Decking Slippery, Soft And Bouncy

What's happening?

A deck is a timber structure kept alive by staying dry and ventilated, and both failures you can feel come from the same cause. The slippery surface is a biofilm — algae and mould growing on damp timber, which is why it is worst on the shaded side and after rain. The soft, bouncy feel is decay in the structure below: joists and, above all, the posts and joist ends where water sits and cannot dry. Decking almost never rots first in the middle of a board; it rots at the end grain, at cut ends that were never re-treated, and where a post sits in soil or in a pocket of trapped water. That is why a deck can look sound from above and be well gone underneath, and why the springy patch matters more than the stained board.

What usually causes it?

• Algae and mould biofilm on damp boards, making the surface slippery in wet weather
• No ventilation gap under the deck, so the structure never dries out
• Decay at joist ends, cut ends and notches where the preservative treatment was breached and never renewed
• Posts set directly into soil or standing in water rather than on a base above ground
• Boards laid with no gap or with the gaps silted up, so water and debris sit on the joists below
• Joists spanning too far or spaced too widely for the board thickness, giving a bouncy feel with no decay at all
• Corroded fixings letting go so boards lift and the frame loses its stiffness, or the deck built hard against the house wall, holding water and debris against the brickwork

When should you call a landscaper?

Call a landscaper if any post, joist end or handrail post is soft, if boards flex underfoot, if the deck is raised and guarding is missing or loose, or if you cannot get under it to check. Treat a raised deck as urgent rather than routine — stop using it and get it looked at. Where the deck abuts the house, ask them to check what the levels are doing against the damp-proof course at the same time, because a deck built up against the wall is a common cause of damp inside.

What will the landscaper actually do?

• Lift boards and inspect the frame, probing posts at ground level and every joist end for decay, and check joist span and spacing against the board thickness to separate a bouncy deck from a decayed one
• Replace decayed posts onto bases that hold the timber clear of soil and standing water
• Renew failed joists and noggins, re-treating every cut end rather than relying on the original treatment
• Re-lay boards with consistent drainage gaps and corrosion-resistant fixings
• Form or reinstate guarding and a handrail where there is a drop, and ensure step risers are consistent
• Open up ventilation beneath the deck, pull it clear of the house wall so nothing bridges the damp-proof course, and finish with a slip-resistant treatment or grooved boards where it stays wet