Oblique Roofs

Over the past two days at Tafe we have been working on some advanced roofing booklets that Paul had handed out to each of us. So far in the booklets we have been learning about how to make precise cuts and measurements for oblique end roofs. An oblique end roof is a roof where there are two parallel walls but one stretches out further than the other, so the wall that joins between them is not at a 90 degree angle. Think of a parallelogram. The hips always bisect the angle of the two wall plates that it sits on, and the rafters are always perpendicular to the wall plate.

Where the short and long hips meet at the crown end junction with the ridge, the two hips are always 90 degrees apart. Because of this we can use pythagoras thereom to figure out other types of cuts on the expanded plumb cuts diagram. From the brown paper drawing we drew at Tafe, we can set bevels to all the different cuts nescessary such as the short hip plumb cut, long hip foot cut and the under purlin down cut.

By following the instructional booklet we received from paul, we made our own oblique roof cuts diagram with our own different pitch angle. I found this quite an involved drawing as you used previous triangles to help calculate the next triangle you were drawing in. One thing that was a bit difficult was that the triangles continually over lapped one another, so if you were to look at the finished diagram, it just looks like a jumble of triangles that don't mean anything. Other than this, the instructional booklet was very easy to follow and quite mind blowing because i always wondered how people worked out the underpurlin cuts and so on, and this diagram showed me one way of doing that.

After this we moved on to a calculations booklet where we had to find the lengths of the short hip and long hip and other parts of an oblique roof. All the different calculations stacking ontop of one another made it hard at times to concentrate on what you were actually trying to figure, because the length of one hypotenuse of a triangle was the length of an adjacent length you needed for another different triangle. I enjoyed these excersises because they were a logical challenge that i could work my way through. It also stretched my methods of using trigonometry too, which i consider the holy grail of roof carpentry because trig can solve most roofing problems.

One thing that was a real mind opener was learning that the square root of the (actual rafter length squared) + (the offset between the hip and rafter squared) = the actual length of the hip.