The point about the span needed to be covered between the outer circle and the inner trilithon horseshoe has been discussed before (A. Vayson de Pradenne, Antiquity, Vol 11, pp87-92). He suggested that a distance of about 9 metres (with a pitch of roof at ~40%) would be needed between the two sets of stones. However, there would still be the question of how to get a beam across the open arms of the horseshoe, unless the area within the horseshoe was uncovered.....