Roof lantern help please

[Riggly]

I am trying to build a roof lantern for my extension and am struggling with the hip rafters. I have constructed the cill and put a ring beam on top with a 25 degree bevel the same pitch as the roof. I have bevelled the ridge with a 25 degree bevel on both sides and i have placed some temporary timbers at 25 degrees to get the correct height of the ridge. My thoughts were to now construct the jack rafters and leave the ridge overlong at present. I have tried to cut the hips using a roofmaster square the hips are mitred together where they meet at the top and then are plumb cut as they meet the ridge. Where the ring beam meets the angle is not 25 degrees it is more like 18 degrees. I am cutting the hips on a table saw with a slider and to get the mitre for where the hips meet i am seting the fence to 45 degrees and the blade to 18 degrees which seems to work. To get the plumb cut i made up the hip and made a plumb line square from the ridge which doesn't relate to the plumb cut indicated by the roofing square. I now have a number of questions should i be setting the roofing square to 25 degrees for the common rafters and use that position for the hips or should it be reset to 25 for the hip. i need to bevel the hip rafter both sides should the bevel be 25 degrees or 18 and finally when i cut the hips to length how do i make the plumb cut can i rest the hip above the ring beam and above the ridge take a line up from the inside of the ring beam or will this make the hips either too long or too short when it moves downwards

i apologise for the ramble it is difficult to explain so here are a few photos

 

[Chems]

There are some long sentences in there!

What your asking is about compound mitres which are a serious headache. And I think if you use a calculator etc to get the angles they apply to a chop saw not a table saw. In my experience that is anyway. The only real help I can give you is that its easiest to make your cuts if you can jig your workpiece so its passing over the saw at the angle your going to be working at. So like 18 degrees.

I'm sure someone else will be more helpful!

 

[merlin]

The angles for a common rafter and a hip (or valley) rafter are completly different this is because the hip travels a further distance, you are quite right in that the plumb cut is 18 degrees for the hip.
I have never used that type of roofing square before as I use a steel square but the idea must be the same, there must be a different "set point" so that the angle changes.
To get the hip length on site I never take measurements from the square I always take a direct measure from the roof, if you set the end common rafters in from the end of roof so that the centre of rafter is the centre of span and plumb up from there you should be able to measure the hip length, make sure the backing of the birdsmouth (measurement from corner of birdsmouth to top of rafter) is the same as the common rafter.
To get the back bevel for the top of hip, draw out a birdsmouth - right angle triangle with the hip angles 18 and 72, the lines all carry on past the triangle,the drawing will have the right angle at the bottom left, you need the angle that is formed by the line passing through the vertical line at the top(obtuse angle on the left).
Hope this helps rather than confuse, Merlin.

 

[Riggly]

Thanks Merlin you have answered quite a few questions and given me an idea about the others. I am in the workshop again today i shall see if i can get it to work. I did cut the roof on the extension but the hips on the lantern have no flexibility about their position.