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What to check, and in what order

My deck boards have cupped or curled

Which WAY the boards cupped tells you which face is wetter. Cupping upward at the edges means the underside is wetter than the top; cupping downward means the top is. Almost every cause below is a way one face stays wetter than the other, and the direction points straight at it.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

Timber moves across its width with moisture, and it moves unevenly because a sawn board's two faces have different grain orientation and different exposure. Cupping is the visible result: the board becomes a shallow channel or a shallow crown.

The direction is the diagnostic. If the edges are HIGHER than the middle — a channel that holds water — the underside is wetter than the top and has swollen relative to it. If the edges are LOWER — a crown that sheds — the top face is wetter or the underside has dried harder. On a deck, the first is far more common, because the underside is shaded, close to the ground and slow to dry while the top is rained on and then dried by sun and wind.

That is why most of the causes below are about the UNDERSIDE and the space beneath the deck rather than about the boards or the finish. A deck built close to damp ground with poor airflow beneath it will cup whatever is done to the top surface.

The related question is which face was up. There is a long-standing argument about laying boards bark-side up or down; the more useful point is that the two faces of any board respond differently, and a board finished on one face only will always be the more unstable for it.

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.

  1. The underside stays wet — poor ventilation or clearance

    Most common

    Edges higher than the middle, boards forming channels that hold water, and worse where the deck is closest to the ground, most shaded, or most enclosed at the perimeter.

    How to check: Look underneath. Damp ground, no airflow, or a skirt board closing the perimeter each identify themselves.

  2. Finished on the top face only

    Most common

    Cupping across a whole deck laid at one time, with a coated top surface and a bare underside — the two faces exchanging moisture at different rates by design.

    How to check: Look at the underside of a board at an edge. Bare timber under a coated top is the commonest build sequence and the commonest contributor.

  3. Boards laid at the wrong moisture content

    Common

    Movement that appeared within the first season, together with gaps that have widened or closed noticeably since installation.

    How to check: Compare the gaps now against what they were when laid. Large changes in both gap and cup point at the timber arriving wet or very dry.

  4. Fastened in a way that prevents movement

    Common

    Splitting at the fasteners as well as cupping, or boards that have cupped between fixings while staying flat at them.

    How to check: Look at the fasteners. One fixing per bearer per board edge behaves differently from a single central one, and both differ from hidden clips.

  5. Water trapped on the surface

    Common

    Cupping concentrated where water stands — under planters, along a wall, at a shaded end — with staining or growth in the same places.

    How to check: Note whether the affected area is where water sits. A pattern that matches the puddles is a drainage issue rather than a timber one.

  6. Bearers too far apart

    Less common

    Boards that both cup and dish between supports, with noticeable flex underfoot between bearers.

    How to check: Measure the bearer spacing against the board thickness and the species. A board spanning too far deflects as well as cups.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    It identifies which face is wetter, which is the single most informative observation on the page and separates the underside causes from the surface ones immediately.

  2. 2

    Most cupping is an underside moisture problem, and ground clearance, ventilation and a closed perimeter are all visible without disturbing anything.

  3. 3

    A board coated on one face only exchanges moisture unevenly for its whole life, and it is the contributor most often built in and least often noticed.

  4. 4

    It tells you whether the timber arrived wet and has shrunk, or arrived dry and has swelled, which changes whether the movement is finished or ongoing.

  5. 5

    Local cupping that matches the puddles is a falls and drainage problem, and no amount of work on the boards will fix it.

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.

Deck Board Calculator →Deck Joist Span and Size Calculator →Deck Stain Calculator →

When to stop and get somebody in

  • Where boards flex noticeably underfoot as well as cupping, which points at the bearers and their spacing rather than at the boards.
  • Where the structure beneath is in contact with damp ground and the timber there is soft to a probe, which is a structural condition rather than a surface one.
  • Where the deck is raised and any part of its support, its posts or its connections to the building show movement, splitting or corrosion.
  • Where a large area needs replacing, since the cause is likely in the ventilation, the clearance or the falls, and relaying boards over the same conditions repeats the failure.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.