To build a cabin, part 5
Trip Start
Nov 27, 2009
1
78
146
Trip End
Ongoing
Aigurande, Centre, France
Tuesday, December 6, 2011
Part 5
Roof raising!!! Peter, Julie, Laura and I took the dogs to the woods to lift the main rafters into place. This is the bit when we have to figure out the reciprocal frame. Tony Wrench's book, "Building a low impact roundhouse", gives lots of info on it but there are some differences between the materials we are using. Whereas the book shows the wall plates and uprights being pegged and strapped with metal before the rafters go on, we're using wooden pegs. The book shows how to make a shallow slope for the turf roof which doesn't want to shed too much water but we're making a steep apex so the rain runs away.
The essential principle is that rafters (roof beams) sit on the one before in the centre so there is no central pillar in the house. In order to put the rafters all up, the first one gets propped up on a forked stick. Then we go round the circle putting up the rafters in turn and the last one completes the circle going under the first one to support it and sitting on the previous one. Our dummy run on the ground ended up with a very tight ellipse in the middle. It was a bit squiffy but it stood.
When the forked stick is pulled out from under the first rafter, the whole roof drops a bit in the middle as it settles into place. The beams squeeze onto each other and the angle of the roof reduces slightly. We'd thought that we'd peg each rafter into the wall plate and upright as we went along. The dummy run educated us.
We tried lifting a rafter onto the forked stick in the middle and resting it on the wall plates. We wondered about just putting all the rafters up and then fixing them. It became clear very quickly that this wasn't going to work either.
The problem is that if we pegged it as we went along, then when the forked stick in the middle is taken out, the roof drops a bit and this changes the angle of the joint between the rafter and the wall plate. If there is a peg connecting them, it'll get bent, maybe it'll snap and maybe it'll hold. If it holds, will it prevent the rafter from dropping fully into place?
Then we had another idea - we'd put the rafters up, pegging as we went. Before removing the forked stick from the middle, we thought we'd smack the ends in the middle where they met to push them off centre and tighten the circle. Our dummy run educated us. We were pitching a steep roof so when a beam was pushed away from the centre line, it slipped down the rafter it was sitting on. This in itself would change the angle at the joint at the wall plate and we would have the same problem with snapping pegs.
Finally we decided to put the first rafter up and peg it. Then place the other rafters into position. Each rafter was notched to fit on the wall plate. The sticky out bits of the joint of the wall plates was trimmed. Chainsaw action again. Once the rafter sat fairly well on the joint, Peter lashed the rafter at the top to the previous one and I lashed the rafter to the post at the bottom. The wall plates are still not secured so they can move around under the rafters. The rafters are about 12cm diameter beams of sweet chestnut about 4 metres long. By the end of the day, there were 12 of them positioned on unsecured wall plates just tied on with bits of baling twine. We don't know how close to breaking strain we came, we only know that it didn't fall down as we were putting it up. It's a lot more nerve wracking putting together the tree trunks than our discussions over matchstick models.
At last we'd got all the rafters up but the first one was still supported on a forked stick in the middle and it did not rest on the one beneath. Also none of the joints were pegged except the first - so if the wall plates were hit with sufficient force, they could theoretically be knocked off the uprights. Currently everything was sitting in place held by the weight of the 12 rafters but we suspect it was fairly precariously held!
The rafters were supposed to be too long. We worked out the length we needed and added an extra half metre. Not sure what happened but they were only just long enough. In places we may not have as much overhang as we'd like to protect the cob walls. We think we've enough to balance the roof properly though. Time will tell if we're right on that one. So even though the beams were awkward and heavy to lift and manoeuvre, we'd go for much longer. They can always be trimmed.
Removing the roof support
One of the best things we did (all credit to Peter for thinking of this in advance) was to put a plank down on the ground underneath the forked stick that held the whole thing up. A rope around the base of the stick was all that was needed to tug it out and it slid along the plank bit by bit whilst we were outside of the circle. None of us were sure if it would stay up and we were now working in twilight. We'd been chasing the daylight and the sunset had beaten us but we wanted to see what would happen when we took the prop away.
It creaked and shifted. The top dropped a bit which it needed to do as the first rafter was much higher than the last. The twine holding the ends of the rafters became tighter. Finally the forked stick was no longer holding the roof up and the roof was still up! A few celebratory dances around and we examined as best we could the seating of the roof. The first rafter was still not sitting on the last rafter and the peg was bent. Not much we could do without light so we headed to the supermarket to buy some fizzy plonk. 99p gets a bottle of very drinkable fizz. A mini celebration at the end of a very successful day!
