The mistake is ordering a sail the size of the gap
A 5 metre by 4 metre gap between posts does not take a 5 × 4 sail. Every corner has a turnbuckle and a shackle between the sail and the post, and every one of them eats length. Order the sail at the size of the gap and it will not reach, or it will reach with no tension left — and an untensioned sail flaps, and a flapping sail destroys itself and its fixings inside a season.
So the sequence runs the other way round from what people expect: set the posts, measure point to point, subtract your hardware, then order the sail. If the sail is already bought, the posts go in to suit the sail plus the hardware — never the sail alone.
Three points or four
| Three points | Four points | |
|---|---|---|
| Tensioning | Always pulls flat. Three points define a plane. | Needs two high and two low corners, or it sags in the middle. |
| Water | Runs off the low corner. | Runs off the low edge, if you built the twist in. |
| Shade for the area | Less per metre of perimeter. | More — a rectangle covers a rectangular patio. |
| Hardest part | Getting the posts in the right triangle. | Getting the height difference right. |
A four-point sail installed with all four corners at the same height is the single most common domestic failure. It ponds, it stretches, and after the first real storm it never comes tight again.
Two worked examples
A rectangle: posts 5.0 m × 4.0 m, 300 mm of hardware a corner, 15 degrees of fall.
- Cut size: 5.0 − 0.6 = 4.40 m, and 4.0 − 0.6 = 3.40 m
- Post-to-post diagonal: √(5² + 4²) = 6.40 m
- Sail diagonal: √(4.4² + 3.4²) = 5.56 m
- Height drop across that diagonal at 15°: 6.40 × tan 15° = 1.72 m
- So if the low posts finish at 2.4 m, the high ones want to be about 4.1 m out of the ground
A triangle: fixing points 6 m, 5 m and 4 m apart, same 300 mm allowance.
- Cut sides: 5.40 m, 4.40 m and 3.40 m
- Still a valid triangle — 4.40 + 3.40 = 7.80, comfortably longer than 5.40
- Area covered: about 7.47 m² by Heron’s formula
- Height drop along the 6 m side at 15°: 1.61 m
The bit that is not a calculation
Post size, footing depth and fixing capacity are wind-load questions. A shade sail is a large sheet in the wind, and it pulls hard and sideways at the top of a post — which is the worst place to load one. The forces depend on the sail area, the site’s wind region, terrain category and shielding, and how high the sail sits. That is what AS/NZS 1170.2 exists for, and it is why the honest answer to “what size post do I need” is: ask an engineer, or buy a kit whose supplier has had it engineered and will give you the certificate.
Two things that follow from that, and both cost money when ignored:
- Fixing to the house is a structural decision. A sail corner bolted to a fascia is pulling on something never designed for it. The load goes to a rafter or a proper wall bracket, not to the gutter board.
- Removable in a storm is a real design choice. Many domestic sails are specified to come down in high wind, and that is only possible if the hardware makes it a five-minute job. Decide that up front.
Does it need approval?
Depends on the state, the size, the height and how close it is to a boundary. A small sail between two posts over a courtyard is usually exempt development; a large structure, one over a certain height, or one attached to the house can tip over into needing consent. Our when a structure needs approval guide walks through the thresholds state by state — and your council can be stricter than the state rule, so ring them before you dig the holes, not after.
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General information for Australian homeowners and trades, and an estimate only — not legal advice, and not planning, building or engineering advice. The geometry here is exact; the hardware allowance and the fall angle are typical values, not a standard — we found no Australian Standard fixing either and could not read a manufacturer’s published install guide on 3 September 2026, so both are inputs you should replace with your own supplier’s figures. Post size, footing depth and fixing capacity are wind-load questions for an engineer under AS/NZS 1170.2 and this page does not attempt them. Shade structures can need council approval depending on size, height and setback, and your council can be stricter than the state rule. No regulator has reviewed this page.