cut rafter roofs

[mark1980]

i am putting a 40 degree hip roof on my garage and have a couple of questions. Firstly is the common rafter plumb cut 50 degrees and secondly how do i determine the birds mouth size over the bottom wall plate( the wall is at a fixed hight and cant be changed). obviously i will want the common rafters to run inline with the edge of the hip eventually. The garage is detached so i cant take any measurements off existing rafters.

Thanks in advance.

Mark.

 

[noseall]

what do you mean by plumb cut?

a 40 degree roof will mean that the rafter ends (bottom), the rafter top and the vertical cut at the birdsmouth will all be 40 degrees.

even the splayed jack rafter cut, joining the hip blade (rafter) will have a 40 degree cut, albeit splayed at 45 degrees.

the hip rafter cuts do not comply with the rest of the angles though. the hip rafter rises the same amount as the common rafters but its going is a lot further, making it an altogether different angle to the rest of the roof.

 

[mark1980]

Cheers noseall thats great. I still don't understand how you determine the height of the plumb cut at the back of the wall plate. am I right in saying that you will adust the pitch of the roof depending on the size of the birds mouth?

 

[noseall]

the birdsmouth should be no deeper than 1/3rd of overall rafter depth.

 

[noseall]

the back cut is exactly the same cut as the roof pitch.

 

[noseall]

 

[gangman]

Mark
If your pitch is 40 degrees then your plumb cuts will be 50 degrees as you say. On the birdsmouth the heel cut will also be 50 degrees and the seat cut 40 degrees.

The birdsmouth is usually cut at one third or less along the heel cut line, though this is not always the case. If you are not too worried about being too precise, and if your timbers have enough thickness use a carpenters square to mark a line, about 50mm, for the intersection of the heel and seat.

Consider buying a Stanley quick square, (not the pocket square) I think B&Q sell them for under £20. Once set it gives you your plumb, seat and heel cuts.

 

[mark1980]

Cheers gangman,

Thanks for the reply. I am getting my head round it, the only bit im struggling with now is with the birdsmouth. If i have a 50 degree plumb cut at the top of my rafter and have the correct rafter length am i not changing the pitch of the roof by deepening the birdsmouth? Sorry if im missing something but any advice would be great.

Mark

 

[gangman]

It depends on how precise you want to be. The depth of the birdsmouth can affect the pitch, but then a rise or dip in the plate will also affect the pitch. Are all your timbers the same width, are the plates perfectly parallel. At the end of the day it' a garage roof. I've just finished a loft conversion on a hipped roof, one hip was 38.2 degrees and the other was 37.6 degrees, who would know if I hadn't checked it.

 

[noseall]

is it me?

i'm not getting this 40 degree 50 degree thing at all.

the birdsmouth back cut is the same as the ridge cut or the fascia end cut.

 

[gangman]

Noseall

I bet you slept in the geometry classes at school.

On a right angled triangle, if the angle between base and hypotenuse is 40 degrees then the angle between perpendicular and hypotenuse is 50 degrees. They add up to 90 degrees.

 

[noseall]

Noseall

I bet you slept in the geometry classes at school.

On a right angled triangle, if the angle between base and hypotenuse is 40 degrees then the angle between perpendicular and hypotenuse is 50 degrees. They add up to 90 degrees.

gotcha.

i am a fan of trig, i just could not get my head around the relationship part, that you two were getting at.

it seems an odd way of doing things. i would just set my bevel and go.

 

[AtoZ]

i never did trig either but i can cut a roof.
i reckon gangman has written it down the wrong way.

i do know that roofing carpentry used to be very confusing but its actually very easy.

if mark sets his chop saw to 40 degrees and cuts his plumb cut then it will work and he will then cut all his seat cuts at 50 degrees =90 degrees.

i know that you can use a bevel and measure these angles both at 40 0r 50 depending how you hold the bevel but from marks point of view, the question he asked, the answer is
plumbs 40 and seats 50

the vertical plane of a rafter is affected by altering the seat cut but the pitch angle is not affected.

hips for this will be 60 plumb and 30 seat.

i am quite prepared for this to be proven wrong because it might be taught a certain way and my apprentice is on holiday so i cant look at his book.

my way works and is exactly how the framing square told me to do it.

i apologise in advance to gangman if he is saying the technically correct way and i certainly dont mean to give it the big-un but i can see mark cutting his rafters wrong and it costing him dosh.

noseall
i do it like you.

 

[noseall]

noseall
i do it like you.

and i thought it was me!

i still can not see the relevance of what the other two guys were talking about.

there is no need for the 'other' dimension when cutting the rafters.

once you know the pitch then all cuts are determined from the pitch.

the seat cut will be 90 degrees from the plumb. no need for this 50degree nonsense at all.

i also realise that the hip is a totally different pitch and i also know how to use the scale to determine this cut.

 

[AtoZ]

it is possible to read a 40 degree cut as a 50 if you hold the bevel in an odd way, but you are right in saying you dont need to worry about the seat angle if you just follow that 90degree rule.

i know good carpenters who cannot cut a roof, even a simple one.
i would go as far as saying that most cant cut one! i presume they learnt how as apprentices but cant be bothered or dont do enough, to warrant keeping the skills.

hope mark gets his roof sorted.