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.
Worked example
Suppose the building span is 28 ft, the roof is 40 ft long, and the pitch rises 6 in per 12 in of run. Each side has a run of 14 ft; the slope length works out to about 15 ft 8 in. Two slope lengths plus 2 ft of eave allowance on each side gives a tarp width of about 35 ft 4 in. Adding 2 ft of gable-end allowance at both ends gives a length of 44 ft. Rounding up, a cover around 36 × 44 ft handles the simple case. Chimneys, valleys, or dormers change the geometry and must be planned separately.
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.
Frequently asked questions
How do I measure a roof for a tarp without climbing on it?
Use building plans, county records, or ground-level measurements of the building footprint plus the roof pitch from documents or a safe vantage point. Binoculars and a tape measure from the ground cover most simple gables. Never measure a storm-damaged roof by walking on it.
How much bigger than the damaged area should a roof tarp be?
Large enough to carry water past the damage onto sound roofing and over the eaves, with allowance at the gable ends. A cover that ends exactly at the damage edge simply moves the leak. The right margin depends on the roof layout and where water can be directed safely.
What size tarp do I need for a typical gable roof section?
Compute the sloped path: two slope lengths from eave to ridge plus eave allowance, times the roof length plus end allowance. A 28 ft span at moderate pitch with a 40 ft roof length commonly lands near a 30 × 45 ft cover before allowance. Always compute from your own measurements.
Can I use several smaller tarps instead of one large one?
Yes, if the overlaps run downhill so upper sheets shed onto lower sheets like shingles. Plan the overlap direction before buying, because a seam that faces uphill channels water under the cover. Multiple smaller sheets are also much easier to handle safely.
How long can a tarp stay on a roof?
A roof tarp is a temporary measure. UV, wind, and chafe degrade it, and a loose sheet can cause more damage than the leak. Treat it as protection while a proper repair is scheduled, and inspect it after every storm.
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.