just about top start laying the joists for the decking,it extends 3.6 from house-joined to back wall with joist hangers with supporting walls at 1.8m and 3.8m the joists are 8x2 ,the decking is just over 1 inch thick,would 20 inch centers be adequate,will save me going and buying a few more joists if i could get away with this size,done a quick test run and it does seem pretty solid,or am i asking for trouble in the future with this spacing?