Glossary
Mil (membrane thickness)
One thousandth of an inch — the unit used for single-ply membrane thickness. Commercial single-ply membranes are commonly produced in 45-, 60-, 72-, and 80-mil thicknesses.
Plain-English definition
One thousandth of an inch (0.001 in.) — the unit used to state single-ply membrane thickness. A 60-mil membrane is 0.060 in. thick, about the thickness of two credit cards and a half. Commercial single-ply membranes are commonly produced in 45-, 60-, 72-, and 80-mil versions.
Why it matters to you
Verified: Thickness is one of the few specification lines an owner can check directly on a bid, and manufacturers produce the same product families in several thicknesses at different price points (Professional Roofing — membrane thicknesses). But total mils are only half the story: for reinforced membranes, what wears away over the years is the polymer above the scrim, so the “thickness over scrim” figure on the product data sheet is a better durability indicator than the headline number. ASTM product standards set minimums for this — for example, ASTM D4434 requires PVC membranes to have at least 16 mils of material over the scrim (Carlisle — understanding ASTM standards). When comparing bids, confirm both numbers match the specification (see 45 mil vs. 60 mil).
Technical detail
Single-ply membranes are specified under ASTM product standards — D6878 (TPO), D4434 (PVC), D4637 (EPDM) — which set thickness classes, tolerances, and minimum thickness over scrim, measured per ASTM D7635. In a 2009 NRCA review, published thickness-over- scrim values ranged roughly 15–19 mils for 45-mil TPO and 21–27 mils for 60-mil TPO; current values vary by manufacturer, so check the specific product data sheet rather than assuming a ratio.