Looking along the cladding, some boards have a visible curve to them rather than lying flat, and where they've bowed the fixing heads are standing slightly proud of the surface, sometimes with a faint rust stain running down from each one.
Timber cladding moves with moisture by its nature, so some seasonal movement is normal. Cupping specifically — bowing across the width of the board — is a sign that the two faces of the board are experiencing different moisture conditions, and that points to how the boards and the wall behind them were detailed.
What cupping actually tells you
A board cups when one face absorbs or releases moisture at a different rate to the other. If the outer face is exposed to weather while the inner face sits against a sealed or badly ventilated wall, the two faces are always going to be at different moisture levels, and the timber bows towards whichever side is drier.
Causes, in order of likelihood
Several detailing choices at installation typically combine to produce this fault, and it's rarely down to a single cause alone.
- Boards fixed too tightly together with no allowance for the board width expanding and contracting with moisture
- One face of the board sealed or treated while the other was left raw, creating an imbalance in how each face takes on and releases moisture
- No ventilated cavity behind the boards, so moisture that gets behind them has nowhere to dry out to
- Fixings placed in the wrong position across the board's width, restraining it in a way that encourages cupping rather than allowing it to move evenly
How to check which cause applies
Check whether there's a genuine air gap behind the boards by looking at the detailing where the cladding meets a sill or corner. A cavity that's been closed off, or one that was never formed with vents at top and bottom, will trap moisture behind the boards.
Look at the back face of a board if you can access one, ideally where a board has been removed or is loose. An untreated back face next to a treated front face is a clear and common cause.
Check where the fixings sit relative to the board's edges. Fixings placed too close to one edge, or in a pattern that doesn't allow the board to move as a whole, will encourage distortion concentrated around those fixing points.
What a survey of the cladding should record
A useful inspection should note the pattern of which boards are cupping — whether it's isolated to one elevation, one orientation, or spread evenly — since a fault confined to the most weather-exposed side of the building points more strongly to moisture imbalance from exposure than a systemic detailing fault.
It should also record whether a ventilated cavity is present and functioning, by checking for vents at the top and bottom of the run, and whether the back face of a sample board is treated to the same standard as the front.
Seasonal behaviour worth noting
Cupping often becomes more visible after a run of wet weather, as the outer face absorbs moisture faster than the inner face can equalise with it, and may appear to improve slightly in a dry spell without the underlying imbalance actually being resolved.
Boards that seem to flatten out over a dry summer and cup again each winter are not fixing themselves — that seasonal cycle is itself evidence of the ventilation and treatment imbalance behind them, and it tends to worsen gradually over several cycles rather than staying the same each year.
Common questions
Is cupping just cosmetic, or does it cause further damage?+
It's more than cosmetic — cupped boards create gaps that let water track behind the cladding, and popped fixings leave holes that let water in directly, both of which accelerate further deterioration once they start.
Will the boards flatten out again if conditions change?+
Some minor seasonal movement can improve or worsen with the weather, but boards that have genuinely cupped from a moisture imbalance rarely return fully flat, and the underlying cause needs correcting rather than waiting it out.
Does the type of timber affect how likely this is?+
Yes — some species and board profiles are more prone to movement than others, but the fixing pattern and ventilation detailing behind the cladding matter more than the species alone in most of the cases we see.
Book a free inspection of your cladding
We'll check the cavity, the board treatment and the fixing pattern, and tell you what's actually driving the cupping.