Three deductions between the post and the fabric
The distance you can measure is between the fixing points. The sail is smaller than that, by three separate amounts, and every one of them is easy to leave out.
The first is height. A sail with any useful fall in it has corners at different heights, so the real distance between two fixings is the hypotenuse of the plan distance and the height difference. Sixteen feet on the ground between a fixing at 10 ft and one at 8 ft is 16.12 ft between the fixings. Small, but it is in your favour and people who measure with a level tape miss it.
The second is hardware, and it is the big one. Between the eye on the post and the ring at the corner of the fabric there is a turnbuckle, a shackle and usually a snap hook, and assembled and part-wound that stack is commonly six to ten inches. At eight inches a corner that is sixteen inches, or 1.33 ft, off every single edge, at both ends.
The third is the slack the sail needs to be tensioned at all. Fabric that arrives exactly filling the remaining gap cannot be pulled tight, because there is nothing left to pull. A few percent is normal and the maker of the cloth publishes theirs.
| Step, on a 16 ft plan span between fixings at 10 ft and 8 ft | Length |
|---|---|
| Measured on the ground | 16.00 ft |
| True distance between the fixings | 16.12 ft |
| Less 8 in of hardware at each end | 14.79 ft |
| Less 3% slack for tensioning | 14.35 ft to order |
That is more than a foot and a half smaller than the span, and it is the whole reason a sail that looks too small when it comes out of the bag is usually the right one.
Why the edges are cut curved
A straight fabric edge under tension flutters, because a straight line between two tensioned corners has no way to carry load along its length. So sails are cut with a scoop out of each edge — commonly several percent of the edge length at the deepest point — and the cable or webbing in the hem pulls that curve out into a straight line, putting the fabric into tension across the whole panel instead of just at the corners. The visible sag along the edge of a hung sail is that curve doing its job.
It costs area. A parabolic scoop of depth d over an edge of length L removes about two thirds of L times d, and on the default triangle that is a scoop of 0.86 ft in the middle of each edge and 24.56 sq ft off a sail whose straight-edged area would be 88.63, leaving 64.07 sq ft of actual shade out of the 112 sq ft the three fixing points enclose. The calculator reports both, because the difference is what you notice standing under it at two in the afternoon. It also means that comparing sails on their nominal dimensions overstates the shade you get, by an amount that depends on a cut nobody quotes.
Fall is not a style choice
Corner heights that are all the same give a flat sail, and a flat sail ponds. Rain collects in the middle, the weight stretches the fabric, a bigger pond collects, and the end of that sequence is either a permanently baggy sail or a fixing pulled out of a post. Every sail maker publishes a minimum fall for their cloth and it is not a small number. The calculator reports the difference between your highest and lowest fixing and expresses it as a percentage of the longest span so you can see what you have got.
The forces involved are the part of a shade sail that deserves real caution. A tensioned fabric plane forty feet square is a sail in the nautical sense, and a gust puts a large load into three or four points, which then go into posts and footings or into a wall. What size the posts have to be, how deep they go, what they are set in, and whether any part of a house will take the pull are engineering questions with local answers, and this page deliberately gives none of them. Sails come down before a storm because that is what the makers say to do.
Related
For a solid cover instead of fabric, the patio cover headroom calculator is where the geometry goes, and the pergola shade calculator covers a slatted roof. To find out where the shade will actually land at a given hour, the overhang shade calculator gives the sun altitude and azimuth for your latitude and date. Posts and the concrete around them are on the post and concrete calculator, and if the sail is going over a triangle you want to check, the triangle calculator will close it.
Questions people ask
Why is a shade sail smaller than the space it goes in?
Because the hardware lives in the gap and the fabric has to arrive slack enough to be tensioned. A turnbuckle, shackle and snap hook assembled at each corner commonly take six to ten inches, which comes off both ends of every edge, and then a few percent of what is left is the slack that lets you pull the sail tight. On a sixteen foot span with eight inch hardware and three percent slack the fabric edge works out around 14.35 ft. A sail ordered at sixteen feet will not go up.
Do I measure the spans on the ground or between the posts?
Between the fixings, along the line the sail edge will take, because that is the distance the fabric plus hardware has to cover. If you measure on the ground with a level tape you get the plan distance, which is shorter whenever the fixings are at different heights. This page will do that conversion for you if you give it plan distances and the height of each fixing, and on a sail with a real fall in it the difference is worth measuring properly rather than estimating.
How much fall does a shade sail need?
Enough that it never holds water, and the number belongs to the fabric maker rather than to a general rule. What matters is understanding why: a flat sail ponds in the middle, the weight of the pond stretches the fabric, the stretch makes the pond bigger, and it ends with either a permanently loose sail or a fixing torn out. A visible difference between the highest and the lowest corner is what stops that, and the calculator reports the fall you have as a percentage of the longest span so it is a number rather than an impression.
Why does my four-cornered sail need a diagonal measurement?
Because four side lengths do not define a quadrilateral. Hold four sticks loosely pinned at the corners and you can push the shape flat or square without changing a single side. A triangle is rigid, which is why three measurements are enough for one; a quadrilateral needs a fifth. Without the diagonal there is no area, no flat pattern, and no way for a maker to cut the thing, which is why every made-to-measure order form asks for it.
Can I fix one corner to my house?
People do, and it is exactly the corner to be careful about. A tensioned sail concentrates a large wind force into each fixing, and a wall fixing has to reach real structure rather than sheathing or a masonry veneer, in a place that is not going to leak afterwards. Whether a given wall will take it, and how, is a structural question for an engineer or the building department, and the sail maker publishes the corner load their fabric develops so someone can answer it. This page gives dimensions and areas and no loads at all.