
glass fibers bonded into a sheet with resin and suitable for impregnation with asphalt in the manufacture of bituminous waterproofing, roofing membranes and shingles.
glass felt: fiberglass mat for bitumen-based waterproofing
Glass felt is a nonwoven fabric made from continuous or chopped glass fibers bonded together with a thermosetting resin, typically phenol-formaldehyde or polyester. The result is a sheet material that typically weighs 10 to 40 pounds per 500 square feet, with a thickness around 0.015 to 0.025 inches. It serves as the structural reinforcement layer in built-up roofing membranes and modified bitumen sheets, where it prevents bitumen from flowing under heat and provides tensile strength to resist tearing and puncture.
Glass felt replaced organic asphalt-saturated felt in the 1960s and 1970s because fiberglass does not rot, absorb moisture, or degrade when exposed to water. It maintains its integrity when saturated with hot bitumen during manufacture, and it bonds reliably to both asphalt and modified bitumen systems. The material comes as wide rolls, typically 36 inches wide, which the manufacturer impregnates or saturates with asphalt at the mill before shipping.
Common variants and applications
Glass felt appears in three main configurations: interply sheet used between layers of built-up roofs; base sheet nailed or torched directly to the roof deck; and as the core reinforcement in single-ply membranes like SBS and APP modified bitumens. Lighter grades (around 15 pounds) are used where flexibility is important; heavier grades (30 to 40 pounds) provide more puncture resistance. Selvedge edges, reinforced marginal strips about 2 inches wide, overlap at seams to create mechanical strength at lap joints.
The performance of glass felt depends on proper saturation. Under-saturated felt leaves dry glass fibers exposed, which can cause delamination and accelerated weathering. Over-saturation wastes asphalt and can cause the material to become brittle when it cools. Quality control during manufacture involves checking weight, tensile strength (typically 40 to 80 pounds per inch of width), and tear resistance.
Problems arise when glass felt is installed in conditions outside its design range. Excessive heat causes bitumen to soften and flow; excessive cold makes the bitumen too brittle. Punctures and tears during installation allow water infiltration that propagates along the felt-asphalt interface. Exposure to standing water or prolonged moisture, though the felt itself resists degradation, can cause the bonded resin to blister if moisture becomes trapped beneath waterproofing layers.