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How to fit twinwall polycarbonate

Updated: October 2026

The sheet is on the bench, and the job fails in four places if you rush it: the UV face, the flute direction, the end tapes, and a screw pulled tight. This is how to fit twinwall polycarbonate on a UK greenhouse pane or a small canopy, in that order. The [twinwall polycarbonate comparison](/hub/diy-supplies/twinwall-polycarbonate) holds the listings, and the [buying guide](/hub/diy-supplies/twinwall-polycarbonate/sub/twinwall-polycarbonate-buying-guide) is where thickness was chosen. Peter F. used manufacturer install notes and the listing extracts, dated 16 October 2026. None of these packs lists breather tape, so the tapes are a separate buy. If you buy through our links, a qualifying purchase can earn us a commission at no extra cost to you.

The sheet is on the bench, and the job fails in four places if you rush it: the UV face, the flute direction, the end tapes, and a screw pulled tight.

Our top picks

Warmfay Warmfay 10-pack 4 mm

Best overall

Warmfay Warmfay 10-pack 4 mm
5.6/10about £54.99
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TEOV TEOV 14-pack 4 mm

Rank 2

TEOV TEOV 14-pack 4 mm
6.8/10about £62.29
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CHEOAR CHEOAR 10-pack 4 mm

Alternative

CHEOAR CHEOAR 10-pack 4 mm
6.0/10about £61.99
View offer
On this page
  1. 1.Face and flutes
  2. 2.Tape the ends
  3. 3.Holes and clamping
  4. 4.When this is no longer a DIY fit

Face and flutes

The coated face goes outwards. Marlon ST notes tell you to peel about 50 mm of film while you fit, then take the rest off. Leave it on and it bakes. Fit the marked face downwards and the sheet yellows. A bullet that only says UV coated does not name the face, and a Palram-style ten-year warranty does not transfer to these packs. The flutes run down the slope, not across it. Water in a channel has to drain. On a greenhouse, the ribs point from ridge towards the gutter. On a lean-to, they follow the fall. A 1210 mm pane is usually turned so the long side follows the slope and the shorter side sits in the bars. Check that against your frame before you tape.

Tape the ends

Close the top. Leave the bottom able to breathe. The install notes call for blanking tape, or aluminium tape, on the high end, and breather tape on the low end, then a U-profile over the breather tape. Seal both ends with closed tape and the flutes sweat and grow algae. Seal neither end and dirt moves in. Buy the tapes with the sheet. They are not in these boxes. Use a low-modulus neutral-cure silicone the sheet maker allows. Bathroom sealant is often acetic, and it crazes polycarbonate. A bead is not a clamp. If you pull the bar down onto a thick line of sealant, the sheet cannot move when the temperature changes. Sealant does not replace the top tape. Clips do not close open flutes. The clip pattern is on the polycarbonate greenhouse panels page.
Warmfay Warmfay 10-pack 4 mm

Testsieger

Warmfay Warmfay 10-pack 4 mm
5.6/10about £54.99
View offer

Holes and clamping

Polycarbonate moves more than glass. Brett Martin allows about 3.5 mm per metre, with holes drilled between the ribs, at least 40 mm from the edge. The Marlon fixing note describes an 18 mm hole for that system. Quinn’s multiwall manual uses about 3 mm per metre and a hole at least 3 mm larger than the screw, 45 mm from the edge. Those are two brands, not one UK law. If your pack has no brand guide, drill a hole larger than the screw, between ribs. On a 2 m run, 3.5 mm per metre is about 7 mm of movement. Stop when the sheet is held, not when the flute squashes. Do not walk on the sheet. Marlon ST uses a minimum slope of 5 degrees. Corotherm’s guide says 10 degrees for that system. A nearly flat roof is outside both notes.

When this is no longer a DIY fit

Stop when the sheet is over a room, a stair, or a space you heat, or when span or snow sits outside a manufacturer table the listing never named. Stop for a listed building or a structure past permitted development. In England, Class E of the GPDO 2015 means eaves of 2.5 m, 4 m on a dual-pitch roof or 3 m otherwise, 2.5 m within 2 m of a boundary, and no more than half the curtilage. Scotland, Wales, and Northern Ireland differ. Twinwall is not automatically TP(a). Approved Document B can treat solid polycarbonate of at least 3 mm as TP(a) rigid, and it excludes multi-skinned sheet. A Class 1 bullet is not that approval. If you are still unsure it should be twinwall, see twinwall vs solid polycarbonate.

Common questions

Which side faces the sun?
The marked UV face faces the sun. Peel about 50 mm to fit the edge, then remove the rest of the film. If the listing never says which side is coated, you cannot aim a warranty face you were never shown. Fitting the coated face down is how these sheets yellow.
Which way do the flutes run?
Down the slope, so water that enters a flute can drain to the bottom end. Ribs running across the fall hold that water. On a greenhouse, the long side of a 1210 mm pane usually follows the roof slope. Check the frame before you cut or tape.
Which end gets breather tape?
The low end gets breather tape, then a U-profile. The high end gets blanking or aluminium tape. Both ends closed, or both ends open, is the mistake. These tapes are a separate purchase. They are not in the packs in this comparison.
Can I use bathroom silicone?
Use a low-modulus neutral-cure silicone the sheet guide allows. Bathroom sealant is often acetic, and that cure attacks polycarbonate. Sealant does not replace the blanking tape on the top end, and a thick bead must not clamp the sheet so it cannot expand when the day warms up.
How big should the screw hole be?
Follow the brand. Brett Martin describes about 3.5 mm of movement per metre and, in the Marlon note, an 18 mm hole, at least 40 mm from the edge, between ribs. Quinn uses about 3 mm per metre and a hole at least 3 mm larger than the screw. With no brand guide, make the hole clearly larger than the screw. Do not treat 18 mm as universal.
When do I stop and call a fitter?
Call one for a room or stair, for snow or span outside a manufacturer table, for a listed building, or when you need a fire class and the seller has only a Class 1 line. A single pane in existing 4 mm clips is the job this sequence is for, if you can reach it from the ground.

Related pages

Best twinwall polycarbonate in this set

Twinwall polycarbonate

Twinwall polycarbonate sheets: what to order in the UK

Polycarbonate greenhouse panels that fit the frame

Twinwall vs solid polycarbonate: which sheet to buy

Sources

Public UK sources on the product standard, fire classification, outbuildings, and the description of goods. Prices, stars, and review counts are the amazon.co.uk figures in the extracts. The Polycarbonate Store prices are from 16 October 2026. Prices move.

  1. BS EN 16153:2013+A1:2015, light-transmitting flat multiwall polycarbonate sheets (retrieved 16 October 2026)
  2. Approved Document B, 2025 amendments (retrieved 16 October 2026)
  3. GPDO 2015, Schedule 2, Class E, buildings incidental to a dwellinghouse (retrieved 16 October 2026)
  4. Consumer Rights Act 2015, section 11, goods to match the description (retrieved 16 October 2026)
  5. Palram SUNLITE technical guide (retrieved 16 October 2026)
  6. The Polycarbonate Store, twinwall polycarbonate (retrieved 16 October 2026)
  7. Marlon ST installation guide (retrieved 16 October 2026)
  8. Omega greenhouse glazing W spring clip, pack of 100 (retrieved 16 October 2026)