What are composite doors made of?

Four layers, broadly. A structural core (48–70mm — laminated hardwood on premium doors like Endurance, high-density polyurethane foam on budget ones), a reinforced timber or composite sub-frame that the hinges and multipoint lock bite into, an insulating layer, and a coloured GRP skin moulded with a woodgrain or smooth texture. The colour runs through the skin rather than sitting on top as paint, which is why composite doors don't need repainting.

How long does a composite door last?

A quality composite doorset has a 30-year design life, and most manufacturers back the slab with a 10-year guarantee covering colour, warping and delamination. In practice the locking mechanism is usually the first thing to need attention, typically after 10–15 years, and it's a straightforward replacement rather than a new door.

Are composite doors worth it?

If you're replacing a tired uPVC or timber front door, yes — mainly for security and maintenance. Composite doorsets are PAS24:2016 tested as standard, hit U-values around 1.0–1.4 W/m²K, and need nothing more than a wipe down twice a year. The cost difference over uPVC is typically £300–£600 fitted, spread across a door that lasts roughly twice as long.

Do composite doors need painting or maintenance?

No painting, ever. The colour is part of the GRP skin, not a coating on it. Maintenance is a wipe with warm soapy water twice a year, a light oil on the hinges annually, and graphite (never WD-40) in the cylinder if the key starts to feel sticky.

Are composite doors any good in coastal weather?

They're the best mainstream option for it. GRP is inert in salt air, so it doesn't corrode like aluminium or rot like timber, and the dense core doesn't swell and stick the way a timber door does in wet-then-dry cycles. In Cornwall we spec A4-grade stainless hinges and fixings on properties within about three miles of the coast, because the door skin will comfortably outlast standard hardware.

What's the difference between a composite door and a uPVC door?

A uPVC door is a hollow plastic frame with a thin infill panel, typically 28–44mm thick. A composite door is a solid 48–70mm slab of bonded materials. That thickness and density is where the security, thermal and acoustic difference comes from — and why a composite door feels heavy when you swing it.

Buyer's explainer

What is a composite door?

A composite door is a solid 48–70mm slab built from four bonded layers — an insulating core, a reinforced sub-frame, a steel-reinforced outer frame and a coloured GRP skin. Here's what each layer actually does, and how long the whole thing lasts.

Get a free quote Composite vs uPVC

Anthracite Grey fully-glazed composite back door on a red-brick home

Green cottage composite door with three diamond glass panels

Black traditional composite door with arched fanlight on a Victorian red-brick terrace

The short answer

Several materials, bonded into one slab — each covering the others' weaknesses.

Every door material has one thing it's bad at. Timber looks superb and insulates well, but it swells, sticks and needs repainting. uPVC is cheap and maintenance-free, but it's hollow, flexes, and yellows in sunlight. Aluminium is strong and slim, but it conducts heat straight out of the house and pits in salt air.

A composite door sidesteps all three by not being made of one material at all. The insulating job, the structural job, the security job and the weatherproofing job are each handed to the material best suited to it, and the whole lot is bonded and pressed into a single slab typically 48–70mm thick. That's roughly double the thickness of a uPVC door, and it's why a composite door feels noticeably heavy when you swing it.

It's sold as a doorset, not a door — slab, frame, cill, hinges, letterplate, handle and locking cylinder all manufactured and fitted together at the factory, then installed as one unit. Nothing critical gets assembled on your driveway.

The four layers

What's actually inside a composite door.

GRP outer skin

Glass-reinforced polyester, moulded with a woodgrain or smooth texture and coloured all the way through. Waterproof, UV-stable, and the reason a composite door never needs painting.

Insulating core

The bulk of the door. Premium doorsets use laminated hardwood (48mm, up to 17 cross-orientated layers); budget doors use high-density polyurethane foam. This is where the U-value comes from.

Reinforced sub-frame

A timber or composite perimeter frame the hinges, letterplate and multipoint lock screw into. Without it, hardware pulls out of the core under load.

Steel-reinforced outer frame

The frame the slab hangs in, with galvanised steel keeps at every locking point, so the lock can't simply be kicked out of soft plastic.

The core is where brands genuinely differ. Endurance uses 17 cross-orientated laminated hardwood layers so the grain of one layer cancels the movement of the next; Solidor uses a solid single-piece 48mm timber core. Budget composite doors use foam, which is lighter and cheaper but flexes under wind load — something you notice on an exposed Cornish frontage.

Performance

What the layers add up to.

Measure	Typical composite doorset
Slab thickness	48 – 70mm solid
Thermal U-value	1.0 – 1.4 W/m²K
Security standard	PAS24:2016, Doc Q compliant
Cylinder	TS007 3-star anti-snap on quality doorsets
Design life	≈ 30 years
Slab guarantee	10 years typical
Maintenance	Wipe twice a year — no painting
Fitted price (Cornwall, 2026)	£1,395 – £3,400

For a full cost breakdown see our composite door cost guide, or the like-for-like composite vs uPVC comparison.

In Cornwall

Why the material mix matters more here than most places.

Cornish weather is unusually hard on front doors: driving rain off the Atlantic, salt-laden air well inland, and a wet-then-dry cycle that makes timber move all year round. GRP is chemically inert in salt air, so the skin itself is effectively immune to the thing that destroys aluminium and timber doors here.

The hardware is the weak point instead, which is why on properties within about three miles of the coast we upgrade to A4-grade stainless hinges, fixings, letterplate springs and cylinder cams as standard. See where we work on the areas we cover page, or read the Cornwall-specific detail on composite doors in Cornwall.

FAQs

Composite doors — the questions people actually ask.

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