Decking joists should be laid on a solid flat surface. Uneven paving, sand or grass could cause the installation to move which could in turn cause damage.It is recommended that joist centres are no greater than 300mm, except when fitting boards on the diagonal where joist centres are 250mm - you should work from the centre out. If used above ground level 250mm centres are recommended.You should use double joists under butt joints, with one clip each side of the butt joint and an 8mm end to end gap for drainage and expansion.