Composite decking problems: heat, algae, subframe rot and how to avoid them

Composite decking is sold as the deck you never have to think about. Mostly that is true. But there are real problems, most of them come from how it is fitted rather than what it is made of, and almost all of them are avoidable if you know about them before the boards go down. This guide lists the nine we get asked about, why each happens in a British garden specifically, and what to do. We sell composite decking; we would rather you knew this now than found out in July.

The short answer

The problems that matter are heat in direct sun, surface algae in shade, a subframe that rots under a board that does not, expansion gaps that were not left, and colour that shifts in the first months. Choose a mid-tone colour, build the frame from rot-proof material with the right joist centres and gaps, keep the surface swept, and none of these will trouble you.

1. It gets hot in the sun

The problem. Composite absorbs more heat than timber and a dark board in full sun on a hot afternoon can be uncomfortable barefoot. Black and chocolate boards run hottest; ash, silver and grey run coolest.

In the UK. This is a handful of days a year in most gardens. A south-facing deck with no shade in a heatwave is the case to think about.

What to do. Pick a lighter finish if the deck faces south and has no shade; our colour guide ranks them. A parasol or pergola over the seating area fixes it entirely. A hose cools the surface in seconds.

2. Algae and green film in shade

The problem. Any outdoor surface that stays damp grows algae, and a north-facing deck under trees in a wet British winter is exactly that. Composite does not rot from it, but a green film is slippery and looks poor.

What to do. Sweep leaves before they sit, and wash the deck with a stiff brush and a composite deck cleaner or diluted washing-up liquid twice a year, spring and autumn. Improve airflow: the installation guide asks for 40mm of clear space under the frame for this reason.

3. The board lasts, the frame does not

The problem. This is the one that catches people out most. A composite board carries a 10 year warranty and will outlast it. If it is screwed to treated softwood joists sitting on damp ground, the joists rot in eight to twelve years and the deck fails from underneath while the surface still looks new.

What to do. Build the frame to last as long as the boards. On concrete or slabs, low composite battens are rot-proof and cheap. Raised off soil, recycled plastic joists and posts cost about double timber and never rot. If you use timber, use ground-contact treated joists, keep them off the soil on pads or posts, and leave the 40mm airflow gap.

4. Boards that bow, lift or creak

The problem. Composite is less stiff than timber. Laid on joists spaced too far apart it flexes, and flexing boards work their clips loose and creak.

What to do. Joist centres at no more than 350mm, and closer, 300mm, if the boards run at 45 degrees. Every board fixed at every joist with a clip. Store boards flat before laying on battens at 400mm centres, covered, so they do not bow before they are fitted.

5. Expansion gaps that were not left

The problem. Composite moves more with temperature than timber does. A 4.8m board grows a few millimetres between a frosty morning and a hot afternoon. Boards butted tight together end to end, or against a wall, push against each other and buckle or lift at the ends.

What to do. The gap depends on the temperature on the day you fit: 9mm end to end below 4°C, 6mm between 4 and 24°C, 3mm at 25°C or above, and at least 20mm from any wall. These are the figures our boards are made for. Side gaps are set by the clips at 6mm.

6. It changes colour in the first months

The problem. Every composite board lightens slightly over its first three to six months outdoors as tannins in the wood fibre react with light and rain, then settles. People see the shift and think the deck is fading.

What to do. Expect it. Buy for the settled colour, not the day-one colour; when torn between two shades, the darker one usually lands where you wanted. Order all your boards from one batch so they settle together. After the first six months the colour holds; that is what the 10 year warranty covers. The colour guide shows what to expect from each finish.

7. Scratches and stains

The problem. Composite can be scratched by dragged furniture and grit, and grease from a barbecue can stain an uncapped board if left.

What to do. Felt pads on furniture feet. Wipe grease the same day with warm soapy water; a mat under the barbecue. Light scratches on a woodgrain board blend in with weathering within a few months. The grooved face hides scratches better than the smooth woodgrain face, which is one reason our boards are reversible.

8. Hollow boards and water

The problem. Our boards, like most at this price, have a hollow core. Hollow boards are lighter and cheaper. If the cut ends are left open, water and insects get in, and in a hard frost trapped water can crack a board end.

What to do. Cap every exposed end. End caps are £1.49 each in six finishes; where a cap does not exist for the colour, finish the end under an L-trim or behind a fascia board. Lay boards with a slight fall so water runs off rather than sitting.

9. It is not wood

The problem. Some people buy composite expecting real oak and are disappointed up close. It is a very good impression of timber, and it is not timber.

What to do. Order free samples, leave them outside for a week, and decide with the real thing in your hand. If the material itself matters to you, buy hardwood and commit to oiling it. Our composite vs timber guide is honest about where wood still wins.

Problems you will not have

Splinters, rot in the boards, annual staining, warping, and boards going soft. Those are the timber problems composite was invented to remove, and it does. Slips are also rarer than on timber: wet timber with algae in the grain is treacherous, while both faces of a composite board are textured for grip, and the grooved face especially so.

Before you buy

• Decide on the frame material first; it decides whether the deck lasts 10 years or 25.
• Pick a colour for the sun the deck gets, using samples outdoors.
• Read the installation guide and count the clips, caps and trims into the budget. The cost guide does this for three deck sizes.
• Call 0161 399 1792 with your deck size and we will list everything you need, frame included.