Dorma Window Framing and Truncated Hips

Yesterday we finalised our construction of our dorma window. Dorma windows may seem rather simple but they require a hell of a lot of trimming and extra fiddly additions to make it stable and work properly. First of all we determined the overall height of the roof, then roughly how high we wanted the window to be. There wasn't any real limitations on this so we concluded that the top of the window should be about head height as it would be aesthetically pleasing. I then worked out the height of the small roof that would sit above the dorma window frame, 450(halfspan) which equals the height from the plate, then add 90mm (the height of the birdsmouth). The overall roof height was 540mm. Dan and I decided to start the dorma roof 300mm down from the top of the main roof and in the diagram on the previous post you can see that we also set the bottom back 300mm too, to make it look even.

To work out the height of the studs we were going to use we simply did the following simple subtraction equation, 2490 - 540 - 300 - 35 - 35 = 1580mm. We then cut all of our studs out of 90 x 35 to a length of 1580. We shot the studs into the bottom plate so that the last stud on the inside of the roof met up with its edge aligned with the underside of the rafter, this is so that everything met up nicely so it could be lined properly. After this we levelled our front and back studs and nailed them sideways into the rafters so that they remained level. Once this was done we measured some top plates to sit ontop of the studs and tie them all together. When the top plate was nailed on it tied everything together and provided our main part of the dorma window. We could then add our dorma roof and other trimmers to it.

The above picture shows our completed dorma window frame. The dorma window roof is 540mm high from the top plate of the dorma stud walls. The vertical studs are 1580mm high.

The photo above depicts our stud wall going back with the rake of the roof. As the roof was a 45 degree pitch the angled trimmer and the shortened stud were cut with 45 degree cuts. Later we concurred that we should have put an extra stud directly alongside the last whole length stud because it is a more important structural piece.

The picture above shows the actual side elevation view of our dorma window frame. It is a good contrast to the drawn up diagram in the previous blog post, where it shows some of the dimensions.

The picture on the top right shows the front of the dorma window. It has the two main rafters 900mm apart from each other with a horizontal fixing plate spanning between them. From this plate there is a small rafter that runs with the other main rafters, this way the roof can be sheeted and be structurally sound.

The picture on the left shows the valley of the dorma roof and its corresponding valley boards. The bottom of the valley board on the left is supported with a trimmer ontop of the plate.

The picture on the left shows the bottom plate for our truncated hip. A truncated hip is used when the wall plate has an angled section. The jack rafter for this truncated section continues down from the normal hip to the bottom plate. Two other hips, bisecting the angle of the wall plates, run up to meet the normal hip.

The picture above shows the following from left to right - centring rafter, left truncated hip, jack rafter, and the right truncated hip. The edge cuts on the truncated hips worked out to be 17.5 degrees, and logically the edge cut on the centring rafters and any creepers joining onto the truncated hips, worked out to be 17.5 degrees as well.