Short answer
A good capped composite board typically lasts 25 to 30 years or more, and most makers back that with a 25-year warranty. In practice the frame underneath usually decides how long the deck lasts: treated timber joists not rated for ground contact can rot in 10 to 15 years, while an aluminium frame outlasts the boards.
In this guide
Composite decking is sold on its lifespan, and the headline figures are long: 25-year warranties are normal on capped boards, and some makers go further. But a deck is more than its boards. Here is what actually wears out, and in what order.
Typical lifespans
| Part | Typical life | What decides it |
|---|---|---|
| Capped composite boards | 25 to 30 years or more | Quality of the cap, colour, sun |
| Uncapped composite boards | 10 to 20 years | Staining and mould in the grooves |
| PVC boards | 20 to 30 years | UV stabilisers, colour |
| Treated softwood frame, ground contact (Use Class 4) | 15 to 30 years | Treatment level, drainage |
| Treated softwood frame, above ground only (Use Class 3) | Often under 10 years if damp | It was the wrong timber for the job |
| Aluminium frame | 40 years or more | Very little |
| Hidden clips and screws | As long as the frame | Stainless steel or coated fixings |
The frame decides how long the deck lasts
This is the point most people miss. A composite board on a cheap frame is like a new roof on rotten rafters.
Most decks are framed in treated softwood. The treatment is graded by “use class”. Timber for posts and joists that touch the ground, or that stay damp because they are close to it, should be treated to Use Class 4. Timber treated only to Use Class 3 is meant for above-ground use where it can dry out, and in a low deck on wet ground it can rot within a few years.
When you compare quotes, ask:
- What are the joists and posts made of?
- What use class is the timber treated to?
- Will cut ends be re-treated on site?
- Is there a membrane and gravel under the deck so the frame is not sitting on wet soil?
An aluminium frame costs more but removes the question entirely. It is worth it on decks you never want to lift, on decks laid over old paving where water collects, and on low decks where airflow is poor.
What wears out first on the boards
Even good composite changes over the years. In rough order:
- Colour. A slight lightening in the first year, then very little change on capped boards. Uncapped boards keep fading.
- Surface marks. Furniture scrapes and dropped items leave fine scratches. Unlike timber, these cannot be sanded out, though they become less obvious as the surface weathers evenly.
- Grooves and joints. Dirt, moss and algae collect between boards and in grooves, especially in shade. This is a cleaning job, not wear.
- Expansion problems. If the original gaps were too small, boards can bow or push against walls. This is a fitting fault rather than ageing.
What shortens a deck’s life
- Standing water. A frame laid flat in a dip, with no fall and no drainage, rots and moves.
- No airflow. Low decks boxed in on all sides stay damp underneath.
- Joists too far apart. Boards sag and crack at the fixings over time.
- Heavy loads the frame was not built for. Hot tubs, large planters and outdoor kitchens need extra joists or their own base.
- Rough cleaning. A pressure washer held too close can damage the cap.
Signs your deck is reaching the end
- Boards bounce or sag between joists.
- The deck moves or rocks when you walk on it.
- Boards are cracked at the screw holes or clips.
- A screwdriver sinks easily into the joists or posts (on a timber frame).
- Persistent mould that does not clean off, usually on uncapped boards.
If the boards are sound but the frame is failing, it is often possible to lift the boards, rebuild the frame and relay them. With clip fixings this is far easier than with face-screwed timber.
Warranties: read the small print
Board warranties usually cover rot, splitting and structural failure, and sometimes fading and staining beyond a set limit. They usually do not cover the frame, damage from poor fitting, or normal weathering. Most also require the boards to have been fitted to the maker’s instructions, including joist spacing and expansion gaps. Keep your installer’s paperwork and the board details safe.
To see how capped, hollow, PVC and aluminium boards compare on warranty and price, see the comparison on our composite decking Leeds homepage.