What is the usable width of ice and water shield once it is lapped?
Short answer. Ice and water shield rolls are about 36 inches wide, but each row laps the one below by about 3 to 3.5 inches, so every added row covers closer to 32.5 to 33 inches. That usable width is what decides how many rows it takes to cover an eave, a low-slope section or a whole roof.
Roll width vs usable width
- First row: the full 36 inches counts.
- Each row after: about 32.5 to 33 inches, after the side lap marked on the sheet.
- Metric rolls: about 1 meter, close to 39 inches, with similar laps.
How width drives rows
- Eaves: two rows cover about 68 to 69 inches up the slope, which handles most Georgia overhangs where an eave ice barrier is required.
- Low-slope porch roof: a section 12 feet up the slope needs one full row plus four lapped rows, five in all.
- Full coverage: a 20 foot rafter run needs about eight rows.
- Valleys: one 36 inch sheet centered in the valley gives about 18 inches on each side, with no side lap at all.
Narrower pieces
Installers cut rolls into strips for walls and squares for pipes, and narrow self-adhered flashing tapes exist for small details, but those tapes are not meant to line valleys or eaves. When a strip is cut from a roll, the factory edge with the lap line should face uphill so the next piece laps over it correctly.
Why it matters on a quote
Material lists that assume a full 36 inches per row come up short on multi-row areas. For roll sizes and coverage, see how roofers measure a roof and ice and water shield in Georgia.