Roof accessories & ice-and-water

IRC R905.1.2 requires the ice barrier to extend from the lowest roof edge to at least 24 inches inside the exterior wall line of the building — measured horizontally, not along the slope. Add the soffit overhang and wall thickness to that 24 inches, multiply by the pitch's slope factor, and you have the up-slope width the membrane must cover. On a 12-inch overhang, 6-inch wall, and 8/12 pitch that's about 50 inches — two courses of 36-inch roll, not one. The requirement applies where the local adopted code calls for it, typically ice-dam regions.

Standard practice lines valleys with self-adhered membrane — typically one roll-width centered on the valley — because valleys concentrate water flow and are the roof's most common leak path. Many manufacturers require it for warranty coverage even where code doesn't spell it out, and closed-cut shingle valleys rely on it entirely. This calculator figures valley liner at one course of your roll width along each valley's length. Include it as its own line; it's a distinct material and labor operation from field underlayment.

The recurring absentees: drip edge (code-required on new roofs in most adopted IRC versions), starter course, ridge and hip cap as separate lines rather than buried in field squares, valley liner, and enough ice-and-water courses once overhang and pitch are figured correctly. Each is a real material with real installation labor. Take them off with this calculator, then cross-check the estimate with the companion-item checklist — accessories are exactly the class of line item estimates drop.