Confused about birds mouth cut dropping ridge height

[flanagaj]

So I am perfectly fine at using trig to calculate the rafter length, eg hypotenuse of a triangle.  But where my head is hurting is around the birds mouth cut.   The depth of the birds mouth cut will drop the ridge by the same amount.  And to therefore get the ridge height back to where it was before you cut the birds mouth, requires the roof pitch to be increased.

 

What am I missing 🤯

[Temp]

Depends where you measure from..  if you measure from the top of the brick work then the wall plate that fits in the birds mouth raises the rafters by its thickness which is about same as the birds mouth. 

[nod]

Your overthinking it 

[flanagaj]

4 hours ago, Temp said:

Depends where you measure from..  if you measure from the top of the brick work then the wall plate that fits in the birds mouth raises the rafters by its thickness which is about same as the birds mouth. 

Hopefully, the diagram below will help.  So it's easy to calculate ?, but a rafter is 6" or 8" and this will then throw the ridge height up by a given amount.  So although measurement ? is constant so long as the pitch remains constant, then it does not matter  whether you use a 100mm or 250mm rafter, however, the calculation fails to take into account the width of the rafter.  And as I mentioned before, if I cut a birds mouth after I have calculated ?, then my ridge height will drop when I seat the rafter on the wall plate.

You say I am over thinking it, but until I see an example of how I calculate ? with a given size rafter, a birds mouth cut and my ridge height being fixed, then I still don't get how you calculate it.  The key point here, is that if ridge height is fixed then to maintain the same ridge height with a different sized birds mouth cuts or even different rafter sizes, the only variable that you have to change is the roof pitch.

 

If I use a 100m rafter then my roof pitch will be steeper than if I use a 250mm rafter.  Simply due to the fact, that bot ends would have to converge at point RH, but the 250mm rafter will sit higher on the wall plate than the 100mm rafter.

[BotusBuild]

? is the length of the top edge of your rafter. Whether you have a 6" or 8" deep rafter, the top edge length will always be the same.

 

Then cut the ends of the chosen rafter to the correct angles to connect to the ridge beam and where it rests on the top of your walls

[flanagaj]

1 hour ago, BotusBuild said:

? is the length of the top edge of your rafter. Whether you have a 6" or 8" deep rafter, the top edge length will always be the same.

 

Then cut the ends of the chosen rafter to the correct angles to connect to the ridge beam and where it rests on the top of your walls

But how do you calculate said angle?   I can obviously sit down and work it out, but my point is that none of the online rafter tutorials cover how you calculate the angle of your common rafter to ridge plate plumb cut according to your finalised ridge height and the associated birds mouth cut.

[joe90]

4 hours ago, nod said:

Your overthinking it 

I tend to agree, I cut what’s necessary on the first one (guessanology! ) then copy it for the rest. 

[BotusBuild]

@flanagaj,

To answer your question, you already have the formula in your photo above. You already seem to know what x and y will be in that diagram from what you are saying in your messages here. So working out that angle should be straightforward (unless we are all missing something).

However, you'll eventually work out the angles and lengths, do the birds mouth cut, install your ridge beam, cut your rafter with the calculated lengths and angles and then find it doesn't fit as snugly as you hoped. It's a building materials thing. It happens all the time.

What @joe90 says above is the way to go.

If you don't want to use 

27 minutes ago, joe90 said:

(guessanology! )

then use CAD - Cardboard Aided Design - a piece of scrap cardboard, marked up against the fitted ridge beam, then transfer onto the rafter before cutting.

[BotusBuild]

Or does this help - https://www.mycarpentry.com/roof-pitch-calculator.html

[flanagaj]

2 hours ago, BotusBuild said:

then use CAD - Cardboard Aided Design 

 

😆 never heard that one before

[joe90]

40 minutes ago, flanagaj said:

 

😆 never heard that one before

My version is “Crayon aided design” 🤣

[nod]

3 hours ago, joe90 said:

I tend to agree, I cut what’s necessary on the first one (guessanology! ) then copy it for the rest. 

I’d 30 trusses to birds mouth before building the roof An engineers drawing with lots of detail I still ended up adding an extra wall plate either side to give me enough depth on the birds mouth 

It will become apparent as soon as you start 

[flanagaj]

As they say.  Better to cut too long than too short

[Alan Ambrose]

>>> What am I missing 🤯

 

A bit of graph paper, a ruler and a biro maybe?

[Jenki]

6 hours ago, flanagaj said:

But how do you calculate said angle?   I can obviously sit down and work it out, but my point is that none of the online rafter tutorials cover how you calculate the angle of your common rafter to ridge plate plumb cut according to your finalised ridge height and the associated birds mouth cut.

The design will give you the pitch, that's the angle. If you want a set Ridge height then you work back from that height for your wall plate height, (Ridge height - Rise) the angle does not change. The birdsmouth  depth will be similar to the wall plate thickness.

[saveasteading]

There's good reason for roofers to put up temporary timbers and measure the lengths.  Nearly all buildings have twists in them and the cumulative effect can be dramatic. Then allow for the complex abutments at both ends.

I'm no expert but I've seen seasoned joiners get it wrong.

 

I wonder if it is safe to mass produce after the first one? I suspect not.

 

It is much worse at hips. Even with a precision made steel building (computers all the way) we knew to allow sliding joints to make sure they fitted.

[joe90]

1 hour ago, saveasteading said:

wonder if it is safe to mass produce after the first one? I suspect not.

Well, when I have done a cut roof I use the “template” one and make sure it fits everywhere first, if you offer it up and it’s 10mm out make that adjustment for that one (not that any of mine were out of course 🙄)

[Temp]

I have a copy of Carpentry and Joinery by Brian Porter. Excellent books I recommend. Small extracts from the relevant pages are below 

 

[oldkettle]

On 26/03/2024 at 19:49, joe90 said:

Well, when I have done a cut roof I use the “template” one and make sure it fits everywhere first

 

If you watch any of Robin Clevett's many roof cut videos, this is exactly what he does.