Composite Decking Went Black. Now What?

Composite decking does not rot and does not need staining, which is why so much of the island's rebuilt deck stock is composite. What it does do is grow mould in the surface texture — the deliberate grain pattern pressed into the board — and on a shaded, humid, bay-facing deck that happens within a few seasons.

Why it is not a stain

The black is organic and it is living in the texture, not soaked into the material. That is good news: it comes off with a cleaning solution and dwell time. It is also why scrubbing at one board tends to lighten it slightly and leave the rest.

The mistake that cannot be undone

A pressure washer on composite erodes the surface unevenly, and it leaves stripes in the pattern of whatever the wand was doing at the time. Those stripes are permanent — you cannot sand a composite board the way you can a wood one, because the colour and texture are the surface. Most composite manufacturers publish a maximum pressure and it is far lower than a typical consumer machine delivers.

This is the single most common piece of self-inflicted deck damage seen on this island, and it usually happens on a Saturday in June with a rented machine.

What the right job looks like

Low pressure, a cleaner suited to composite, enough dwell time for it to work, and a thorough rinse. It is a slower and less dramatic process than blasting a wood deck and the result lasts considerably longer.

If your deck is actually a mix

Plenty of island decks are composite boards on a wood frame with a wood or cable rail, and some have a wood top rail on composite posts. That matters, because the rail may want stripping and re-coating while the deck wants nothing of the sort. Worth establishing before anyone quotes — and worth asking the contractor to say in writing which surfaces are getting which treatment.

The slippery part

A green slick on a north-facing deck is not a cosmetic issue, it is a fall. On a raised island deck with steps down to grade it is worth dealing with before the season rather than after somebody notices.