Shade Sail Planner

How big a sail to order

A sail is never as big as the space between its fixings. Each corner needs room for a turnbuckle and a shackle, and the cloth stretches as it is tensioned. Coolaroo's installation guide recommends "allowing an additional 10% of the sail length on each corner for accessories and fabric stretch." ShadeSails.com puts it the other way round: make the footprint of the fixings 18 in (46 cm) larger than the sail, so small differences in the sail's size do not matter and there is space for rope, chain, turnbuckles and shackles.

The two rules land close together. For fixings on a 4 m (13 ft 1 in) square, Coolaroo's 10% gives a 3.50 m (11 ft 6 in) sail with each corner 35 cm (1 ft 2 in) short of its fixing, and ShadeSails.com's 18 in gives 3.54 m (11 ft 7 in). The planner offers both and draws each corner in along the line from the sail's centre to its fixing, which is where ShadeSails.com says the fixing belongs: "Imagine a straight line from the center of the shade sail through the corner ring, that is where you need to put the attachment point."

Ready-made sails come in set sizes, so pick the nearest size down and take up the difference with chain or rope. ShadeSails.com warns against going too far: do not add more than 24 in (61 cm) on more than one corner, because a sail on long leaders moves too much in wind, and the shock loads shorten its life. One corner may be extended a long way; two or more may not. Its sails' edges are also cut in a curve that "deflects about 10% over the length of each edge," so the shade is a little smaller than the straight-sided shape the planner measures.

How much fall a sail needs

A flat sail holds water. Coolaroo's rule: "To enable suitable water run-off the height difference from the low points to the high points should be at least 16% of the longest side," and its Table 2 lists the figures for regular shade fabric. ShadeSails.com sets a steeper minimum on its design parameters page: "A slope of 1:4 is the recommended minimum," more for larger triangles and flat sails, because "failure to have enough slope will cause puddling and excessive sag which can cause a membrane to fail." The planner checks both: the height difference against the longest side, and the slope from the highest corner to the nearest lowest one.

Longest side	Height difference, Coolaroo (16%)	1 in 4 along that side
3 m (9 ft 10 in)	0.5 m (1 ft 8 in)	0.75 m (2 ft 6 in)
3.6 m (11 ft 10 in)	0.6 m (2 ft 0 in)	0.9 m (2 ft 11 in)
5 m (16 ft 5 in)	0.8 m (2 ft 7 in)	1.25 m (4 ft 1 in)
5.4 m (17 ft 9 in)	0.9 m (2 ft 11 in)	1.35 m (4 ft 5 in)
6.5 m (21 ft 4 in)	1.0 m (3 ft 3 in)	1.63 m (5 ft 4 in)

Shade cloth is knitted and lets some water through. ShadeSails.com's FAQ describes its cloth as "about 10% porous": canted, most rain sheds to the edge with a mist coming through; flat, the water puddles and then drains through the knit. A waterproof sail has no such escape, so every drop has to run off. Neither maker publishes a separate figure for waterproof fabric, so the planner holds a waterproof sail to the steeper of the two, ShadeSails.com's 1 in 4, as its own rule; the maker of your sail has the final word. No shade sail is built for snow: ShadeSails.com says they "do not shed snow well" and should come down where it snows.

Triangle, square or twist?

Four corners shade more than three. An equilateral triangle covers 43% of the square on its longest side, and a triangle can only tilt, so its cloth stays flat. A sail with four or more corners can twist: set the high corners opposite each other and the low corners on the other diagonal, and the cloth takes a saddle shape, the surface mathematicians call a hyperbolic paraboloid, or hypar. ShadeSails.com prefers it "when possible," because it "imparts a 3rd dimension of tension into the membrane": a twisted sail spreads its load more evenly, while "flat 2 dimensional sails tend to concentrate forces at the corners and edges" and do not last as long in the same conditions. One corner much higher than the rest, a high-point sail, gives the same third dimension. Its hints sum it up: "Three dimensional membranes move less, sag less and look better," and "the more height variation the better."

Its design parameters set the limits for a sail without engineering: up to 30 ft (9.1 m) on the longest side of a triangle and 40 ft (12.2 m) for a four-cornered sail, and no long, skinny shapes, meaning a shortest side under half the longest. Sails that overlap need at least 18 in (46 cm) between them so they do not rub in wind. The planner warns when a sail is past the size or shape limits.

A worked example

The planner opens on a twisted square: two fixings on a house wall 4 m (13 ft 1 in) apart, at 2.9 m and 2.2 m (9 ft 6 in and 7 ft 3 in), and two posts 4 m out from the wall at 3.2 m and 2.2 m (10 ft 6 in and 7 ft 3 in), the high corners opposite each other. The ground under it is 16 m² (172 sq ft); measured along the twisted cloth it is 16.3 m² (175 sq ft). With Coolaroo's 10% a corner the sail to order is about 3.55 by 3.6 m (11 ft 8 in by 11 ft 10 in), 12.5 m² (134 sq ft) of cloth. The corners are 1.0 m (3 ft 3 in) apart in height, 24% of the longest side, so it clears Coolaroo's 16%; from the 3.2 m post to the nearest low corner the fall is exactly 1 in 4, ShadeSails.com's minimum.

Posts and footings

Coolaroo's guide sizes posts and footings by the sail and the height of the fixing. Each footing is a hole 450 mm (18 in) across, filled from the bottom up: a 100 mm (4 in) concrete pad, left to set; 100 mm (4 in) of 20 mm (3/4 in) gravel, which the post stands on; then concrete to the top. The post is braced leaning out from the centre of the sail before that last pour. The guide labels the lean "10° (1 in 10)" and draws it as 100 mm out over 1,000 mm, which is 1 in 10, about 6°; the planner uses the drawn 1 in 10 and tells you how far in to set each post's foot. Its Table 1 for square sails:

Fixing height	3.6 m (11 ft 10 in) sail: footing deep, post long	5.4 m (17 ft 9 in) sail: footing deep, post long	Square steel post
2 m (6 ft 7 in)	800 mm (2 ft 7 in), 2.9 m (9 ft 6 in)	900 mm (2 ft 11 in), 3.0 m (9 ft 10 in)	100 x 3 mm
2.5 m (8 ft 2 in)	900 mm (2 ft 11 in), 3.5 m (11 ft 6 in)	900 mm (2 ft 11 in), 3.5 m (11 ft 6 in)	100 x 3 mm
3.5 m (11 ft 6 in)	1,200 mm (3 ft 11 in), 4.8 m (15 ft 9 in)	1,200 mm (3 ft 11 in), 4.8 m (15 ft 9 in)	100 x 4 mm

Its treated timber posts are 150 mm, or 200 mm for the 5.4 m sail at 3.5 m, and its 5 x 3 m rectangle needs deeper footings, 1.1 to 1.4 m. ShadeSails.com's post guide gives a US rule of thumb: 4 in schedule 40 galvanized steel pipe or treated poles 6 in or more across, in concrete footings 3 to 6 ft deep and 1 to 1.5 ft across, with "generally a third of the total length of the post" in the ground, "so for example, a 10' high post should have about 5' in the ground." It has posts taller than 12 ft (3.7 m), or posts that carry more than one sail, engineered, and charges $800 to $2,500 for its own engineering service. "If you skimp on depth the post will get pulled over."

Both are general guides. Coolaroo says installation needs "can vary depending on factors such as soil types, anchor points, wind conditions, local planning and building regulations," and to consult a qualified builder or engineer and the local council before starting; in the US that means your local building department, which may want a permit. Have a large sail, tall posts or a fixing on the house checked by a structural engineer.

Fixing a sail to the house

The wall has to be strong enough for the pull. ShadeSails.com says to fix to the frame of the house, not to siding or fascia, and to keep off the roof, which can leak: a pad eye lag-bolted into the header over a window or door, or into the top plate of the wall, works. Coolaroo uses a fascia support where a sail meets the eaves, which carries the load back to the rafters. ShadeSails.com puts the wind on a sail at 5.5 to 15 lb a square foot, so the 12.5 m² (134 sq ft) example pulls on its four fixings with roughly 740 to 2,010 lb (330 to 910 kg) between them.

Hardware

Each corner needs a fixing (a pad eye on a wall, an eye bolt through a post), a D-shackle, and a way to pull it tight. Coolaroo asks for a turnbuckle on at least two corners, and lists its fittings in 316 stainless steel: turnbuckle, D-shackle, pad eye, S hook, eye bolt and snap hook. ShadeSails.com pre-tensions its sails to about 100 lb, pulling equally on each corner, and warns that "nothing causes faster wear and tear than loose sails moving and banging in the wind." Coolaroo's limit on the other side: stop when the sail "is rigid with little or no creases and does not sag," because over-tensioning strains the fabric and stitching. Snap hooks make a sail quick to take down for storms; ShadeSails.com says its standard sails should come down in high winds. How to install a shade sail walks through the job.

Where the shade falls

A sail's shade is under it only when the sun is high. The 3D view uses NOAA's solar equations to place the sun for your town, date and time, and drops each corner's shadow height ÷ tan(sun's altitude) away from the sun. In London at 3 pm on 21 June the sun is 54° high in the southwest, and a sail 2.7 m (8 ft 10 in) up casts its shade 1.96 m (6 ft 5 in) to the northeast; at the same hour on 21 December the sun is only 5° up and the shade lands about 29 m (95 ft) away. In Phoenix at 3 pm on 21 June the sun is 56° high in the west and the shade sits 1.83 m (6 ft) east of the sail. Set the time to when you sit outside and move the corners until the shade covers the table. How the sun moves across your garden explains the sun's path, and the best shade for a south-facing garden and shade for hot climates cover the choices for a sunny plot.

A sail is one of three common answers to a sunny patio; shade sail vs awning vs pergola compares them. Coolaroo's care advice is a mild detergent and water, left on for 10 minutes and rinsed off with a hose; how to clean outdoor fabric has more.

Sources: Coolaroo Shade Sail Installation Guide (Gale Pacific, April 2026: sail allowance, fall and Table 2, posts and Table 1, hardware, care); ShadeSails.com design parameters, helpful hints, post recommendations and FAQ (slope, twist, size limits, posts, wind, porosity), all read 19 October 2026. The sun's position is from NOAA's solar calculation equations. The planner is a guide, not an engineer's design.