Saturday, 26 March 2011

Roof going round in circles

The foundation is racing ahead, but it's still not clear what is happening on the roof.  This needs to be decided so that the order can go to pre-cut the wood, and until that happens there will be no pillars, which will be needed to hold the roof up.

The South-facing roof is solar. Until recently solar roofs meant building a normal roof (compliant with the regulations for fireproofing and waterproofing) and then solar installers coming along and drilling holes in it to fit on their panels. Drilling holes is fair enough for retro-fits, but for new builds this seems foolish, especially if you're trying to get a highly insulated, airtight roof. Using a different construction does not meet their installation requirements and you lose their guarantee, and the panels themselves don't qualify as roofing. 

Last July one roof maker, Caname, who started out making roofs for temples, introduced a solar roof that meets the regulations for roofing, so there is no need for a double roof and, at least in terms of design, a more elegant solution is possible. Economically this should also make sense, but design simplicity does not always correlate with economics.

So, all we have to do it build up to the rafters, then Kaname will put their roof on top of it.  As long as they've got something they can put a waterproof sheet over, and into which they can screw the corrugated steel roofing which they mount the panels onto, everything will be fine.  The Kaname construction allows air flow under the panels, which is important to keep temperature down on the panels, which keeps efficiency up. Corrugated steel should work well for this, increasing the surface area and taking heat away by convection and conduction.

It's usually practice in Japanese building to have an air gap between the insulation layer and the external wall or roof.  As Kaname are putting corrugated steel on top, as well as providing air flow for the panels, this should also provide an air gap for the insulation. The reason for the air gap is in case humidity builds up and needs somewhere to go. Not everyone believes this is a good idea, and any gaps potentially can attract insects or even bats. The practice seems to have evolved to cover up for any problems with humidity, although, in theory at least, a well designed wall should not allow humidity to build up to turn to condensation. Also in theory, the gaps around the corrugated sheet should work both ways, but I've been battling both the roofer and the insulator (two different contracters) who seem keen to have things done their own way, and are looking at an assembly of parts rather than the whole, which was the main reason why Kaname seemed so appealing. 

However, there's also the west wall. In our wisdom, we decided that the house should not be square.  As the plot is not square, and non-square rooms do interesting things with space perception, this seems to be the correct decision long term, but when it comes to this issue, (and no doubt countless others that will emerge) it's causing problems. Kaname only make square roofs, so there is going to be a one metre overhand at the South west corner that needs to be supported somehow. 

One possibility is to get the rafters to stick out of the side of the house to support the overhand.  There are two problems here. In the original plan the rafters run up and down, north-south. Changing all of the rafters to run side to side, east-west seems to have been too difficult for the architect to adapt to without changing the whole structure. More seriously the rafters sticking out will lead to thermal losses and thermal bridge effects, sucking the heat out of the house. 

So we reached a plan to put the waterproof roofing sheet on top of the rafters (where the insulation layer ends) then put horizontal beams on top of that, which can stick out to the west and take the load of the overhanging corner. The panels need to be fitted with screws at horizontal intervals of 160mm and vertical intervals of 830 mm. Horizontal beams can cope with this, but if they are mounted horizontally, there are fears of beams twisting and the roof rolling off. The thinner the beams are, the less of a problem this is, but the beams need to all be the same thickness as the solar roof is flat, and they need to be thick enough to support the overhang. Extra beams can be added to the west to make this stronger. 

Another problem is that the solar roof installers need to add the roofing sheet, which would mean them making two trips, with some carpentry in between, rather than just coming and doing the whole job in one day. It's also not clear how they would feel about people making holes in the waterproof layer. 

The builder seemed much happier with vertical beams and some construction board mounted on top, although that will mean another air space and another layer of tyvec or some barrier sheet to stop any moisture that gets in there.  They pointed out that this would stop overheating in the summer, but we already have solar panels on the roof, and almost half a metre of insulation, so over-engineering is a much bigger concern to me than overheating!

So far all attempts to keep the design simple seem to be riddled with complications, and "simple" means very different things to different people!