Millboard Decking Expansion and Contraction
October 26, 2020
Millboard decking does expand and contract, although very minimally in comparison to both solid timber decking and traditional composite decking. Millboard typically expands/contracts at a rate of 0.1% of the length of the board, although it may reach 0.2% in some exceptions. Please note that two boards may move at the same end, so the end gap can be larger/smaller than normal.
You should also be aware that the polyurethane surface of Millboard tends to contract in hot weather and expand in the cold weather. This means that in warmer weather you may experience some gaps appearing in your deck. These should close up again once cooler temperatures return.
Thanks to the minimal expansion and contraction, Millboard planks (176 and 200mm wide) can be installed with as little as 4mm gap between lengths, which is very minimal for the width of the planks. In comparison, a 6-8mm gap between lengths is recommended for most solid timber decking planks between 130-140mm wide, and a 5mm gap between lengths is commonly recommended for traditional composite decking between 136-140mm wide. With Millboard you get wider boards and narrower gaps - a better look overall!