To describe the bow, or bend, as a result of shrinkage. Usually applied to timber planks that have been exposed to damp, excess humidity, or flood- the water damage causes the planks to cup across the width.
Timber cups with the dip of the cup towards the bark side – so the curved top of the cup will be towards the centre of the tree.
It’s said now that timber should always be installed with the bark side down. ( Look at the sawn ends and imagine the tree rings, and bearing in mind the way timber is cut, leaving a square core – each piece has the growth rings showing – so the widest curve will be the bark side.) This is because, apart from water damage, as the wood ages and dries, natural shrinkage creates some cupping
In the past, pre- underfloor insulation and central heating, timber floors were often damp and cold. To avoid the cupping they were laid alternately – one side bark down,then the next bark up. They worked with and against each other, to minimise unevenness. The tongues – whether loose or machined- and the particular nailed fixing further helped to keep them in place.
I’ve been told by several retired joiners that the practice was always to pre-lay the floor boards, upside down, at the outset of any building project – new or re-furbishment. This gave them the best chance to weather in, to take on the humidity of the place, to adapt, before they were turned over and fitted in place. All shrinkage was taken up within the skirting board or wall panelling kick boards, and all damage kept to the underside.
With a solid timber floor that has been well fixed, the result of any serious cupping can be sanded out and the surface re-finished.
There is nothing much to stop a floating floor from cupping, which is fine in a controlled environment, but creates major problem with any accidental water ingress.
Planks can be ordered or specified with – what the best companies offer this as standard – grooves along the bottom side, which prevent or at least limit the cupping – for two reasons, firstly to make space for the cup, the concave side to move into and also to reduce the density than can be affected.