Shade sails are tensioned fabric sheets fixed to posts, walls or trees. They are cheap, look good and block most of the sun’s UV, but they are sized differently from the space they cover. Because the sail needs room for turnbuckles, shackles and chains at each corner, you order it smaller than the distance between the anchor points.
The rule
Sail side = anchor distance × (1 - allowance)
Most manufacturers recommend leaving 5 to 10% of each side, or at least 12 in (30 cm) at each corner, for tensioning hardware. Many sails also have curved (catenary) edges, cut inward to tension the fabric evenly; their listed size is the corner-to-corner distance.
Area
- Rectangle or square: length × width.
- Triangle: with Heron’s formula, from the three sides: s = (a + b + c) / 2 and area = √(s(s - a)(s - b)(s - c)).
The shade on the ground depends on the sun’s height and the sail’s tilt. At midday in summer it is close to the sail’s horizontal projection; in the evening the shadow moves well away from the sail.
Example
Three posts 16 ft apart call for a triangular sail of 14.4 ft sides with a 10% allowance. Its fabric area is about 90 sq ft and its perimeter 43 ft.
Triangle or rectangle?
Triangles are easy to fit between three existing anchors and look striking when overlapped at different heights. Rectangles give more shade for the same edge length: for the same perimeter, a square covers about 30% more area than an equilateral triangle. For wide spaces, two or three overlapping sails often work better than one large one.
Installation tips
- Twist it: set one or two corners higher than the others. The hyperbolic twist makes the sail stiffer and lets rain run off; a flat sail collects water and sags.
- Strong anchors: posts take large loads in wind. Steel posts are usually set in concrete footings about a third of their height deep. Fix to walls only through solid masonry or structural framing.
- Take it down before storms and for winter in snowy areas. Most fabrics are not designed for snow loads.
Fabric
Knitted HDPE fabric lets air and some rain through and blocks 80 to 95% of UV. Waterproof PVC or polyester fabric stays dry underneath but catches more wind and needs more slope.