Begin installing composite joists by laying your joists directly onto your concrete base. The spacings (or centres) of your joists should be 250-350mm, and they should ideally run at right angles to the direction of your deck boards. Ensure that your joists are orientated with the guidance channel facing upwards before attaching them to the concrete base (this will assist with screwing and water drainage). A gap of approximately 8mm should be left between joins in joists, allowing for expansion. Double joisting is required where you expect deck boards to meet at butt-joins.