EPDM Thickness Guide: 1.0mm vs 1.2mm vs 1.5mm

EPDM Thickness Guide: 1.0mm vs 1.2mm vs 1.5mm

Rubber Roof UK stocks EPDM rubber roofing membrane in three thicknesses: 1.0mm, 1.2mm and 1.5mm. This guide compares what each is actually for, what changes between them, and what does not, using the real specification of the membrane we sell rather than generic industry figures. Prices below are our own live cut-to-size rates, inclusive of VAT, correct as of the date this page was last updated.

The short answer

Thickness	What it's for	The one thing to know	Price per m² (inc. VAT)
1.0mm	Sheds, summerhouses, log cabins, small outbuildings	Not for Building Regulations work, and cannot be joined with seam tape — the roof must be covered by one sheet	£21.12
1.2mm	Extensions, garages, garden rooms, dormers, porches — most roofs	The standard specification for domestic and light commercial flat roofs	£24.48
1.5mm	Roofs with regular foot traffic, commercial spans, ballast or paving on top	About 25% thicker than 1.2mm — tougher, not more waterproof	£33.58

Per-m² rates are the same at every stock width for a given thickness, taken directly from our own 1.0mm, 1.2mm and 1.5mm product pages. Full certification, guarantee and warranty details are on each product's own page rather than repeated here.

Does a thicker membrane make a roof more waterproof?

No. All three thicknesses are the same EPDM rubber, fully waterproof, and unaffected by standing water. Thickness changes how well the membrane resists punctures, tearing and foot traffic, and how forgiving it is to install over an imperfect deck — it does not change whether the roof keeps water out. A leak comes from poor bonding or poor detailing at an edge, upstand or penetration, never from the sheet being "too thin." Choosing a thickness is about durability and use, not waterproofing.

1.0mm EPDM — sheds and small outbuildings

The light-duty grade, and the one we would fit on a shed ourselves. It comes in 8 widths from 1.40m to 5.00m, and the largest single sheet is 5.00m wide by 20m long, supplied as one continuous piece with no seam.

• Not intended for buildings that must comply with Building Regulations — that rules it out for extensions, most attached garages and habitable garden rooms.
• Cannot be joined with seam tape. The roof has to be covered by a single sheet, so a roof wider than the largest stock width needs 1.2mm instead.
• Bonded with water-based deck adhesive rather than the contact-adhesive system used on the thicker grades.
• Not intended for foot traffic — fine for the occasional visit to clear the gutter, not for a roof anyone stands on regularly.

On a shed, summerhouse, log cabin or store, none of those limitations matter and 1.0mm is the sensible, most economical choice.

1.2mm EPDM — the standard specification

The thickness most UK flat roofs are built with. It comes in 20 widths from 0.53m up to 15.24m, and the largest single sheet is 15.24m wide by 30m long — wide enough that the great majority of domestic and light commercial roofs need no join at all.

• Suitable for Building Regulations work: extensions, garages, garden rooms, dormers and porches.
• Can be joined with seam tape where a roof genuinely exceeds the largest single sheet — overlap the pieces by 75mm, prime both faces and roll the tape from the centre outwards.
• Bonded with water-based deck adhesive across the flat field and contact adhesive at the perimeter and any upstands.
• Not intended as a walking surface for regular traffic — fine for occasional maintenance access in soft-soled shoes.

If you are not sure which thickness a project needs, 1.2mm is the right default question to start from: it is enough unless the roof will be walked on regularly or the roof is a shed you would rather cover more cheaply with 1.0mm.

1.5mm EPDM — the heavy-duty grade

About 25% thicker than 1.2mm, in the same 20 widths from 0.53m to 15.24m, with the largest single sheet at 15.24m wide by 30.5m long. The extra thickness buys resistance to punctures, tearing and impact, and makes the sheet more forgiving to lay over a slightly uneven deck — it is not a different, more waterproof rubber.

Worth specifying when:

• The roof gets regular foot traffic — a terrace, balcony or roof garden.
• It's a commercial or large-span roof.
• Pavers, ballast or decking will sit on top of the membrane (use a protection layer or walkway pads between the membrane and whatever is laid over it).
• There's realistic impact risk — overhanging trees, ladders, an older or uneven deck.
• A specifier or building control officer has asked for it by name.

For a standard extension, garage or garden room roof that nobody walks on, 1.5mm is a genuine option but not a necessary upgrade over 1.2mm — you would be paying roughly 37% more per m² (see below) for toughness the roof will not use.

What actually changes, side by side

Property	1.0mm	1.2mm	1.5mm
Typical use	Sheds, summerhouses, outbuildings	Extensions, garages, garden rooms, dormers, porches	Foot traffic, commercial, ballast/pavers over the top
Building Regulations	Not suitable	Suitable	Suitable
Stock widths	8 (1.40m–5.00m)	20 (0.53m–15.24m)	20 (0.53m–15.24m)
Largest single sheet	5.00m × 20m	15.24m × 30m	15.24m × 30.5m
Can be joined with seam tape	No — single sheet only	Yes	Yes
Colour	Black	Slate grey	Slate grey
Deck compatibility	OSB3, WBP plywood, sound timber	OSB3, WBP plywood, sound timber	OSB3, WBP plywood, sound timber — more forgiving over slight unevenness
Recommended fall	Minimum 1:80	Minimum 1:80	Minimum 1:80
Standing water	Unaffected	Unaffected	Unaffected
Price per m² (inc. VAT)	£21.12	£24.48	£33.58

How much more does a thicker membrane actually cost?

Using our own live per-m² rates: stepping up from 1.0mm to 1.2mm adds about 16% to the material cost; stepping up from 1.2mm to 1.5mm adds about 37%. On a 20m² roof — a typical single-garage or garden-room deck — that is roughly the difference between £422 for 1.0mm, £490 for 1.2mm and £672 for 1.5mm before adhesive, trims and sealant. The right comparison is not the cheapest sheet, though — it's whether the roof qualifies for 1.0mm at all (Building Regulations rules most projects out) and whether it will see foot traffic (which points to 1.5mm).

Which thickness do you need?

Choose 1.0mm when:

• It's a shed, summerhouse, log cabin or similar outbuilding.
• The building does not need Building Regulations sign-off.
• The roof is 5m or narrower in one dimension, so it fits on a single sheet.
• Nobody will walk on the roof except to clear leaves occasionally.

Choose 1.2mm when:

• It's an extension, garage, garden room, dormer or porch.
• The project needs to satisfy Building Regulations.
• The roof is only accessed occasionally for maintenance.
• You want the standard, most widely specified thickness for a domestic flat roof.

Choose 1.5mm when:

• The roof is a terrace, balcony or roof garden with regular foot traffic.
• Pavers, ballast or decking will sit directly on the membrane.
• It's a commercial or large-span roof, or a specifier has asked for it.
• The deck is older or slightly uneven and the extra forgiveness is worth the cost.

Installation is the same method across all three

Whichever thickness you choose, the method does not change: a dry, swept, sound timber deck (OSB3 or WBP plywood), a minimum fall of 1:80 towards the drainage edge, water-based deck adhesive across the flat field, contact adhesive at the perimeter and any upstands, and at least 100–150mm of membrane turned over every edge. Work needs a dry deck, a dry day and a temperature of 5°C or above so the adhesive can cure properly. The full installation guide covers the method step by step, and the adhesives guide explains which adhesive goes where and how far a tin covers.

Frequently asked questions

Is 1.5mm more waterproof than 1.2mm, or 1.2mm more waterproof than 1.0mm?

No. All three are the same EPDM rubber and all are completely waterproof. Thickness buys resistance to punctures, tearing and foot traffic, not extra waterproofing — that comes down to bonding and detailing.

Can I use 1.0mm EPDM on an extension or garage?

No. 1.0mm is not intended for buildings that must comply with Building Regulations, which covers extensions and most attached garages. Use 1.2mm.

My roof is bigger than the largest single sheet — what do I do?

On 1.2mm or 1.5mm, two sheets can be joined with seam tape, overlapped by 75mm and primed on both faces. 1.0mm cannot be taped at all, so a shed roof wider than the largest stock sheet needs 1.2mm instead.

Will people be walking on the roof — does that change the thickness I need?

Yes. Neither 1.0mm nor 1.2mm is intended as a walking surface for regular traffic. If the roof is a terrace, balcony, or will have pavers or ballast laid on top, specify 1.5mm and use walkway pads or a protection layer over the membrane.

Is 1.2mm enough for a garden room?

For most garden rooms, yes — 1.2mm is the standard specification for exactly this kind of build. Step up to 1.5mm only if the roof will be walked on regularly or something will sit on top of it.

Does the colour tell you the thickness?

Roughly, on our stock: 1.0mm is black, while 1.2mm and 1.5mm are both slate grey, so colour alone won't distinguish 1.2mm from 1.5mm — check the product label or your order reference.

Do I need a different adhesive for a thicker membrane?

No. The adhesive system is chosen by where it's used — water-based deck adhesive for the flat field, contact adhesive for the perimeter and upstands — not by the thickness of the sheet. 1.0mm does not use the contact-adhesive system since it isn't taped or turned up a wall in the same way.

What size membrane should I order, whichever thickness I choose?

Measure the deck, then add at least 100mm per side on a simple shed roof (1.0mm) or 150mm per side on a larger roof (1.2mm/1.5mm) so the rubber can dress over every edge, adding more where there's an upstand, kerb or wall to climb. Take the next stock width up and round the length to whole metres — between two sizes, always take the larger, since cut membrane cannot be returned.

Which thickness costs the least to run over the life of the roof?

We don't have measured lifespan data to compare thicknesses on cost-per-year, so we won't put a number on it here — see each product's own page for its specific warranty and guarantee terms. What we can say from our own pricing: 1.0mm costs about 16% less per m² than 1.2mm, and 1.2mm costs about 27% less per m² than 1.5mm.

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