Where this starts
A deck is timber outdoors with air beneath it, which is the best place timber can be outdoors — as long as the water leaves. Every common failure is a place where it does not: a joint that holds water, a joist top that water sits on, a gap too small to drain, or a deck too close to the ground for air to move underneath.
PROBE THE JOISTS, not the boards. The boards shed water and dry on both faces; the joists are flat-topped, take the run-off from every gap, and are covered by the boards so they never see the sun. Push a bradawl into a joist top near a fixing and into the joist ENDS where they meet a beam or a wall. That is where a deck fails, and it fails invisibly until someone puts a foot through it.
SLIP IS A SEPARATE MATTER and it is worth being clear about: algae and moss on damp timber are among the most slippery surfaces in a domestic setting, more so when wet, and a deck in shade will grow them every year regardless of how sound the timber is. It is a cleaning and drainage question, not a decay one — but a deck that stays wet enough to be slippery is also a deck whose joists are staying wet.
AIRFLOW UNDERNEATH is the thing that decides both. A deck with clear ventilated space under it dries between showers; one sitting close to the ground, or with the perimeter closed in with solid boarding, holds damp air permanently. Closing in the sides of a deck for appearance is one of the most reliable ways to shorten its life.
THE FIXINGS TELL A STORY TOO. Corroded screws or nails, black staining around them in oak or cedar, or boards that have lifted at the ends, point at the wrong fixings for the timber — some timbers attack ordinary steel, and some treatments attack it faster still.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Rot in the joists rather than the boards
Most commonBoards that look sound but feel springy, movement near a wall or a beam, and soft material when a joist top or end is probed.
How to check: Probe the joist tops near fixings and the joist ends at the beam. That is where run-off sits and where decks fail.
Algae and moss on a surface that stays wet
Most commonA green or black film, dangerously slippery when wet, worst in shade and under trees, with the timber beneath still sound.
How to check: Whether the timber under the film is hard. Growth on sound timber is a cleaning and drainage matter, not decay.
No airflow beneath the deck
CommonA deck close to the ground or closed in around its perimeter, damp and musty underneath, with the underside of the boards darker than the top.
How to check: Whether air can move under it. A closed-in deck holds damp air permanently and dries only from above.
Gaps too small, or blocked with debris
CommonWater standing on the surface after rain, gaps between boards packed with leaf litter and grit, often on an older deck that has never been cleared.
How to check: Whether water drains through the gaps. Blocked gaps turn a drained surface into a flat one.
The wrong fixings for the timber
CommonCorroded screw heads, black staining spreading from each fixing in oak or cedar, and boards lifting or splitting at the ends.
How to check: Look at the fixings themselves. Some timbers and some treatments attack ordinary steel quickly.
Ledger or wall connection failing
Less commonMovement where the deck meets the house, rust staining at the wall, or a gap opening between the deck and the building.
How to check: Whether the deck moves relative to the house. A ledger connection failing is how decks collapse and it gives little warning.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Joists take the run-off from every gap, are never in the sun, and fail first and invisibly. Sound boards over rotten joists is the normal state of a failing deck.
- 2
A deck that dries between showers lasts; one sitting near the ground or closed in at the perimeter holds damp air permanently, and boarding in the sides is a common self-inflicted cause.
- 3
Gaps packed with leaf litter turn a drained surface into a flat one, which is both the slip problem and the rot problem.
- 4
Growth on sound timber is a cleaning matter; growth on soft timber means the same damp has already done structural work.
- 5
The ledger connection is the one part whose failure is sudden rather than gradual, and it is the least visible.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where any joist, beam or post is soft to a probe, since the deck's structure rather than its surface is affected and a foot can go through a sound-looking board.
- Where the deck is raised, since a raised deck failing is a fall rather than a nuisance and the posts and the ledger carry everything.
- Where there is movement or rust staining where the deck meets the house, because a ledger failure is sudden and is the way decks collapse.
- Where the deck has a balustrade that is loose, which is the component people lean on and is subject to a loading requirement.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.