The handy Multi-Spacer Decking Tool makes quick work of accurately spacing Millboard Decking boards at 3, 4, 5 or 6mm gaps. While a 4mm gap between boards is recommended, there are always times when you need to juggle dimensions to fit a space, and with this tool every gap-width required will be professionally consistent, however large your installation. To ensure consistency, the multi-spacer should be used above every other joist.