Capped vs uncapped composite decking: what the cap changes

The cap on a composite board is a layer of plain polymer with no wood fibre in it, fused to the core while the board is extruded. It changes how the board takes a spill and how far it fades after the first months, and in the documents cited here, only capped ranges carry a fade or stain warranty. It won't make a board scratch-proof, and it won't keep a dark board cool in the sun. If you're choosing between the two, those are the lines I'd draw.

What composite decking is made of, and where the cap goes

An uncapped board is one mix from face to face. One maker's uncapped board is about half reclaimed timber fibre and the rest mostly recycled polyethylene, with UV stabilisers and bonding agents, and the maker sells it on being the same colour all the way through². ModWood says the wood fibre at the surface of its uncapped boards is what bleaches in the sun³, and that the exposed fibre picks up fatty stains but lets a weathered board take some decking oils⁴. That maker sells both kinds. Its uncapped range is Natural Grain, whose brushed face has the fibre exposed. Its capped range, Xtreme Guard, is capped on all four faces⁶.

A capped board starts with a similar core and gets a layer of plain polymer fused over it in the same production run, which the makers call co-extrusion. On a cut end, the cap is the outer layer around the core, and that layer is the whole product difference.

Caps don't all cover the same faces. One maker's shell covers the top surface and both sides of the board and leaves the underside bare¹¹. Others wrap all four faces, groove included⁸, and one says its coating shields the core from moisture. On a low deck with little airflow, or over a damp slab, the four-sided kind is the one I'd want. On a normal raised deck I wouldn't let the cap layout decide the board.

Stains and mould

This is the difference I'd weigh most. On an uncapped board, oil is the hard one. ModWood says oil-based stains on its uncapped boards can be difficult to remove⁴. On Whirlpool in 2012, Syameese described a test patch of ModWood that "got badly marked by dirt and water from pot plants", marks that never came out properly¹⁵. That was years before the maker launched its capped range⁶. ModWood's warranty for its uncapped range lists staining, mould, fungi and algae among the things it doesn't cover⁵.

A cap is meant to keep a spill on the surface, where it washes off, though Ekodeck still says its capped boards "can stain, especially if not cleaned regularly"¹. The capped ranges with stain warranties cover food and drink stains only if you clean them within the time allowed, which is a week in one document⁹ and 72 hours in another⁵. Leave a spill for a fortnight and you've lost the cover.

A cap doesn't stop mould. Mould can grow on any outdoor surface, capped boards included. It feeds on the dirt and pollen left lying on a deck. Ekodeck says it comes from constant moisture and shade, with a dirty deck making it worse. The cap changes where the mould sits. One capped maker says surface mould can't get into its boards and washes off⁷, and two of the stain warranties cover mould staining if you clean it off within their time limit⁹.

Owners still see it. A ModWood owner on ProductReview wrote two months ago that their deck "turns green and extremely slippery every winter" despite regular cleaning¹², and someone who had built five NewTechWood decks wrote six years ago that mould formed on all of them¹³. On a shaded south-facing deck I'd pick capped and still plan to wash it, and the routine is in how to clean composite decking.

Fading after the first season

Both kinds shift colour when they first meet the sun. ModWood says its uncapped boards finish weathering two to three months after installation, with the smooth face fading more than the brushed one³. Ekodeck warns of a "slight colour shift upon initial exposure to UV" on its capped boards¹.

After that early shift, the cap matters. On an uncapped board the change can be big. A ModWood owner on ProductReview wrote that a Jarrah deck "faded to a soft pink" within three months¹². Jarrah is a Natural Grain colour. A cap has no wood fibre in it to bleach, and the capped makers put a number on how far the colour may move, which is no more than 5 Delta E over the fade term⁹.

Capped boards still shift. A NewTechWood owner on ProductReview wrote nine months ago that the boards had faded¹³, and that maker caps its boards on all four sides. So I'd leave a sample of the exact colour outside for a month before ordering. Composite decking fading and scratches has more on what the fade warranties cover.

Scratches: the cap isn't armour

Here the cap can make things worse. On an uncapped board a scratch is a change of texture in a material that's the same colour all the way through, and the makers say you can sand it² or wire-brush the brushed face so the repair blends in³. On a capped board, Ekodeck's manual says not to sand at all, because it can do irreparable damage¹. NewTechWood says plainly that composite can't be sanded back⁷.

Owners notice. On Whirlpool in December 2022, strawb said they had scratch-tested samples of several brands before leaving them outside for six months. The Ekodeck sample did worst, and its "outer layer scratches very easily"¹⁶. A four-star Ekodeck reviewer on ProductReview found the thin plastic coating gouged when a tool was dropped on it¹⁴, and a NewTechWood owner wrote a month ago that the top layer scratches easily and sheds a furry layer when cleaned¹³.

Caps differ. One maker ranks its own lines for scratch resistance and puts its top line first¹⁰, but the warranties exclude scratching either way⁵. My advice is felt or rubber feet on the chairs and a mat under the barbecue, whichever board you choose.

Heat

None of the makers' documents cited here gives a figure showing that a cap on its own makes a board hotter or cooler. The makers do say that darker colours absorb more heat. One capped maker says its boards get hotter than timber in direct sun⁷, and another sells a capped line with heat-mitigating technology as its coolest board¹⁰. So I'd choose on colour first and treat a cooler-cap claim as something to check with a sample. If bare feet matter, read does composite decking get hot before you look at caps.

Warranty and price

The warranty gap between the two is easier to pin down than the price gap. ModWood sells both kinds. Its uncapped range carries 10 years against defects on a house deck, with no fade or stain cover, and its capped range carries 25 years plus a separate fade and stain warranty⁵. Another maker's uncapped range carries 15 years², and its capped range 25, though that capped warranty excludes natural colour change and staining¹.

One capped maker says the boards at the bottom of the market aren't capped at all⁸. Prices change by state and by month, so compare the capped and uncapped ranges from the same maker on the prices page. I've broken down what the warranty years actually pay for in lifespan and what warranties really cover.

When uncapped is fine

Under a roof, uncapped boards make sense to me. ModWood designs the brushed face of its uncapped boards for sheltered decks and says it shows minimal fade⁴. Uncapped also suits a utility deck behind a shed or around a water tank, where a stain is nobody's problem. It suits a job you plan to oil, too, because a weathered uncapped board can take some decking oils while the capped makers advise against any coating¹. Can you paint composite decking covers that in more detail.

I'd pick capped for a deck in full sun, on the shaded side of the house, or anywhere a barbecue lives.

Before you order

Cut a sample of each with a handsaw and look at the end. Drop a teaspoon of olive oil and a splash of red wine on both, leave them a day, then wash them with dish soap. Leave both in the sun for a month beside a fresh sample you kept inside. Scratch both with a key. By then you'll know which one you want. Hollow vs solid composite decking covers the other half of the choice. Once you've decided, the board calculator will tell you how many boards you need.