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How to fit a polycarbonate sheet

Updated: 23 October 2026

A polycarbonate sheet can shrug off a knock that would shatter glass, and still fail in the first winter because the coated face was inside, the flutes were open, or the screws were tight. If you are fitting a multiwall side or a solid pane into a rebate, you can do that on the ground. If the next step means standing on the sheet, stop. This is how to fit polycarbonate sheet when the board is the right thickness. The sequence follows UK merchant guidance and Brett Martin's Marlon ST notes, not a screw layout we invented for any pack in our [polycarbonate sheet comparison](/hub/diy-supplies/polycarbonate-sheet). A qualifying purchase can earn a commission at no extra cost to you.

A polycarbonate sheet can shrug off a knock that would shatter glass, and still fail in the first winter because the coated face was inside, the flutes were open, or the screws were tight.

Our top picks

Panda Ant Panda Ant 4 mm solid sheet

Best overall

Panda Ant Panda Ant 4 mm solid sheet
7.6/10about £24.99
View offer
CHEOAR CHEOAR 4 mm twin-wall 10-pack

Best value

CHEOAR CHEOAR 4 mm twin-wall 10-pack
7.4/10about £61.99
View offer
SANJIAOJIA SANJIAOJIA sheet, 1 m × 2 m

Alternative

SANJIAOJIA SANJIAOJIA sheet, 1 m × 2 m
5.5/10about £32.90
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On this page
  1. 1.Before you peel the film
  2. 2.Cut, drill, and clear the flutes
  3. 3.Tape, fall, and fixings
  4. 4.When to stop

Before you peel the film

Leave the film on until the sheet is cut, drilled, and fixed. On outdoor multiwall it labels the UV face, and that printed face goes to the sun. Peel it only when the fixings are in, because film left in the sun can bake on. If you ordered a thin SANJIAOJIA roll, reviews describe curl that does not relax. This method is the wrong one for that product. The CHEOAR 10-pack's included-components field is NO, so buy closed tape for the top and breather tape for the bottom. Whether 4 mm is the right sheet is the polycarbonate sheet buying guide, not a decision to make with the saw running.

Cut, drill, and clear the flutes

Support the sheet close to the cut, film still on, and use a fine-tooth blade. A coarse blade leaves a ragged edge that cracks later at the rebate. Do not rush the last centimetre, which is where an unsupported corner snaps. Drill fixing holes larger than the screw, between the ribs, not through a rib. On Brett Martin's Marlon ST guidance the holes are 18 mm and the drill stays at least 40 mm from the edge. Those figures belong to that system, not to every Generic pack. The hole must let the sheet move, or a tight screw will split it on the first hot afternoon. Clear the swarf before you tape. Quinn's manual tells you to vacuum the flutes, not to wash water down them.
Panda Ant Panda Ant 4 mm solid sheet

Testsieger

Panda Ant Panda Ant 4 mm solid sheet
7.6/10about £24.99
View offer

Tape, fall, and fixings

Ribs run down the slope, so water that gets in can leave. On a roof, Professional Building Supplies and Brett Martin both want at least a 5 degree fall. A vertical side still needs closed tape at the top and breather tape at the bottom. The same tape on both ends either traps condensation or lets the rain in. Fix through the oversized holes and do not clamp the sheet tight. Quinn's rough guide is about 3 mm of movement per metre. A pinched sheet will buckle. Stop when the cap touches. Neutral-cure silicone is the sealant these guides specify. A vinegar-smelling sealant attacks the plastic. Take the film off once the sheet is held. The crop side of fitting is on the polycarbonate sheets for a greenhouse page.

When to stop

You can cut and fix a greenhouse pane, a cold-frame lid, or a shed window from the ground or a stable platform. You stop when the work is at height, when the sheet is the roof of a room, or when the only foothold is the plastic. Health and Safety Executive roof-work guidance treats these sheets as a fragile surface. Call a competent person for a conservatory, a carport, or any slope where a fall would matter. Also stop if the film has no writing. You cannot see a UV coating, and an unmarked outdoor face is a guess that yellows the sheet. If the sheet is 1 mm or curled, fitting technique will not rescue it.

Common questions

Which side of a polycarbonate sheet faces the sun?
The face marked on the protective film. Outdoor multiwall is coated on one side, and that side goes out. Peel the film only after the sheet is fixed, or sunlight can stick it to the plastic. If there is no print, you cannot tell the coated face by looking. Do not guess on a roof.
Do I tape both ends the same way?
No. Closed tape goes at the top to keep rain out. Breather tape goes at the bottom so condensation can leave and insects stay out. The ribs run downhill. The CHEOAR pack does not include these tapes, so buy them with the sheet. Using one tape at both ends traps water or lets the rain in.
How much of a gap do I leave for expansion?
Quinn's multiwall manual uses about 3 mm of movement per metre as a rough guide, so a metre of sheet wants a few millimetres of room, not a pinched screw. Brett Martin's Marlon ST notes use 18 mm holes, at least 40 mm from the edge, for that system. Those hole sizes are not a rule for every pack.
Can I use any silicone?
Use a neutral-cure silicone, which is what the UK merchant guides and Brett Martin specify. A builder's silicone that smells of vinegar can attack the edge and leave it cloudy. Silicone is a seal at the frame. It is not a substitute for the closed tape and the breather tape on the open flutes.
What saw will not shatter the edge?
A fine-tooth saw, with the sheet supported and the film still on. Keep the offcut from dropping as you finish the cut. A coarse blade or an unsupported corner is what shatters the edge. Vacuum the swarf out of the flutes afterwards. Do not wash it in with a hose, because the water stays in the channels.
Can I stand on the sheet to fit the next one?
No. Treat the sheet as a fragile roof surface, in line with Health and Safety Executive roof-work guidance. A greenhouse side you can reach from the ground is a reasonable DIY job. A room roof or a slope at height is the point where you bring someone in.
Why is my sheet still curled?
Because a thin roll is not a glazing sheet. Reviewers of the SANJIAOJIA listings describe curl and a plastic that is too thin for a shed window. Leaving it in the sun is not a promised cure, and we do not claim a roll will flatten. If you need a flat pane, use a 4 mm solid sheet or a twin-wall board that was packed flat.

Related pages

Best polycarbonate sheet

Polycarbonate sheet vs acrylic

Polycarbonate sheet

Polycarbonate sheet buying guide for span and type

Polycarbonate sheets for a greenhouse: 4 mm or 6 mm

Sources

Public UK sources and the amazon.co.uk listings compared on this page. Listings captured 19 October 2026. Updated 23 October 2026.

  1. The Glazing Shop thickness guide (retrieved 19 October 2026)
  2. The Glazing Shop buyers guide (retrieved 19 October 2026)
  3. BSI, BS EN 16153:2013+A1:2015 (retrieved 19 October 2026)
  4. Approved Document B amendments (retrieved 19 October 2026)
  5. Brett Martin TB413 (retrieved 19 October 2026)
  6. Brett Martin Marlon ST (retrieved 19 October 2026)
  7. RHS: choosing greenhouses (retrieved 19 October 2026)
  8. Class E, GPDO 2015 (retrieved 19 October 2026)
  9. The Polycarbonate Store twin-wall (retrieved 19 October 2026)