How to Measure for Replacement Greenhouse Film, Panels or Shade Cloth

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Measure the surface the covering must wrap, not the footprint of the greenhouse. For film, run a tape or string over the arch or roof and down the sides. For panels, measure every opening between frame members, one by one. For shade cloth, measure the area to cover and then add what you need to attach it. Then compare your numbers with what the listing really sells.

This is an ordering guide: how to get the right size on the first try. The install steps are covered in our replacement covering guide.

Why the footprint is the wrong number

A greenhouse that is 10 feet wide on the ground does not take a 10 foot wide cover. The cover has to go up one side, over the top and down the other, and then reach whatever holds it at the base. On an arched or hoop house, that path is the length of a curve. On a gable, it is two slopes and two walls.

Ends are separate pieces. Ordering one length of film for the whole structure assumes you can wrap the ends from the same sheet, which depends on the design. Work out in advance whether you are ordering one piece, an end-wall piece, or both.

Film: the measuring steps

1. Choose a calm day. Wind makes a tape or string hard to hold.
2. Measure over the frame. Run a flexible tape or a length of string from the base on one side, over the top and down to the base on the other side. Follow the surface the film will lie on. Then measure the string against a rigid tape.
3. Measure along the length. Measure the full length of the frame from end to end, including any overhang you plan at the ends.
4. Add an allowance for fastening. The film needs extra to reach the fastener at the base and to be pulled tight. Check the film maker's or the kit maker's guidance, and note it as a separate line on your worksheet.
5. Do the ends separately. Measure each end wall, including door and vent cut-outs you plan to leave open.
6. Measure again. If two measurements differ, use the larger and find out why.

For a double layer, ask whether the two layers are ordered at the same size and whether an inflation kit needs a particular arrangement. Our double-poly guide covers the system.

Read the listing before you trust the size

The size on a listing may not be the size of the sheet you can use. Check these points:

• Folded or flat. Some film is sold folded, so the width listed may be the folded width or the full opened width. Ask the seller or read the description.
• Nominal or actual. Listed sizes are sometimes rounded. Look for a stated tolerance.
• Units. Check feet against inches and metres. Convert your numbers before comparing.
• Thickness and type. Size is not the only variable. Thickness, additives and the coating change the product. See our glazing types guide.
• Returns. A cut or opened roll is often not returnable. Read the policy first.

Panels: measure every opening

Frames are rarely perfectly square, and neighbouring openings can differ by a small amount. Measuring one and assuming the rest are the same is the easiest way to buy panels that do not fit.

1. Measure the width at the top, middle and bottom of each opening, and the height at the left, middle and right.
2. Measure the diagonals. If they differ, the opening is out of square and you need to know by how much.
3. Measure the groove or rebate depth that holds the panel edge, so the panel is not too small to sit in it.
4. Check the panel thickness against the frame's channel or glazing bar.
5. Label each opening on a sketch and write its numbers beside it.

Follow the panel maker's instructions on how much clearance to leave, how to seal cut ends and which way to orient twin-wall panels, since the answers differ by product. Our guide to polycarbonate replacement panels and the comparison of twinwall and triplewall help with choosing the type.

Shade cloth: size, overhang and edges

Shade cloth is usually simpler, but a few details are easy to miss. Measure the area you need to cover and decide how much overhang you want. Then check how the cloth is finished: hemmed edges, grommets at set intervals, or a raw edge you will cut and finish yourself. Check whether the listed size is the finished size or the size before finishing.

Percentage matters as much as size, because the same width at a different density behaves differently. Our shade cloth percentage guide explains the choice.

Where measurements usually go wrong

• Measuring the ground. The footprint is shorter than the path the cover travels.
• Forgetting the fastening edge. The film has to reach the channel, board or clamp, and be gripped. A cover that just reaches the base cannot be pulled tight.
• Ignoring door and vent cut-outs. Decide whether you will cut around them or lap the film over them, and measure for that choice.
• Assuming square. Diagonals tell you if an end wall or opening is out of square.
• Mixing units. One column in feet and another in inches leads to expensive mistakes. Pick one unit and convert at the start.
• Trusting an old photo or a kit page. Frames get modified, extended or repaired. Measure the structure you have.

Take a photo of each measurement with the tape visible. It is a quick way to settle a doubt a week later when the order arrives and one number looks odd.

A worksheet to fill in before you order

Keep the worksheet. If you reorder in a few seasons, you will not need to climb up and measure again. Also check the old covering: laid flat it gives a cross-check on your numbers, though it may have stretched, shrunk or been trimmed, so do not treat it as the measurement.