# Forum Home Renovation Roofing  Birdsmouth Heel Cut for Hip Rafter

## cminto

I recently completed my hip roof over my day bed. See photo. I still have to put up flashing and asphalt/fibreglass tiles over the painted plyboard roof decking. 
I made extensive use of blocklayer.com's hip roof online calculator.
However, the first hip rafter that I made using this calculator sat too low, due to the bird's mouth heel cut being too deep. It seemed to me that there might be an error in the online calculator.
I watched several videos online, notably by Joe Fusco, about common and hip rafters, then went out and bought a metric framing square to try this method.
My common rafter rise/run is 400/1000, or 21.8 degrees.
I did the following (I already had the common rafters attached to the top plate).
1) Set the metric framing rafter to 80/283 and drew the plumb building line on the hip rafter.
2) Measured down this plumb line from the top of the hip rafter the HAP as measured on the common rafter (along the plumb line from the top of the common rafter to the seat cut of the common rafter) as in the diagram below.  
3) As I was going to chamfer the hip rafter, I cut out this seat line (actually, I also shorted the seat line by a distance of 1/2 the hip rafter thickness, so that I could create a nice pocket for the corner of the top plate).
3) Determined the "hip drop" measurement by coming in along the hip  bird's mouth seat line a distance equal to half the hip rafter  thickness, and measuring down to the bottom of the hip rafter.
4) I checked this measurement mathematically (on spreadsheet) using: =0.5 * hip rafter thickness * TAN(RADIANS(hip angle))
5) I then chamfered (bevelled) this off the top of the hip rafter. 
This worked beautifully. You can see the top of the hip roof in the next photo.   
The slight error in the cheek cut was due to the fact that I didn't realise until after I had complete the cuts, that with a circular saw I should have set the circular saw to a 45 degree angle rather than the angles shown on the blocklayer.com site (which would be correct for a manual cut, or a compound mitre saw). 
The reason for this post:
Blocklayer and I have had many email exchanges over the past few days.  We cannot at this stage agree on the correct way to do cut the hip rafter bird's mouth cuts. 
My experience of one roof tells me that the calculation for the depth of the heel cut (whether with the "drop hip" option or with the "chamfer hip" option) on the blocklayer.com site results in a hip rafter that is too low, whereas the method explained by Joe Fusco (of transferring the measured HAP from the common rafter to the hip rafter and then either increasing the heel cut depth by the "hip drop" amount or chamfering the top of the hip rafter by the same amount) gave me a perfect fit. 
I would be happy to hear from anyone who either agrees or disagrees with the idea of transferring the HAP measurement from the common rafter to the hip rafter as the first step in calculating where the seat cut of the hip rafter bird's mouth goes. 
Many thanks.

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## intertd6

> I recently completed my hip roof over my day bed. See photo. I still have to put up flashing and asphalt/fibreglass tiles over the painted plyboard roof decking. 
> I made extensive use of blocklayer.com's hip roof online calculator.
> However, the first hip rafter that I made using this calculator sat too low, due to the bird's mouth heel cut being too deep. It seemed to me that there might be an error in the online calculator.
> I watched several videos online, notably by Joe Fusco, about common and hip rafters, then went out and bought a metric framing square to try this method.
> My common rafter rise/run is 400/1000, or 21.8 degrees.
> I did the following (I already had the common rafters attached to the top plate).
> 1) Set the metric framing rafter to 80/283 and drew the plumb building line on the hip rafter.
> 2) Measured down this plumb line from the top of the hip rafter the HAP as measured on the common rafter (along the plumb line from the top of the common rafter to the seat cut of the common rafter) as in the diagram below.  
> 3) As I was going to chamfer the hip rafter, I cut out this seat line (actually, I also shorted the seat line by a distance of 1/2 the hip rafter thickness, so that I could create a nice pocket for the corner of the top plate).
> ...

  the plumb & level bevel of the common rafter & the hip are different angles, the edge bevel of a hip is greater than 45', normally a hip length is measured by tape, normally a hip is 25% deeper than the rafters, to get the correct height of the hip above the plate the correct bevels have to be used on each member.
regards inter

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## cminto

Thanks. I have the different angles accounted for. 
Main question for a roof framer is: do you measure down from the top of the hip rafter the common HAP measurement as the first step to locate the level cut of the hip rafter's bird's mouth when using a framing square?
Thanks.

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## barney118

1. Rise/m calc = tan 21.8 = 0.39997
2. true length/m = sq root (0.39997 sq + 1.41 sq) = sqrt(0.15999+ 1.998) = sqrt (2.159) = 1.469m for every m of 1/2 width of top plate. 
3. so if your pergola is 3m wide = 1.469x1.5 = 2.204m set out to birdsmouth. 
4. Mark plumb cut on rafter to 21.8 deg
5. Mark again the thickness of COMMON rafter parrallel to this mark. 
6. Now on edge of timber from second mark draw 90 deg line across thickness of rafter.
7. draw across the 2 marks
should look like this  note the hip bevel is shallower than the common rafter. 
6. Measure down 2.2m
7. Mark plumb bevel hip bevel on mark.
8. cut out birdsmouth not more than 1/3 of member, 25mm is generally a good guide.

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## intertd6

> Thanks. I have the different angles accounted for. 
> Main question for a roof framer is: do you measure down from the top of the hip rafter the common HAP measurement as the first step to locate the level cut of the hip rafter's bird's mouth when using a framing square?
> Thanks.

  I can't ever remember hearing the abbreviation of HAP or know what it is ( probably lost those brain cells retaining information like that years ago ), you tape measure the hip length normally as once the common & crown end rafters are fixed nothing can move. I could calculate it but measuring is the easiest & quickest way, what maybe happening is your not allowing for the bevel being removed from the hip top edge which will leave the hip low at the plate end.
regards inter

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## Blocklayer

Cminto is referring to the 'Hip Drop', which drops the hip down (with a birds-mouth) so its top edges don't protrude higher than the plane of the commons. 
I think the procedure to set it out with a roofing square is best explained by (the late) Joe Fusco (as Cminto mentioned) in his videos *Hip Rafter Layout Part 1 - YouTube* and *Hip Rafter Layout Part 2 - YouTube* 
You can either drop the hip with a birds-mouth, or bevel its top edges to get it in plane with the commons.
The calculations for the amount to drop are pretty basic (there's lots of formulas), but you need to take into account the different plumb cut lengths of the hip and common in the adjustment to the hip birds-mouth plumb cut length.
As the hip is at a lower angle, its plumb cut length is slightly shorter than the plumb cut on the commons. 
My calculator didn't adjust for this (slight) difference, so the hip birds-mouth depth was a few mill out. It does now however, and matches Joe Fuscos results. I think Joe knew what he was doing.
Excellent videos to learn from by the way. 
The 'HAP' stands for 'Height Above Plate' and is pretty widely used in the US. 
Thanks to Cminto for his persistence in convincing me it was out.

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## cminto

Thanks to Blocklayer for his great online calculators and the post above clarifying the issue.

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