Rafter Design
Now that the walls of the inner room were assembled, it was time to put the roof load calculations into practice and actually build the ceiling rafters. As I have mentioned several times before, the "room within a room" design requires a completely standalone inner room - so you are essentially building a standalone building that just happens to sit inside another building. Therefore, given the substantial weight of the plasterboard that would be fixed to the rafters' underside, I had to use 2by8 timber. This meant that the rafter members were pretty heavy, and required some clever support structures to allow me to put them up unassisted. Here's a diagram showing the required shape for each complete rafter:
As you can see, each complete rafter would be made up of three individual sections of 2by8 in order to achieve the required vaulted shape. This presented the question of how to join together the three sections to give maximum strength. Normally, in a rafter configuration, the two slanted pieces would continue past the crosspiece and meet at what is called a ridge beam (or ridge board) at the apex, as shown below:
However, in my case this wasn't possible because of the existing outer room!
Joining Rafter Sections for a Vaulted Ceiling
Due to my vaulted ceiling, therefore, I had to work out the best way to connect the three pieces of the rafter. To begin with, I did some testing using one bolt and a metal restraint strap (the same ones I used to strengthen the existing structure) - you can see this below using a scrap piece of 2by4:
It quickly became clear that this would never provide the required rigidity in the joint, so I then moved onto using two bolts in the joint. You can see a test of this method below (ignore the weird shapes of the scrap pieces):
When trialling this, I read somewhere that you should keep a minimum of five bolt diameters between holes, and between the holes and the edges - so in this case 6cm.
The Birdsmouth Joint
Now came the time to decide how to actually manufacture the rafter sections. The central section was clearly easy: just a straight length cut to the correct dimension (although slightly fiddly to work out the dimension!). The right and left sections, however, needed what is known as a birdsmouth joint, which allows for them to sit properly onto the top of the walls, on the crown plate. This joint is also marked in the diagrams above for clarity. Not knowing how best to approach this, it took me several attempts at working out the exact joint shape which I would then transfer to all the other rafters. Also, due to the asymmetrical nature of my roof, I needed two different birdsmouths, as the roof was slightly shallower on the right hand side of the building, impacting the shape.
Eventually, I worked out that I could use a laser line to draw the horizontal part of the birdsmouth (as this would give a perfectly horizontal line) while holding the rafter section as the angle that I wanted: parallel with the outer room roof. After a bit of trial and error, I eventually reached a birdsmouth that would be suitable, but even cutting this repeatedly proved challenging, and I ended up having to use plastic shims on some of the joints that didn't sit as well as I wanted.
Rafter Installation
Once I had decided on the joining method, I had to work out how to actually install the rafters. Given that it wasn't possible space wise to assemble them on the floor and then manoeuvre them into position, I worked out that we could use two improvised "guide rails" consisting of lengths of 2by4 to provide supports for the central rafter section. This allowed me to position the central section into position, then to attach the two side sections. Once all the rafters had been constructed, I could could then simply take down the guide rails and the rafters would stay in place. Here's what I mean - you can see the left hand side guide rail supporting one complete rafter at the back, and one central rafter section (labelled R3):
This method also made it easy to match the exact shape (i.e. lengths and angles of each section of the rafter), so that they would be as level and aligned as possible when it later came to fixing plasterboard to their undersides - not something to be underestimated!
After a while, I realised that it would be very easy to replace the right hand guide rail with an actual permanent purlin, made from two sections of 2by8 bolted together. This would both dramatically increase the roof's strength, and also provide a nice wooden "feature" in the finished room that would otherwise just end up being white painted plasterboard. Therefore, I constructed a support structure from 2by4 (ensuring to account for any lateral loads with diagonal supports), and replaced the right hand guide rail with a purlin as you can see below:
As you can see, the purlin replaced the temporary guide rail, and provided additional support. Here's a closeup of the structure I built up to reinforce the stud wall where the purlin would be imposing a heavy point load on it. The white marks one rafter, the red is the purlin, the blue are the horizontal supports to restrain any lateral or twisting forces on the purlin, and the green are the additional studs that I added to reinforce the stud wall:
Once I had installed one, it was just a case of repeating this all the way down the room. I marked out the position of each rafter (at 40cm centres starting from the back wall). Here's some pictures from the installation process:
As you may have noticed, I decided to add some mini joist hangers at the join between each rafter and the wall, to ensure that they didn't move side to side, or slip out of position. I then also added noggins (made from 2by8) between each rafter and the next, to stop them from pivoting, or collapsing. Here's picture of the noggins, marked purple (this picture was taken slightly later on in the project). Similar to the marked up picture above, the rafters are in white, and the purlin is red.
And that's it! Building the inner room ceiling was definitely one of the most challenging parts of the entire build, but the result was extremely satisfying, and by this point the build was really starting to feel like a room.