A gable cover must travel over both sloped faces and extend beyond the damaged area. Measure or calculate the sloped path, then add eave and gable-end allowance.
Roof tarping is temporary water management, not a structural repair. Work at height and storm-damaged buildings can be deadly.
Safety comes before tarp size
OSHA’s roof-tarping guidance warns against installing tarps during storms and identifies falls, damaged surfaces, wind, and electrical hazards. Use qualified evaluation, proper access, and required fall protection. From the ground, use building plans or safe remote measurements when possible.
Never turn a sizing result into an installation plan
This guide does not select anchors, design fall protection, assess structural stability, or calculate wind forces.
Measure the coverage zone
- Record roof length along the ridge direction.
- Record the horizontal building span from eave to eave.
- Identify rise per 12 inches of run, or safely obtain the actual rafter/slope length.
- Add coverage beyond the damaged area so water is directed onto sound roofing and past the eaves.
- Account for chimneys, valleys, dormers, vents, and non-symmetrical faces separately.
Simple symmetrical gable calculation
For each side, run is half the building span. Slope length is found from run and rise. The tarp width over the building equals two slope lengths plus the chosen eave allowance on both sides. Tarp length equals roof length plus gable-end allowance at both ends.
Plan overlap for drainage
If several sheets are unavoidable, upper sheets should overlap lower sheets in the direction water flows. Avoid creating dams, pockets, or seams that direct water under the cover. Actual attachment and safe wind performance require a qualified plan.
When the calculator does not apply
Do not use the simple gable mode for hips, valleys, flat roofs, complex intersections, curved roofs, unstable structures, or a permanent roofing system. Those cases require project-specific measurement and design.
Safety source: OSHA Roof Tarping (Blue Roof) Safety Fact Sheet.
How this guide was prepared
We separate geometry and unit conversions from product claims, use exact standards where available, and flag specifications that vary by manufacturer. See our editorial policy for the review process.