Wether you go with trig or tables.. or from the speedsquare i'm a bit muddy on this point. As soon as you add a birdsmouth you have a different angle/triangle which is either not square, or if it is then the bottom of it is lower than the plate. When/where is that allowed for in calculations given that not every roof would be potentially using the same spec of timber.. also when i see people marking the birdsmouth out for a third of thickness are they allowing for the angle of cut? i.e. obviously when pitched the corsscut is longer then when square..
Clarification on calculating pitche angles..
- Thread starter kaseryn
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the starting point for roof is the pitch angle of the common rafter.
its not that hard to start from that to work out birdsmouth size etc.
Noseall quite recently posted a sketch showing how to work it out.
There is an app for android called: right angled triangle solver which you can use to find the angles / sides of a triangle.
its not that hard to start from that to work out birdsmouth size etc.
Noseall quite recently posted a sketch showing how to work it out.
There is an app for android called: right angled triangle solver which you can use to find the angles / sides of a triangle.
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I just did a little sketching out and see i was overthinking it. In general now would you expect to be given a specific angle of pitch or is it more likely to be imperial? Also.. are hip/valley rafters almost always at 45 or no?
Their lateral position relative the rafters is 45 degrees. Their pitch differs from the commons as in they are always shallower due to the length travelled.
Oops, thanks again
I'm still struggling with something here though. If you had to work the pitch out for yourself the actual line would run from flush with the ridge beam at the top to edge of plate if there were no birds mouth.. but as soon as you add the birdsmouth you're changing the pitch right? The actual line on top of the rafter is running some 2/3rds above the edge of plate so the triangle of ridge beam to plate edge + 90deg doesn't worK? WHat am i missing.. how do you get that pitch and allow for the check?
What is your start point?
Are you trying to build to a given pitch, for example: architects drawing says 40 degree pitch.
Or is your ridge height already determined and you want to know what the pitch works out at?
Either way, top of ridge to outside top corner of wall plate is not the pitch.
You could work it all out with trigonometry using a series of right angled triangles.
The hip angle can also be worked out that way. (If the hip is 45 deg on plan).
Are you trying to build to a given pitch, for example: architects drawing says 40 degree pitch.
Or is your ridge height already determined and you want to know what the pitch works out at?
Either way, top of ridge to outside top corner of wall plate is not the pitch.
You could work it all out with trigonometry using a series of right angled triangles.
The hip angle can also be worked out that way. (If the hip is 45 deg on plan).
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is this a lean too or pitch roof
adding birdsmouth dosnt change pitch it lowers the rafter a plumb cut to plumb cut is your length and pitch then you add your seat cut 1/3 is max so as long as your not over you can be under
also hips and valleys are 45 to rafters on standard same pitch roof but not on dual pitch roof as they then become a bastard hip
rafter is a 12" rise hip 15" rise
measure as shown in nosealls link but where he shows x (the right way) this confuses most people just measure on the top of the rafter
adding birdsmouth dosnt change pitch it lowers the rafter a plumb cut to plumb cut is your length and pitch then you add your seat cut 1/3 is max so as long as your not over you can be under
also hips and valleys are 45 to rafters on standard same pitch roof but not on dual pitch roof as they then become a bastard hip
rafter is a 12" rise hip 15" rise
measure as shown in nosealls link but where he shows x (the right way) this confuses most people just measure on the top of the rafter
Thanks all. This isn't for any project i have on, i'm just trying to get my head around it all and be as ready as possible for different scenarios. I guess you'd mostly have the pitch to begin with making it all much simpler, if not tho and you want a birds mouth then i guess you could just mark back the depth of cut from plate edge and that angle to top of ridge would give your pitch.
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