What field dimensions are
Field dimensions are the measurements of your space taken from attachment point to attachment point: every side, both diagonals, and the height of each attachment point. They describe the real three-dimensional shape the sail has to fit, which is why we build every made-to-fit sail from them.
“Point to point” means from the exact spot the hardware will connect — the pad eye, the eye bolt, the hole in the post cap — not from the edge of the patio and not along the ground.
How to measure
- Label each attachment point with a letter, going around the shape in order: A, B, C, D (and E, F for five or six points).
- With a helper, pull a tape tight in a straight line between neighboring points: A–B, B–C, C–D, D–A. Don’t let it sag or follow the ground.
- Measure the diagonals A–C and B–D. For five or six points, measure from A to every corner it isn’t next to (A–C, A–D, and A–E on a six-point sail).
- Measure the height of each attachment point above the ground or patio.
- Take a few photos that show all the points, and text or email the numbers with the letters, for example “A–B 16′ 4″”.
A long tape and a ladder are usually all you need. If points are high or hard to reach, a laser measure from point to point works well. In the Phoenix area we can also measure for you.
Why the diagonals matter
Four sides alone don’t fix a shape: the same four sides can lean into many different parallelograms. The diagonals lock the shape in, and they let us check your numbers.
When we get your field dimensions, we rebuild the points in 3D. If the sides, diagonals and heights can’t all be true at once, one of them is off, and we’ll ask you to re-measure before anything is cut. It’s much cheaper to re-check a tape measurement than to re-make a sail. A few inches of tape error is normal; a foot usually means a number was written down wrong.
Heights, twist and “flattening” the sail
A four-point sail looks and performs best when two opposite corners are higher than the other two. That twist turns the fabric into a saddle shape (a hyperbolic paraboloid, or “hypar”), which stiffens the sail, sheds water and handles wind better than a flat sail. As a starting point, plan on a couple of feet or more of difference between the high and low corners.
Because the corners aren’t level, a sailmaker “flattens” the sail to pattern it. The height difference is split around a middle plane: if two diagonal corners are 4 ft higher than the other two, the high corners are treated as 2 ft lower and the low corners as 2 ft higher. Measuring point to point already includes the height in each side, which is why we ask for those straight-line measurements rather than distances along the ground.
The heights also check the tape: on a twisted sail the two diagonals don’t cross, and the gap between them should match the height difference you gave us. If it doesn’t, a height or a diagonal is off.
A triangle is the exception: three points always make a flat plane, so a triangle can’t twist. Set one corner lower so rain runs off.
Field size versus sail size
The finished sail is always smaller than the field dimensions. We deduct for the corner hardware and for tension, so the fabric can be pulled tight without the corners touching the anchors. You give us the field; we work out the sail.
Stock sails work the other way: they’re sold by their finished size, so your anchors need to sit farther apart than the sail. See the planning guide for that rule.
Corner hardware grows with the sail
The load at each corner rises with the size of the sail, so the corner hardware does too. As a rule of thumb:
- Smaller and standard sails (up to about 200 sq ft): welded stainless D-rings sewn into the corners.
- Medium sails (about 200 to 400 sq ft): Pro-Rig style corners (any brand): a heavier D-shaped ring with a groove the perimeter cable slips through.
- Large membranes (over about 400 sq ft): sandwich brackets, also called commercial brackets, which clamp the corner between two plates. Some customers choose them on smaller sails too, for the clean look compared with a turnbuckle or a block-and-tackle pulley.
Between the sail and the anchor, each corner gets a turnbuckle, a block-and-tackle tensioner, or a shackle with a lead line where the sail is shorter than the reach to the structure. See hardware.
Barrel vaults and conics
Not every fabric structure is a flat-edged sail.
Barrel vault. The fabric is stretched over a frame rolled into an arch. For the curved sides, measure the straight span across the frame (the chord) and the rise of the arch at its middle, plus the frame’s length the other way. From the span and rise we work out the arc length, the length along the curve that the membrane must cover. (The area between the straight chord and the curve is called a segment.) A 20 ft span with a 3 ft rise, for example, has an arc length of about 21 ft 2 in.
Conic. A center mast or ring pushes the middle of the fabric up, or a tie-down pulls it down, so the membrane forms a cone. Conics are patterned in panels for each project. Send photos and the outer measurements, and we’ll design it with you. See commercial structures.
What to send us
- Each side and each diagonal, labeled by letter (A–B, A–C…).
- The height of every attachment point.
- What each point attaches to: a post, a wall, a beam or fascia, and what it’s made of if you know.
- Photos showing all the points, and the address where the sail is going.
- Your fabric color, if you’ve chosen one (see colors).
Text it to (480) 399-7516 or use the quote form.