House Wrap Coverage Calculator

Understanding House Wrap Coverage

House wrap (also called weather resistive barrier or WRB) is installed on the exterior sheathing of a building before siding is applied. It prevents water from penetrating the wall cavity while allowing water vapor from inside the house to escape outward. This prevents mold, rot, and structural damage within the wall framing. Properly calculating coverage ensures you buy enough material to wrap the entire house with the required overlaps.

Standard Roll Sizes:

Calculating Wall Area:

To find the total wall area, add together the area of all four exterior walls. For each wall: multiply the wall length by the wall height. Do not subtract window and door openings from the total because the house wrap overlaps around these openings and is cut out after installation. Including the openings in your calculation automatically provides a small material buffer.

Total Wall Area = (Wall 1 Length x Height) + (Wall 2 Length x Height) + ...

For a simple rectangular house: Total = 2 x (Length + Width) x Wall Height

Do not forget the gables. A gable-ended roof leaves a triangle of wall above the top plate at each end of the house, and it gets wrapped like everything else. Each triangle is half the house width times the rise from the plate to the peak, and there are two of them, so:

Gable Area = Width x Rise

On a 30 foot wide house with a 6 foot rise that is another 180 square feet, easily enough to push a job from one roll to two. A hip roof has no gables and needs nothing extra; a saltbox or a cross-gabled house has more than two and needs measuring properly.

Overlap Requirements:

House wrap must be installed with proper overlaps to maintain the moisture barrier. These overlaps reduce the effective coverage per roll significantly.

• Horizontal overlaps (between courses): Minimum 4 inches (10 cm), with the upper course overlapping the lower course
• Vertical overlaps (between sheets): Minimum 6 inches (15 cm), overlapping in the direction of prevailing wind if known
• Window and door integration: At least 4 inches (10 cm) overlap onto window and door flanges
• Corner wraps: At least 12 inches (30 cm) around each corner

Effective Coverage After Overlaps:

Due to overlaps, the effective coverage per roll is typically 80 to 85 percent of the total roll area. For a standard 9 ft x 150 ft roll (1,350 sq ft gross area), the effective coverage is approximately 1,080 to 1,150 square feet (100 to 107 square meters). Always use the reduced coverage figure when calculating the number of rolls needed.

Narrow rolls lose more, and the reason is arithmetic. The 4 inch horizontal overlap between courses is a fixed width, so it eats a fixed 4 inches out of whatever roll width you bought. On a 9 ft roll that is 4 inches out of 108, about 4 percent. On a 3 ft roll it is 4 inches out of 36, over 11 percent, nearly three times the loss for the same barrier. A 3 ft roll also means three times as many horizontal seams to tape. It is the right choice for a small addition, a shed, or working alone where a 9 ft roll is unmanageable, and the wrong choice for a whole house. The calculator uses about 78 percent effective coverage for 3 ft rolls against 85 for the wide ones.

Practical Example:

A house measuring 40 feet by 30 feet (12.2 by 9.1 meters) with 9-foot (2.74 meter) walls has a perimeter of 140 feet (42.7 meters). Total wall area is 140 x 9 = 1,260 square feet (117 square meters). Using 9 ft x 150 ft rolls with 85 percent effective coverage (1,148 sq ft effective per roll), you need 1,260 / 1,148 = 1.1 rolls, so buy 2 rolls. The extra material covers waste from cuts around windows, doors, and corners.

Installation Tips:

Start at the bottom of each wall and work upward so that upper courses overlap lower ones like shingles. This ensures water always flows over the overlaps, not behind them. Secure the wrap with cap nails or plastic-cap staples every 12 to 18 inches (30 to 46 cm) along studs. Tape all seams with manufacturer-recommended seaming tape (not generic duct tape or masking tape). Complete installation within 30 days before applying siding, as UV exposure degrades most house wrap products over time. Some premium wraps are rated for up to 6 months of UV exposure.

Vapor Permeability:

A good house wrap has high vapor permeability (allows water vapor to pass through) while being water-resistant (blocks liquid water). This is measured in perms. Standard house wraps have a perm rating of 50 to 60. Higher is better for allowing walls to dry outward. Plastic sheeting (polyethylene) has a perm rating near zero and should never be used as a house wrap because it traps moisture inside the wall cavity.