Flat and low-slope roofing — TPO, EPDM, modified bitumen, and built-up (BUR) for low-slope sections

Low-slope (often called "flat") roofs are a different system class from the pitched, shingled roof on most of a Houston home — they appear on porches, additions, dormers, and modern flat-roof houses, on any plane too shallow for shingles (roughly below 2:12). Shingles are not appropriate there; low-slope roofs use a continuous waterproof membrane instead. Four systems are common. TPO (thermoplastic polyolefin) is a single-ply, usually white reflective membrane (commonly 45, 60, or 80 mil) whose seams are HEAT-WELDED into one continuous sheet — a sound weld is stronger than the membrane itself, and the reflective surface is a genuine cooling advantage in Houston heat; plan on roughly 20 years, noting TPO has a shorter field track record than the older systems. EPDM ("rubber roof") is a single-ply synthetic-rubber membrane, usually black, whose seams are GLUED or taped rather than welded — historically its weak point; it has the longest real-world track record of the single-plies (plan ~25-30 years), but its dominant failure mode is shrinkage — the membrane tightens over time and pulls at edges and flashings, which Houston heat accelerates, and the black surface absorbs heat rather than reflecting it. Modified bitumen ("mod-bit") is a polymer-reinforced asphalt membrane installed in two or three rolled layers — by torch, hot mop, cold adhesive, or self-adhered peel-and-stick (the no-flame self-adhered version avoids open flame on an occupied home); plan ~20 years, with the redundancy advantage that damage to the top layer still leaves plies protecting the deck. Built-up roofing (BUR, "tar and gravel") is the oldest system — alternating layers of felt and bitumen topped with gravel for UV protection; it is largely a legacy system now (plan ~15-20 years) and is usually replaced with single-ply or mod-bit when it fails. Across all four, the enemy of a low-slope roof is standing ("ponding") water and the details at seams, flashings, and penetrations — the field membrane rarely fails first. Positive drainage — water gone within about 48 hours, the NRCA benchmark — is the single most important performance factor, and on a dead-flat deck a contractor builds slope with tapered insulation or crickets. What to ask a low-slope contractor: what membrane and what thickness in mils, and whether that thickness qualifies for the warranty term quoted; how seams are joined (heat-welded versus adhesive) and who inspects them; how water will be made to drain within about 48 hours so it does not pond; how every penetration and the wall and curb junctions will be flashed; whether the job is a full tear-off or a recover over the existing roof (recovering can trap moisture, and many manufacturers will not warranty over an old roof); and what the manufacturer (material) warranty covers versus the contractor's own workmanship warranty, and for how long each. Specific cost and the right system for any roof depend on the deck, the slope, and local code; confirm any drainage requirement and the warranty's ponding-water language in writing. [Source: NRCA low-slope membrane roofing guidance; Asphalt Roofing Manufacturers Association (ARMA) modified bitumen and ponding-water technical bulletins; IIBEC drainage and ponding guidance; GAF single-ply membrane comparison; InterNACHI EPDM inspection guidance]