How thick is roof underlayment in millimeters?
Short answer. Synthetic roof underlayments are roughly 0.2 to 0.8 millimeters thick depending on the product, while self-adhered ice and water membranes must be at least 1.0 millimeter (40 mils) under ASTM D1970, and some products are thicker. Felt is graded by weight as #15 or #30 rather than by a stated thickness.
Mils to millimeters
Data sheets in the United States usually list thickness in mils, thousandths of an inch. One mil equals 0.0254 millimeters:
- 10 mils is about 0.25 mm.
- 20 mils is about 0.51 mm.
- 30 mils is about 0.76 mm.
- 40 mils is about 1.02 mm.
Typical ranges by type
- Spun or lightweight synthetics: toward the thin end, around 0.2 to 0.3 mm.
- Coated woven synthetics: thicker, up to roughly 0.75 mm.
- Self-adhered membranes: 1.0 mm minimum by standard.
- Felt: sold by grade; #30 is heavier and thicker than #15.
Thickness is not the best comparison
A thin woven synthetic can outperform thick felt. When comparing products, especially imports that list millimeters or grams per square meter, look at:
- Weight per area, such as grams per square meter.
- Tensile and tear strength, which predict how it survives foot traffic and wind.
- UV exposure rating, how long it can sit uncovered.
- Code evidence: an ASTM standard for felt or self-adhered membranes, or an ICC-ES evaluation report for synthetics.
- Approval by your shingle or metal manufacturer for warranty purposes.
If you are comparing flooring underlayment, those products use entirely different specifications; this page covers roofs only.
See synthetic underlayment vs felt and ice and water shield in Georgia.