Glossary
Scrim
The reinforcing fabric layer inside a single-ply membrane. Polymer above the scrim is the weathering surface; when weathering wears down to the scrim, the membrane is at or near end of life.
Plain-English definition
The reinforcing fabric layer embedded inside most single-ply membranes (TPO and PVC in particular). Think of it as the rebar in the sheet: the polymer above the scrim does the waterproofing and takes the weathering; the scrim gives the sheet its tear and puncture strength.
Why it matters to you
Verified: Industry guidance defines “thickness over scrim” as the amount of membrane material above the reinforcement that is exposed to weathering and provides the waterproofing integrity (Professional Roofing — membrane thicknesses), and ASTM membrane standards set minimums for it — ASTM D4434, for example, requires PVC membranes to carry at least 16 mils of material over the scrim (Carlisle — understanding ASTM standards). For an owner, the scrim is the practical end-of-life marker: once weathering has worn the surface down far enough that the fabric pattern is visible, the waterproofing layer above the reinforcement is gone at that spot and remaining service life is short. Visible scrim in an inspection photo is a budget signal, not a maintenance note.
Technical detail
Two membranes with the same total mil thickness can carry different amounts of polymer above the scrim, which is why specifiers read thickness-over-scrim off the product data sheet rather than relying on the headline thickness. In the 2009 NRCA review cited above, published values ranged roughly 15–19 mils over scrim for 45-mil TPO and 21–27 mils for 60-mil TPO; current products vary by manufacturer. Thickness over scrim is measured under ASTM D7635, and each membrane family’s product standard (ASTM D6878 for TPO, D4434 for PVC, D4637 for reinforced EPDM) sets its own minimums.