Wide board ‘cupping’

Q. We are replacing old floorboards and have been reading about timber cupping that could happen to wider timber boards, what would you recommend 130mm x 19mm or 85 mm x 19 mm?

ANSWER:

Wide boards stay flat

Wide boards stay flat in stable conditions – photo: timber+DESIGN

Floorboards will cup if they are exposed to wetting or drying from one side. For example, flooring suspended over a damp site will absorb moisture from underneath and the underside of the board will swell more than the top, making it cup. Similarly, flooring exposed to sun shining on it will lose moisture from the top more than the bottom, making the top surface shrink. Again the result is cupping. Narrow boards will still cup under these conditions, it’s just less obvious than with wide boards. However, under stable conditions (no moisture underneath and no drying from the top) wide boards will stay flat, assuming they have been correctly kiln dried and installed. Many older buildings have wide floorboards that have given years of good service. Note that secret or concealed nailing on only one edge is not recommended for boards wider than 85 mm. Wider boards are required to be fixed with two nails at each point where they cross a joist.