# Forum Home Renovation Pergolas, Gazebos, Strombellas & Rotundas  Angle cut on rafter in joist hanger?

## Colsy

Hello all,
a question has been rolling around in my head while I construct my first pergola, slowly but surely. 
The question is, if my rafters sit at an angle to allow for roof pitch, how do I work out how much/what angle to cut the end of the rafter at so that it sits flush against the wall (ledger) within the joist hanger? 
Thanks for any help.

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## Stan 101

double post

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## Stan 101

You will have defined your roof pitch before construction. You need to know the pitch before you can determine the length of your rafter and it will look something like this when lying on the flat. 
... ___________________________
.../........................................./
./__________________________/  
The cuts you make are refered to as "plumb" cuts. The angle you cut the plumb cuts will be the same as the roof pitch. IE: the timber removed will be the roof pitch. To put the "seat" cut or birdsmouth on the rafter, you will simply cut the timber at the heel end at 90 degrees to the plumb cut. 
..._____________________________
./............................................./
.\___________________________/ 
Not quite 90 degrees on the screen but it's the best I can do on short notice. From cut to cut needs to be 90 degrees. 
To work out the length of the rafter use a calculator and do this.
*Assuming the roof pitch on both sides of the roof are equal.
*A ridge board has the rafters butt into it. A ridge beam has the rafter sit on top of it and therefore carries more roof load.  Span of the roof less the thickness of the ridge board (if using one) divided by 2.Add any overhang dimension using the on the flat dimension.divide this number by the roof pitch and then hit the COS (cosine) button and then =. This is the length of the timber member. 
Here's an example:
I have a simple garage I wish to pitch a roof on. The building is brick and cavity - typical east coast design. The eave is 600mm. The roof pitch is 20 degrees. I am using a ridge beam so the two rafters butt up against each other.
From outside of timber frame to outside of timber frame for the building is 6000mm. The eave is 600mm but we need to take into account the thickness of the brick and the cavity to get back to the outside of the frame. Brick is generally 110mm. Cavity is generally 40mm or 50mm. In this case it is 50mm. 
(6000mm divided by 2 [half span]) + 600mm [eave] + 110mm [brick] + 50mm [cavity] = 3760mm
3760 divided by 20 [roof pitch] COS [cosine] = 4001.3mm
** Some calculators require you to input this way: 3760 divided by COS 20 =.  
Remember with rafters that you need to take into account the extra timber required for the second plumb cut. The length of one side of the rafter (fence length) is 4001.3mm but you could not cut that rafter out of a piece of timber 4001.3mm long.   
cheers,

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## Colsy

Thanks heaps Stan. I think by your answer you may be thinking I am installing a gable roof, however it is a flat roof pergola so there will only be one angle cut at the building end.  
Still, the information you have given me has helped a lot, basically summed up in "The angle you cut the plumb cuts will be the same as the roof pitch". 
thanks again.

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## Stan 101

No problem Colsy, I just got a bit carried away  :Biggrin: 
I did actually think you wanted to know the angle you needed to cut the rafter for a "seat cut" so that the rafter was cut horizontally and vertically into the joist hanger.  
Enjoy the project.   
cheers,

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