How do you measure for ice and water shield?

Step 1: Eaves

Measure the length of every eave along the gutter line. If you are covering eaves, decide how far up the roof you need to go. Where the code requires an ice barrier, it must reach at least 24 inches inside the exterior wall line. With a typical overhang, that distance measured up the slope is often more than 3 feet, which means two rows. Multiply eave length by the number of rows.

Step 2: Valleys

Measure each valley along its actual length, not its plan length. Valleys run diagonally and are longer than they look from the ground. One 3-foot-wide sheet centered in the valley is standard.

Step 3: Walls

Measure every roof-to-wall joint, such as a dormer side or where a lower roof meets a second-story wall. These need a strip running onto the roof and up the wall.

Step 4: Penetrations

Count chimneys, skylights and pipes. Pipes take a small square each; chimneys and skylights take more, depending on size.

Step 5: Add laps and waste

Add about 10 to 15 percent for end laps, side laps and cutting waste. Cut-up roofs need the higher end.

Step 6: Convert to rolls

Multiply each length by its covered width to get square feet, total them, and divide by the square feet printed on the roll label. Round up.

Tips

• Measure along the slope, not across the floor plan. Steeper roofs make a big difference.
• Aerial measurement reports list eave, valley and wall lengths, which speeds this up.

How whole roofs are measured is explained in how roofers measure a roof, and where membrane belongs in ice and water shield in Georgia.