Rafter Collars

 Parts of this article are fairly specific to my exact building, but if you have a similar blockwork construction, or similar rafter arrangement then this will be very useful for you

As part of maximising the internal space in the room, I decided that we would need to cut back one of the block walls at the back of the room, pictured below, just to the right of the red cannister. Although the space loss seems minimal, I was reasoning that by the time we had built our internal room around the protruding wall, leaving the ~30cm air gap that we planned for, we would lose quite a lot of valuable space. 

A closeup of the wall reveals another issue with the existing setup: as you can see, the purlin (the long member that runs perpendicular to the rafters, sitting atop the wall) was not well fixed into the wall, and didn't look at all secure.

This is even more evident in the picture below - it had just been left resting on top of some loose bricks!

Therefore, I planned to both cut back the wall and, in the process of doing so, replace the existing purlin with something more sturdy, and able to support the additional roof load of the false ceiling that we were planning. As I wasn't sure on how to do this, and was not planning on knocking down a structural wall by myself, I got a quote from a builder - Joe - to do the work.

Joe said that he was happy to cut the wall back by two full blocks worth, but that we would have to add a piece of steel box column in front of the wall to bear the weight of the new purlin. This new purlin would then consist of two pieces of 2by8 timber, bolted together with a steel plate in the middle: an arrangement called a "flitch plate". More on Joe's wall slicing antics later on.

Joe also suggested that we should add a second layer of rafter "collars" (the ties between the two sloping rafters that prevent them from splaying open under the roof load). This would be both for additional rafter strength in the long run, and also as a temporary measure to support the roof while we removed the lower level of collars in order to be able to swap out the old purlin for the new flitch plate.

Therefore, I set about buying, cutting and installing a whole new layer of rafter collars on all 17 rafters in the room, out of 2by6 timber. I used M12 high tensile strength bolts to do this, as well as 50mm square plate washers on both sides of the collars. This mirrored the existing setup of the lower rafter collars, however, I decided to swap out the nuts for nylon insert ones given that the nuts on the existing collars were becoming quite loose... 

This picture shows both layers of rafter collars now present, effectively sandwiching the purlin. The blue dot shows the next rafter along, just to get an idea of the orientation of the image.

I then removed the lower level of rafter collars, again so that Joe would be able to take down the purlin. This can be seen below: notice how in this image the collars are now above the purlin as opposed to underneath it as they were in the very first image of this article.

Next up - The Stud Frame