# Where to cut the seat for a birds mouth?



## Bud9051 (Nov 11, 2015)

Sorry, too long and confusing for me to follow what you posted. 
There are SIMPLE ways to get your birds mouth and ridge cut so tell me what you have so far, like is the ridge board/beam in place? Walls in place?

Bud


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## trouts2 (Aug 30, 2011)

Short version.


For a gable roof, house or shed. How is the length of the birds mouth seat calculated?
Some say the length is 1/3 the width of the rafter width. Some say ½. Both can’t be right as that changes the angle of the rafter to ridge board match.


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## Bud9051 (Nov 11, 2015)

Is anything in place now or are you waiting for the calculation?

Bud


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## trouts2 (Aug 30, 2011)

Timber frame shed construction with posts and plates up. The roof will be 2 x 6 rafters with a ridge board. None of that is in place. I’m going through things I need to know before I continue. I’ve got most figured out things but the length of the seat of the birds mouth is a puzzle. But the question would apply to any dimension shed or house with a gable roof.


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## Nealtw (Jun 22, 2017)

Usually it is the width of the wall or beam so if you were putting a ceiling in, the ceiling would meet the wall on the inside. I am sure that may not work all the time so I would want a min. bearing on the rafter so at least make it 2 1/4" long.


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## trouts2 (Aug 30, 2011)

@neaaltw I may have this wrong but what you suggest is exactly what I don’t understand. I think that to get 2 1/4 “ would require to be a certain length up the plumb line for the birds mouth. How will that insure that with that cut the top of the rafter will mate properly with the ridge board? Some say that the length of the seat will be fine if the cut is 1/3 of the width of the rafter. That would be a different length of seat length. It would also be a different length up on the plumb line for the birds mouth. So I think both cuts one for 2 ¼ and one for 1/3 the width of the rafter width would sit at different heights on the plate. It would slide the rafter top up or down from the ridge board. Note: I said in a prior post that different cuts would change the angle of the ridge board. That was not correct. What would happen (I think) is different cuts will cause the rafter to sit higher or lower on the plate. That would shift the plumb cut at the rafter to ridge board end. The rafter would sit higher for a short seat cut and lower for a long seat cut. I would think that there is only one set of numbers for the length of cut on the BM plumb line and one for the length of seat that will make the rafter end just match with the ridge board. 

Put another way, the longer the BM seat causes the cut of the BM plumb line to be longer. That is it is deeper into the plumb line. The rafter will sit lower. The top end will be plumb to the ridge board but sit lower if the cut is further up the plumb line of the birds mouth.


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## Nealtw (Jun 22, 2017)

trouts2 said:


> @neaaltw I may have this wrong but what you suggest is exactly what I don’t understand. I think that to get 2 1/4 “ would require to be a certain length up the plumb line for the birds mouth. How will that insure that with that cut the top of the rafter will mate properly with the ridge board? Some say that the length of the seat will be fine if the cut is 1/3 of the width of the rafter. That would be a different length of seat length. It would also be a different length up on the plumb line for the birds mouth. So I think both cuts one for 2 ¼ and one for 1/3 the width of the rafter width would sit at different heights on the plate. It would slide the rafter top up or down from the ridge board. Note: I said in a prior post that different cuts would change the angle of the ridge board. That was not correct. What would happen (I think) is different cuts will cause the rafter to sit higher or lower on the plate. That would shift the plumb cut at the rafter to ridge board end. The rafter would sit higher for a short seat cut and lower for a long seat cut. I would think that there is only one set of numbers for the length of cut on the BM plumb line and one for the length of seat that will make the rafter end just match with the ridge board.
> 
> Put another way, the longer the BM seat causes the cut of the BM plumb line to be longer. That is it is deeper into the plumb line. The rafter will sit lower. The top end will be plumb to the ridge board but sit lower if the cut is further up the plumb line of the birds mouth.



So you attacked it wrong from the start. 



Before you figger the height of the ridge board you consider the BM. 

So now when you cut the BM with the correct angles for seat and plump you have discovered the ridge is to high.


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## Nealtw (Jun 22, 2017)

Some will use the green triangle to do the math to find the bottom .
I use the blue arrows to find the top
Perhaps the quick fix is to add a 2x4 to the top of the wall.


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## trouts2 (Aug 30, 2011)

Before you figger the height of the ridge board you consider the BM. 

So now when you cut the BM with the correct angles for seat and plump you have discovered the ridge is to high. 
@Nealtw If you match the height of the ridge beam to whatever cut you make to the birds mouth it seems like fudging. It also seems like that would work. BUT that might change your roof pitch or (I hope I have this right) the story pole length. When you first figure the numbers for the rafter you get the run and rise. If cutting the BM first the rise might change a bit depending on the measurements for the BM. That might not be a big deal. I’m not a builder so don’t know this stuff. Please don’t think I’m giving you a hard time. I’m just trying to figure this stuff out. 
I have picked 7:12 for the pitch which is a guess and will probably work. The shed is 8 x 10 and I can add rafters for strength for roof load so not a big deal. If I cut the MB first it may change where the ridge board ends up but that is probably too small a change to make a difference. It would affect the gable end supports and maybe other things but I’m not sure just what.
What you wrote before seems to be another way I’ve seen mentioned. That is cut the MB to the width of the plate and sheathing. That puts a lot of the load on the plate so good. And I guess from there you adjust the ridge height to match where the plumb end of the top of the rafter ends up. 

On the other post I’m a bit confused. What determines the length of the blue line? If the black line above the bottom edge just a line from bottom blue line to bottom blue line? I’m not sure how that got there. It looks like the picture shows two 2 x 4’s at the plate so is it showing the extra 2 x 4? A birds mouth should be able to be cut without having to add height to the plate.

I’m not at the site where the shed is being built (the state of Maine, I’m currently in Massachusetts). Maybe the easiest thing to do is cut some 1 x 6 inch stock that is to finished rafter length and see where the top plumb cut ends up.


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## Nealtw (Jun 22, 2017)

@trouts2 



Sorry I am not understanding the problem and I am not offended by more questions 



In order to set set the height of the ridge you calculated the height of the rafter on the 7 /12 angle, which might be 4" on a 2x4 then you have to lower it the height of the birds mouth. 

If the birds mouth is 3 1/2 by 1 7/8. the top of the rafter is 2 1/8 above the outside of the wall


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## trouts2 (Aug 30, 2011)

The overall issue and reason for the post is this (keep in mind I’m a guy building a shed not a contractor).

I thought the order of things would be calculate all the roof dimensions so you know the height of the ridge board and all the dimensions of the rafter in advance of putting anything up. So I thought a ridge board would go up first to its finished height. The rafter would then be measured and cut by rules and not referenced to anything else. Doing that I thought would have the ridge board fit the rafter board at the right place, no moving it.

BUT

I found that various explanations of how to cut a rafter have different BM dimensions. The YouTube videos show the rafter layout and end with the rafter matching the ridge board. But just how the ridge board match was gotten is not detailed. Maybe they adjust the height as you mentioned after they fix what they want for a seat length. 

There must be a way to calculate numbers for the birds mouth that make it so that it can be cut and match a pre hung ridge board. Maybe they are complicated. No one I have read or video has calculated the birds mouth. They all have some guide they use, 1/3 the width of the rafter board, the plate width or many other ways but none done by calculation only. 

Again, I’ll probably put up the ridge board then cut some scrap 1 x 6 and get the dimensions for the BM that way. I could probably scale the dimensions down and cut it out on cardboard. 
Glad you mentioned the ridge adjustment as that seemed like the only way to get a match. I would guess most builders work of house or shed plans that have the dimensions specified. For builders doing a build on the fly they probably know how to adjust in advance from experience. The width of the plate for the seat length will probably make the ridge board match close enough to not matter if the height is adjusted slightly. If the 7:12 ends up a little more or less I doubt it will make a difference.

Thank you for the help.


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## Nealtw (Jun 22, 2017)

trouts2 said:


> The overall issue and reason for the post is this (keep in mind I’m a guy building a shed not a contractor).
> 
> I thought the order of things would be calculate all the roof dimensions so you know the height of the ridge board and all the dimensions of the rafter in advance of putting anything up. So I thought a ridge board would go up first to its finished height. The rafter would then be measured and cut by rules and not referenced to anything else. Doing that I thought would have the ridge board fit the rafter board at the right place, no moving it.
> 
> ...


 Most of the time, we contractors or working off a set of plans that tell us the height to put the ridge and by surprise the rafter works, So much of the time no thought is given to the height of the ridge and I suspect that is why it is not in the videos. 



On blueprints the height of the ridge and the seat cut are both given. 

So on the job we just have to confirm or adjust for the width of the building. 

Some one posted a good explanation the other day but I haven't been able to find it.


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## Nealtw (Jun 22, 2017)

trouts2 said:


> The overall issue and reason for the post is this (keep in mind I’m a guy building a shed not a contractor).
> 
> I thought the order of things would be calculate all the roof dimensions so you know the height of the ridge board and all the dimensions of the rafter in advance of putting anything up. So I thought a ridge board would go up first to its finished height. The rafter would then be measured and cut by rules and not referenced to anything else. Doing that I thought would have the ridge board fit the rafter board at the right place, no moving it.
> 
> ...


 Skip to 1100 on this video


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## trouts2 (Aug 30, 2011)

The video is excellent but not specific on the BM. He has another video that is also exellent but vague on BM. In the second he only says keep the seat length between 2-4 inches. It would have been nice if he went over what happens at the ridge board for a seat length of 2" cut and a 4" length.



He does explain in both the top of the ridge board being roof shaped or flat and how to calculating the total rise for both a peaked ridge board flat top ridge board.


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## Nealtw (Jun 22, 2017)

trouts2 said:


> The video is excellent but not specific on the BM. He has another video that is also exellent but vague on BM. In the second he only says keep the seat length between 2-4 inches. It would have been nice if he went over what happens at the ridge board for a seat length of 2" cut and a 4" length.
> 
> 
> 
> He does explain in both the top of the ridge board being roof shaped or flat and how to calculating the total rise for both a peaked ridge board flat top ridge board.


 But in his pictures the birds mouth is the width of the wall so 3 1/2 for 2x4 and 4" if you have sheeting. 

So you have the ridge in now and are we looking for a cheat to get it to fit right with out worrying about the pitch?


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## Nealtw (Jun 22, 2017)

I would draw out the wall and the ridge as is on a piece of plywood and lay the would be rafter on it.
When measurement A and B are the same the measurement c and d will be the same.
When that all matches draw you plum lines and the seat cut is 90* from the plumb cut. or subtract the plumb angle from 90.


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## Nealtw (Jun 22, 2017)

I am not above cheating when I build a shed. before the walls are built I draw in the walls and ceiling height and cut the top of 2 rafters and set them in place to measure the birds mouth
Cit the birds mouths and set them back in place and then set a short pies of ridge on top of them. I trace all this on the floor and I can measure stud heights, ridge support and anything else I need as well as trimming the top of the sample rafters to fit the ridge.


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## trouts2 (Aug 30, 2011)

I think I see where I have been messing up on the calculation part after looking at your drawings. 
For calculating with any reasonable length of birds mouth heal you just subtract the heal length from the rafter width and add that to the calculated rise height. If the BM length was the full plate & sheathing or 1/3 of the rafter width that would get two different BM heal lengths and two different lengths of remaining rafter widths above the end of the heal cut. When either length is added to the calculated rise the rafter top end should match the ridge board height.
Said a different way
If the rafter is 6 inches wide and the birds mouth heal 2 inches then 4 inches is added to the calculated rise.
If the rafter is 6 inches wide and the birds mouth heal 3 inches then 3 inches is added to the calculated rise. 
Either way the top of the rafter should match at the same place on the ridge board.
I think this is the same and C and D in your post. When you cut the BM you measure C and add that (at D) to the calculated rise.


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## Nealtw (Jun 22, 2017)

trouts2 said:


> I think I see where I have been messing up on the calculation part after looking at your drawings.
> For calculating with any reasonable length of birds mouth heal you just subtract the heal length from the rafter width and add that to the calculated rise height. If the BM length was the full plate & sheathing or 1/3 of the rafter width that would get two different BM heal lengths and two different lengths of remaining rafter widths above the end of the heal cut. When either length is added to the calculated rise the rafter top end should match the ridge board height.
> Said a different way
> If the rafter is 6 inches wide and the birds mouth heal 2 inches then 4 inches is added to the calculated rise.
> ...


Yes I think you have it, When I do it I start by finding A, that the length of the seat can change to what you want with some min. bearing but what ever you use you find A. Subtract A from the plumb of a 7/12 and you get (c) which is the same as (d).
If I cut the first rafter just like that and then cut it back to fit the ridge, usually 3/4"
Draw the line and square off the top and measure how much lower the ridge should be to allow for no slope on the ridge.


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