Twinwall polycarbonate sheets: what to order

Updated: October 2026

You are replacing a greenhouse pane or roofing a small canopy, and the amazon.co.uk packs all say polycarbonate. Twinwall polycarbonate sheets are the boards with two skins and hollow flutes.

You are replacing a greenhouse pane or roofing a small canopy, and the amazon.co.uk packs all say polycarbonate. Twinwall polycarbonate sheets are the boards with two skins and hollow flutes. This guide is the order of decisions: thickness for the frame, width for the span, colour for the light, then the words the listing must print. The twinwall polycarbonate comparison holds the eight listings. Peter F. used those extracts and public UK trade notes, dated 16 October 2026. We did not weather a sheet in a lab. If you buy through our links, a qualifying purchase can earn us a commission at no extra cost to you.

Our top picks

Badge	Product	Thickness	Width	Colour	Note	Price	Offer
Best overall	Warmfay Warmfay 10-pack 4 mm	4 mm	605 mm	Clear	5.6	about £54.99	View offer
	TEOV TEOV 14-pack 4 mm	4 mm	605 mm	Clear	6.8	about £62.29	View offer
Alternative	CHEOAR CHEOAR 10-pack 4 mm	4 mm	610 mm	Transparent	6.0	about £61.99	View offer

Best overall

Warmfay Warmfay 10-pack 4 mm

5.6/10about £54.99

View offer

Rank 2

TEOV TEOV 14-pack 4 mm

6.8/10about £62.29

View offer

Alternative

CHEOAR CHEOAR 10-pack 4 mm

6.0/10about £61.99

View offer

On this page

1.What thickness the job needs
2.How wide the sheet can run
3.Clear, bronze, or opal
4.What the listing must say

What thickness the job needs

Start with the frame. UK trade notes treat 4 mm twinwall as the usual greenhouse pane, about 0.8 kg/m², about 82% light, and a U-value of 3.8 on the Glazing Shop figures. Those are guides, not a test of a marketplace pack. Six millimetres, about 80% light and a U-value of 3.5, is for a wider slot. A clip sold for 4 mm will not suit it. Ten millimetres is where those guides start a small canopy or carport. Palram’s clear 10 mm line, as UK merchants repeat it, is about 79% light, a U-value of about 2.9, and about 1.7 kg/m². Do not force that board into a greenhouse bar made for horticultural glass. A heated room or a stair wants 16 mm multiwall or thicker.

How wide the sheet can run

Two UK retailers disagree on 10 mm twinwall. The Glazing Shop uses about 600 mm centres, tighter on an exposed canopy. The Polycarbonate Store uses about 700 mm. Do not average them, and neither is a snow-load calculation. Rafters at 800 mm need another bar, or a thicker sheet with its own table. A UK greenhouse module is often 610 mm. A pack that says 600 mm or 605 mm is a different pane, and 4 ft on a title is often 1200 mm in the bullet, not 1220 mm. Measure the opening, then read the millimetres. Bars at 900 mm are a different roof from bars at 700 mm, even at the same thickness. Colour changes the light. It does not change the span.

Testsieger

Warmfay Warmfay 10-pack 4 mm

5.6/10about £54.99

View offer

Clear, bronze, or opal

Clear sheet is the crop choice. Palram-style figures put clear twinwall near 82% light at 4 mm, 80% at 6 mm, and 79% at 10 mm. Bronze on a 10 mm line is about 35% light. Opal is about 30%. Those shares are for glare and privacy on a canopy, not for tomatoes. A marketplace bullet that says 89% light is that seller’s claim. Do not merge it with the Palram 82%. Under an opal sheet the bed is in shade. Seedlings want clear 4 mm or 6 mm that fits the slot. Palram’s SUNLITE note warrants light transmission for ten years when the marked face is outwards. UV coated is a claim until the page says which side faces the sun. A face fitted downwards yellows.

What the listing must say

Read four lines before you compare prices. The page should say twin-wall, a thickness in millimetres, a size in millimetres that agrees with itself, and which UV face goes outwards. In this set, one clear sheet is 1 × 3 m at £51.99 and never says twin-wall or a thickness. On 16 October 2026 The Polycarbonate Store listed 4 mm twinwall at £17.57/m². Packs here land nearer £6–£10/m², with no tape and no named ten-year UV warranty. The cheaper square metre counts only when that size fits. If title and bullet disagree, stop. Measure the slot, pick 4 mm or 6 mm for a greenhouse and 10 mm for a small roof, then clear or opal. Fitting is on how to fit twinwall polycarbonate. Clips and 610 mm panes are on polycarbonate greenhouse panels. The plate-versus-flute choice is twinwall vs solid polycarbonate.

Common questions

Is 4 mm enough for a greenhouse?

For a typical UK greenhouse or cold frame, 4 mm is the thickness trade guides treat as the default, at about 0.8 kg/m² and about 82% light on the Palram-style figures. It is enough when the slot and the clip are made for 4 mm. It is the wrong sheet for a canopy span, and it is not enough when the frame was built for 6 mm. Measure the rebate before you decide the pack is too thin.

When do I step up to 10 mm?

Step to 10 mm for a small canopy or carport when the bars sit inside the guide you are following, about 600 mm centres in the Glazing Shop note or about 700 mm in the Polycarbonate Store note. Do not step to 10 mm to reglaze a 4 mm greenhouse slot. A heated room wants thicker multiwall, not a thicker twinwall from this range.

Is a UV bullet the same as a marked film face?

No. UV coated or 99.9% is a claim. Palram SUNLITE is warranted for light transmission when the marked face is outwards, for ten years, on that brand’s terms. If the listing never says which side faces the sun, you cannot apply that warranty, and you can fit the coated face downwards by mistake.

Why is cut-to-size dearer than a marketplace pack?

Specialist 4 mm was £17.57/m² at The Polycarbonate Store on 16 October 2026, while several packs in this set work out nearer £6–£10/m². You are comparing a cut size and a named trade sheet with a fixed module and a short or shared review file. The pack is cheaper per square metre only when the printed size is one you can use.

What if the listing never says twin-wall?

Leave it out. A clear polycarbonate title can be solid sheet, a film, or a board with no thickness. The 1 × 3 m sheet in this set at £51.99 does not say twin-wall or a thickness. Until those words are on the page, you cannot match it to a 4 mm clip or a 10 mm span.

Best twinwall polycarbonate in this set