How long does decking last? Timber and composite compared

Typical lifespans

How long a deck lasts depends on three things: the boards, the frame under them, and how wet the deck stays.

The Timber Decking and Cladding Association treats 15 years as the minimum service life for a quality installation, and says much longer is achievable with more durable timber. A composite warranty is the manufacturer's own statement about the boards. It usually says nothing about the frame or the fitting, so read what it covers.

The frame decides

Boards are what you see, but the joists, bearers and posts underneath decide how long the deck lasts. The Timber Decking and Cladding Association recommends that all substructure timber is treated to Use Class 4, the level for timber in contact with the ground or providing exterior structural support. Every cut end needs a brush-on end-grain preservative, because cutting exposes untreated wood at the very point water gets in.

Composite boards do not rot, but they are usually laid on a timber frame that can. A composite deck on a poorly treated frame lasts as long as the frame, which is why some fitters use aluminium joists under composite boards. Whatever the boards, ask what the frame is made of and how it is treated.

What shortens a deck's life

Most decks that fail early fail for the same reasons.

• Ground contact. A bearer sitting on soil holds water against the timber all year.
• No air underneath. A deck built close to the ground, with no gap and no drainage, stays damp.
• Shade and debris. Leaves and soil packed into the gaps between boards hold moisture and feed algae.
• Tight boards. Boards need a gap between them to drain and to move.
• Untreated cut ends and wrong fixings. Ordinary screws corrode and stain; decking needs coated or stainless fixings.
• Planters and hot tubs. Constant water and heavy point loads on a frame not designed for them.

A deck that meets the house also needs to sit at least 150mm below the damp proof course and must not cover airbricks. Getting that wrong can damage the house as well as the deck.

Signs decking needs replacing

Walk the deck and look underneath where you can. Replacement is likely when you find:

• Boards that feel soft or spongy underfoot, or that a screwdriver pushes into easily.
• Fungal growth on the frame, or joists crumbling where they sit on bearers or posts.
• A deck that moves or wobbles when you walk across it.
• Screws that turn but no longer hold.
• Posts rotted at ground level.
• Boards splitting along their length or cupping badly.

If the frame is sound and only the boards are tired, re-boarding on the existing frame costs much less than starting again. If the frame has gone, the deck needs rebuilding from the frame up.

Timber or composite for a longer life?

If the aim is the longest life for the least work, composite boards on a well-treated or aluminium frame come out ahead: no rot, no annual oiling, and colour that holds for years. Their weak points are heat, since dark composite gets hot in full sun and expands, and repairs, because a discontinued colour is hard to match. Softwood is cheaper to build and easy to repair board by board, but it only reaches its lifespan with yearly care. Hardwood sits between the two: long-lived and repairable, at the highest price. In a damp, shaded garden, the frame and the ventilation matter more than any choice of board.

Making a deck last

Sweep it often and clean it at least once a year, clearing the gaps between boards. Softwood needs re-treating every year or two with a decking oil or preservative; hardwood benefits from oiling to keep its colour; composite needs only cleaning. Check fixings each spring and replace any that have lifted. Keep soil, plant pots and leaves from piling against the frame, and keep the space underneath open so air can move. Every board type turns slippery in shade, so cleaning is about safety as much as looks.

Replacing a deck: what it costs

Decking typically costs £60 to £110 per square metre supplied and fitted. Treated softwood sits around £60 to £80, composite around £90 to £130, and hardwood around £110 to £170. Composite costs more at the start and much less in upkeep, so over 15 years the gap narrows. A deck raised more than 30cm above the ground normally needs planning permission, so check that before replacing a low deck with a higher one.