When a shade sail installation fails, the fabric is rarely the culprit — it is the posts. A tensioned sail pulls hundreds of pounds sideways on every corner, and a post set like a fence post starts leaning within a season. The fix is not exotic: right material, deep enough footing, a deliberate lean into the tension. This is the post guide we send buyers before they pour concrete, because once the concrete sets, every mistake is permanent.
Both work; they fail differently. Steel pipe posts (galvanized or powder-coated) are slimmer, don't rot, and handle the highest loads — the right choice for large sails, coastal air and commercial sites. 6×6 timber posts look warmer in gardens and cost less, but they must be rated for ground contact and will eventually check and twist over the years. Whatever the material, avoid the two classics: reusing a fence post (fence posts are specced for vertical loads, not constant sideways pull) and thin-walled decorative tubing that buckles at the base the first windy week.
The rule that matters most: bury about a third of the post's total length. A post that stands 2.4 m above ground needs to be roughly 3.6 m long overall — 1.2 m of it in a concrete footing. The hole wants to be around 300–350 mm across, wider at the bottom than the top so frost and uplift can't jack it out. In frost climates, go below the local frost line; in sandy soil, go wider, not just deeper.
| Sail size | Post (rule of thumb) | Footing (approx.) |
|---|---|---|
| Up to ~12 m² | 6×6 timber or 76 mm steel pipe | 300 mm dia × 900 mm deep |
| 12–25 m² | 89–90 mm steel pipe | 350 mm dia × 1000 mm deep |
| Over 25 m² / exposed site | Engineered spec — don't guess | Per local code / engineer |
And the step everyone rushes: let the concrete cure before tensioning. Two to three days minimum, a week is better. Tensioning onto green concrete shifts the post in its footing and no amount of re-tightening ever fixes the lean.
Set each post leaning about 5 degrees away from the sail — toward the outside. It looks wrong before the sail goes up and exactly right after: the fabric's pull draws the post to vertical under load. A post set dead vertical ends up leaning inward once the sail is tensioned, and it reads as a mistake from across the garden. Mark the lean direction before you pour; the sail's corner line tells you which way is "away".
A sail needs a 15–20° slope between its high and low corners to shed water and spill wind — which means your posts are different heights, and the difference is decided now, at the digging stage, not when the sail arrives. Work the geometry backwards: fix the high-corner height (2.4–3 m over a patio is typical), compute the drop needed across the span for your slope, and that gives you the low-post height. The full measuring method — including the 10–15% hardware allowance between anchor points — is in our shade sail sizing and installation guide.
You rarely need four new posts. A solid masonry wall or a structural fascia on a house can carry one or two corners with pad-eye anchors — that is half the digging gone. What a wall cannot do is forgive: pad eyes must go into structural masonry or framing, never into mortar joints, siding or trim. Mixing is normal — two house corners, two posts — and usually the cheapest good layout. If the whole job leans on posts, keep them on the garden side of beds and paths so the footing disappears into planting.
At each post corner: an eye bolt through the post (through-bolted, with washers — not a screw-in eye lag that can pull out of timber grain), then a stainless turnbuckle and D-shackle to the sail's corner ring. The turnbuckle is not optional decoration; it is how you tension the sail now and re-tension it after the first month of settling. Near pools or the coast, use 316 marine-grade stainless for everything. If you are speccing a large or exposed sail, the fabric side of the equation is covered in our heavy-duty shade sail guide.
Be honest about the line. DIY is fine for a garden triangle up to about 12 m² on timber or standard steel posts. Beyond that — commercial seating areas, carports, sails over public space, coastal wind zones, or anything over 25 m² — the posts stop being a gardening question and become a structural one. Get the footings engineered to local code. The few hundred that costs is cheaper than a post through a windshield.
Not every site needs digging. On a sound concrete slab, base-plated steel posts bolted down with expansion anchors work well — provided the slab is at least 100 mm thick and crack-free, and the anchors are sized for pull-out, not just shear. On sloped ground, set posts so the leaning direction follows the slope's fall where possible; a post leaning uphill against the sail's pull fights two forces instead of settling into one. And wherever a post goes in, call before you dig — hitting an irrigation line is annoying, hitting power is a hospital visit.
About one-third of the post's total length, in concrete — so a 2.4 m above-ground post needs roughly a 1.2 m footing. Go below the frost line in cold climates.
Steel handles bigger loads and never rots; 6×6 timber looks better in gardens and costs less. Both fail the same way when under-specced — at the footing — so the hole matters more than the material.
No. Fence posts are designed for vertical loads and shallow footings. A tensioned sail pulls sideways all day, every day — fence posts lean within a season.
Wait at least 2–3 days for the concrete to cure before applying any tension; a full week is safer. Tensioning early permanently shifts the post in soft concrete.
Posts first, sail second: one-third of the post in concrete, a 5° lean into the tension, through-bolted eye bolts and a turnbuckle at every corner — and the concrete cures before the sail goes up. Send us your layout and we will size the shade sail and the posts to match.