This free standing roundhouse is an 8.5mtr outdoor classroom for Brixham Cofe School. The posts sit on stone at ground level. The school hired me to build the timber frame and will be adding the turf roof and membrane themselves.
I recon the buried post method is in appropriate for turf roofed public buildings. Turf roofs are a heavy load to support over the heads of many people and need to be secure for a couple of decades. The buried post method will inevitably rot and fail. It is fine for personal use where the builder/owner is around to keep an eye on it but quite a risk if it’s a pubic building somewhere else. I have built buried post roundhouses for schools before, like the one pictured below for a school in London. This uses light timbers and a soft top. I have written a fact sheet for lili about roundhouses, which talks more about this issue. Download at http://www.lowimpact.org/factsheet_roundhouses.html
All components in the Brixham roundhouse are larch. Larch is at the low end of the acceptable range of out-door durable construction timbers. Larch is however cheaper than chestnut and that is why we’ve had to use in on this occasion. There are 10 up-rights at about 12” thick at the top end to accommodate the ring beam tenons. These tenons are 5’x5”x3” and are pinned in place. The ring beams are about 12”x3”. Every up right is wind-braced with an 8”x2.5” beam mortised into the up right with a side shoulder tenon. Each primary rather is pinned at the apex and housed and pinned at the post end. I dropped this reciprocal roof in the conventional way. I set the rafters up at 18dec and after the drop ended up with 16dec. I tend to work out the roof geometry on paper first, (try it for real at ground level first if you are un-sure). I’m finding now that my roofs are quite predictable dropping about 6” so I can be fairly accurate in setting the final roof pitch. The secondary rafters are pinned to the primary rafters and to an up right mounted on the ring beam. The edge boards are 8”x3” and coach screwed and pinned. We have battened the roof with milling off-cuts.