Extending the rafters

One of the many processes we’re going through in the Westacre renovation project is making rafter extensions. It involves a lot of careful measuring and sawing.

We’re installing external wall insulation, hoping to get our 1930s semi to near passive house standard. It is making our walls 16cm thicker. And that means the roof needs to be extended on all sides to cover that thickness.EWIRafterExtensions

Where the rafters come down to the wall, we have to make them longer. We’re also kicking them out slightly so that making the roof wider doesn’t also mean making the eaves lower, which would cover the tops of some of the upstairs windows.

Because this is a renovation, we have to work with the existing building. Working on some of the earlier rafter extensions, we noticed that just making identical pieces of wood does not make for a straight roof edge. Evidently, the angle of the roof isn’t uniform.

So in the section we are currently working on, we have had to make some extra measurements.

We measured the horizontal and vertical along the joists, so that we could calculate the angle of the roof at each of those points. The angles varied from 31.2 to 32.5 degrees. Over the length of the joist extension (132 cm), that makes quite a difference.calculations

Alex fed the numbers into his computer. It calculated the equations and told us the exact angle of the birds mouths in the rafter extensions, and the height of the blocks that they will rest on. We meticulously measured those out on the wood and tried to cut them as precisely as possible.

The next step was to paint the rafter extensions black. We’ll have to protect the wood somehow, and we’ve decided that black wood against the black roofing felt will probably look better than very blonde pine. It’ll also hide a multitude of sins going on under the eaves.

We hung the rafter extensions from a wire in the garage for easy access and painted them with bitumen paint. That was a sticky couple of hours for Hilde and Roger, Alex’s dad. Sometimes we’re very grateful for small mercies like rubber gloves.blackrafterextension

When we get back from Belgium, we’ll be nailing the blocks and rafter extensions into place. And then we can finish insulating that long wall. That’ll complete roughly 2/3 of the external wall insulation. We’ll be waiting for more clement weather to do the remaining third.

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