plumb line cut depth on birdsmouth ,

[thatbloke]

i will be starting work on a lean-to roof in a few days , its around a 20-25 degree pitch with a 3.12 meter span.

i will be using 175/50 rafters and perhaps a 200x50 ridge plate (would that be about right?) , the rafters will be 600 centres.

anyhow , what i cant get my head around is how deep do i make the plumb line cuts to birdmouth the rafters around the ridge plate?

is there a formula? 50% of the rafter height or something? i dont want to take too much out and weaken the rafter.

also , is a 200 mil ridge plate too deep?

cheers.
thatbloke.

(the cut i am talking about is a in the pic , the vertical cut of the birds mouth).

 

[noseall]

We usually take about 1/3rd of the material out for the b.m.

So on a 7" x 2" you would remove say a 2" plumb cut for the b.m.

The top cut does not need b.m.ing onto the pitching plate.

You don't have a ridge as it is a lean-to?

 

[noseall]

I have re-read.

200mm x 50mm sounds excessive for a pole or pitching plate yes, but too deep?

It can never be too deep.

Go with 450mm centres. I hate 600mm centres.

 

[thatbloke]

so , how do you go about attaching the rafters to the pitching plate?

how big a pitching plate do you think i should use? it will be a lightweight wooden shingle roof.

 

[noseall]

You can cut the rafters so that they touch the wall. Once this happens they can not go downwards unless the (lower) wall plate slips outwards.

So all the pitching plate is doing is keeping the rafters steady.

You will cut all the rafters and put them in place leaning onto the wall. Then lift the pitching plate into place so that it just touches the rafters. Then fix the pitching plate to the wall then just pop a couple of fixings into the plate through each rafter.

Use a 150mm x 50mm plate.

 

[joinerjohn]

Yep 1/3rd depth of the rafter for the birdsmouth. As noseall says just do a plumbcut on the top end of the rafters to butt against the top plate. I'd either screw or nail the rafters to this. (Minimum 3 per rafter).
If the top plate and the bottom wallplate are equidistant over the length then cut one rafter, try it in position and if it's ok then use this rafter as a template for the rest.
Simples

 

[thatbloke]

lovely , thanks guys.
one problem though , its a timber framed house with wattle and daub walls coated in a very weak , old lime plaster. the pitching plate will be screwed to the vertical studs of the frame but i would feel far safer if the rafters were attached to the pitching plate somehow. this old house does tend to move around a fair bit as well.
the wall is far from true so each rafter will have to be cut to fit and it will be a warm roof so no inner ceiling.

 

[noseall]

In your image it shows the pitching plate kind of under the rafters.

You can set the plate so that the rafter ends are seated completely on the pitching plate - i.e. raise it. This scenario mimics a traditional (duo pitch)cut roof but without the rafters the other side.

The image shows a 6" x 1" bearer or pitching plate, but i advise say a 200mm x 50mm in your case (you suggested using 175mm x 50mm rafters) so as to collect the whole plumb cut on the plate.

This will be the best way in your situation.

 

[thatbloke]

cheers fella , so whats the best way to fix the rafters to the pitching plate? this house does move around quite a lot , its been sat empty for many years and has never been centrally heated so as soon as i warm things up it will shift around , ive also got a lot of frame repair work to do so movement through that is also very likely.

i wont be using any ceiling joists , just reed board and lime plaster over celotex on the rafters , not enough head room for a flat ceiling.

 

[noseall]

Screw them.

One through the top and two each side on the skew.

 

[thatbloke]

lovely , thank you , you have been a great help and its much appreciated.
thanks again.

 

[garyo]

Nose,

In your picture above you show the wall plate on the inner leaf of the cavity wall. Is that the normal arrangement, or is either acceptable?

Gary

 

[joinerjohn]

Gary, it's normal for the wallplate to sit on the inner leaf. I wouldn't recommend putting it on the outer brickwork.

 

[noseall]

Nose,

In your picture above you show the wall plate on the inner leaf of the cavity wall. Is that the normal arrangement, or is either acceptable?

Gary

I have seen instances of both but the majority of cases the w.p. sits on the inner leaf.

Strapping (wall plate holding down straps) becomes difficult when situating the w.p. on the outside wall. Also there are other problems that occur like joist height/location, lintelling etc.