Turf roofs are a bit of a scam, because the turf layer isn’t really doing anything apart from looking pretty and making the roof really heavy. What is actually giving you a rain-proof shelter is the membrane that goes under the turf, and in our case that’s heavy butyl rubber, very expensive and normally used as high-end pondliner. There is a cheaper polypropylene material which is supposed to last a lifetime (no-one specifies how long you’re going to live, of course), and we’re going to try that out on the shed-type spaces (mushroom-sheds, garden-tool sheds etc), as it’s a lot cheaper. It’s not completely true, of course, to say the turves are doing nothing, as they should protect the membrane which will eventually perish and go brittle if left exposed; but I’m very dubious about their so-called insulating properties. If you’re a hardy ancient Scandinavian you might notice them insulating you a bit, I suppose. The traditional membrane was made with peeled birch-bark, but we’re not inclined to go decimating what birch-woods we have left around here for the sake of these roofs, as I assume peeling the tree will kill it.
The membrane layer is not, according to the recipes, the only one required. You’re supposed to put on a geotextile layer as an underlay (and as expensive as the rubber layer), and I think another one on top of the rubber. This latter may be to protect the rubber from puncturing (who by? marauding fork-wielding savages? Could be), but it could also be to stop the turf slipping down on the smooth surface of the rubber. As we’re not too keen on paying the extra cost, we’ve tried to counteract this effect on the roof of Robin’s gang-hut by making the pitch very shallow, which means that the supporting timbers of the roof have to be extra strong, as you’re not relying so much on the walls to carry the weight of the centre parts of the roof. On the roof over the little caravan I have a much steeper pitch, and I’ve tried to counteract the slippage tendency there by bracing the turves against the ground – ie. the roof comes right down to the ground on one side. We’ll see how this works. For the “underlay” layer we’ve used a thick layer of straw: this is to protect the rubber from puncturing or point-pressure from the roof-timbers below it, and I guess also to give a bit of insulation.
Additional details of structure: on Robin’s roof we used a main structure of large tree-trunks for beams, these being supported on massive uprights, with the cob wall built around them. They say cob can support a heavy roof like this, but I don’t trust ours to do that (I think Robin’s walls have been made too thin anyway). 3×3 timbers are laid across the big beams at 2- or 3-foot intervals, then threequarter-inch boards over these, then the straw, the rubber layer and the top layer of turf, which is perhaps as much as six inches deep. It’s done ok for the past three years.
The turf-edges on Robin’s roof are held in by what I think’s called a barge-board; I’m going to try something different involving stones on the caravan over-roof. I’ll post details and comments in due course.