A standard 15# felt roll nominally covers 432 ft²—but after a 2-inch side lap and 6-inch end lap on a flat deck or shed roof, effective coverage drops closer to 400 ft². Enter your deck area and lap assumptions to get a real roll count, not the number on the wrapper.
Underlayment cost is roll-coverage math. Overlap loss changes effective coverage and roll count significantly.
Enter values and click Calculate to see roll count and cost.
Start with the area that actually needs felt coverage.
Use the felt/synthetic roll specification planned for this project.
Reduce nominal coverage to effective coverage after overlaps and practical losses.
Round up to whole rolls, then apply price-per-roll for estimate totals.
Recheck valleys, upstands, and edge detailing before procurement.
Lap loss, layers, upstands, and detail rules vary by product/system. Confirm exact install instructions before final procurement.
Most ordering mistakes happen when assumptions are mixed across units, pitch, and coverage rules. Using Flat & Shed Roof Felt Calculator early helps align scope, quantity, and labor planning before supplier pricing or installer scheduling. This reduces reorders, avoids under-counting, and improves quote consistency.
Start with verified dimensions, run conservative waste assumptions, then compare output against product data sheets and field conditions. For cross-checks, pair this page with Underlayment cost calculator and Flat roof cost calculator.
Treat calculator output as a controlled estimate, then validate accessories, overlaps, and edge details separately. Final checks are stronger when you review assumptions with Single ply roofing cost calculator before submitting purchase orders.
It covers felt underlayment and base-ply planning. For the finish membrane (TPO, EPDM, mod-bit cap sheet), use the membrane-specific calculators linked above. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project notes, then confirm drainage falls before final ordering. This keeps your system specification aligned with real site conditions and reduces costly quantity revisions.
Side laps and end laps consume usable area. A 15# felt roll labeled at 432 ft² might deliver only 395–410 ft² of effective coverage after a standard 2-inch lap—multiply that across 20 rolls and you're short by a full roll before you count edge cuts. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project.
Yes. This calculator works perfectly for shed roofs, lean-tos, and any small, simple structures that require standard roofing felt underlayment. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project notes, then confirm drainage falls before final ordering. This keeps your system specification aligned with real site conditions and reduces costly quantity revisions.
Yes. Add 5% minimum for cuts at penetrations and parapets. On a roof with a dozen curbs or tight valleys, add 8–10% on top of the lap-adjusted count to avoid a mid-job trip to the supplier. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project notes, then confirm drainage falls before final ordering. This.
No, this focuses exclusively on estimating the felt underlayment rolls and costs. Use our membrane calculators for the top layer. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project notes, then confirm drainage falls before final ordering. This keeps your system specification aligned with real site conditions and reduces costly quantity revisions.
Not exactly. Overlaps, end laps, and upstands reduce effective coverage, so real installed area per roll is lower than nominal label coverage. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project notes, then confirm drainage falls before final ordering. This keeps your system specification aligned with real site conditions and reduces costly quantity revisions.
Confirm sidelap and endlap requirements for your membrane system and local guidance before ordering. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project notes, then confirm drainage falls before final ordering. This keeps your system specification aligned with real site conditions and reduces costly quantity revisions.
Layer count depends on system specification and exposure needs. Follow product installation guidance and local requirements rather than generic one-layer assumptions. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project notes, then confirm drainage falls before final ordering. This keeps your system specification aligned with real site conditions and reduces costly quantity revisions.
Yes. Perimeter and penetration details consume additional material and must be included in final takeoffs. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project notes, then confirm drainage falls before final ordering. This keeps your system specification aligned with real site conditions and reduces costly quantity revisions.
No. Use it for planning quantities, then finalize with the selected membrane system specification and detail drawings. For better estimating accuracy, cross-check roll coverage, overlap width, and edge detailing with your project notes, then confirm drainage falls before final ordering. This keeps your system specification aligned with real site conditions and reduces costly quantity revisions.
Reviewed by the Roofing Materials Calculator Team
Every calculator on this site is built using manufacturer specifications, industry-standard waste factors, and real-world estimating practices. Our formulas are cross-referenced against supplier data sheets and professional roofing takeoff methods. Calculations are for planning purposes — always verify final quantities with your supplier before ordering.
Use these together for a complete roofing material takeoff.