Abstract: A preformed, unitary composite for reinforcing while suppressing curl in bituminous roofing membranes includes as a first layer, an open, non-woven grid of low shrinkage, continuous filament polyester yarns that are at low tension and adhesively secured together, and as a second layer, a lightweight, preformed fiberglass mat, the second layer being adhesively secured to one side of the first layer. The adhesive secures the two layers together such that at least some of the individual yarns of the first layer are at least partially coated and impregnated by the adhesive without forming a film that closes all openings through the composite. The adhesive that secures the continuous filament yarns of the first layer together and that secures the first and second layers together is a vulcanizable rubber binder, such as cross-linked styrene butadiene rubber, which includes about 50 to about 80% styrene.