Hmmm. Well I did think that the Heather Specification which i buy from the SGD had probably been properly researched and would give a watertight and best practice specification so I based the spec on that combined with modifying the sub-frame spec I had for a Millboard installation. I did look at some span tables too. since the posts are at 1.2m centres in both directions, the maximum span is never reached, so i’m happy with that.
Thanks very much for your input Karl and for getting back to me so promptly.