A drainage mat for use in building structures, particularly in exterior walls and roofing, to improve drainage and ventilation within such structures. The drainage mat includes a web of extruded polymer monofilaments that are heat welded at junctions to form a matrix of tangled monofilaments. The web includes on its front face an alternating array of ridges and planar regions, each ridge having a substantially trapezoidal shape with a top surface and a base, the width of the top surface being less than the width of the base.