Hip Rafter

[MishaZ]

I'm using Blocklayer.com for generating a hip roof plan and have a question on one of the options regarding Hip Rafter.

Drop Hip vs Chamfer Hip...  Can you please explain the difference?  I see that the Drop Hip has a deeper seat (bird mouth) to bring it level with common rafters.  What I don't understand is what I will have to do if I go with Chamfer Hip.

Thank you,

[Tom King]

The important thing is that the vertical height at the outside edge of the wall is the same.  Change the 12 on your framing square to 17 for the hip and you can lay it out easily either way.

[Don P]

If you back the hip (chamfer) then there is no hip drop needed, you make it the same on the corner as the HAP (height above plate) of the commons and jacks. If you do not back the hips then the outer corner is above the plane of the roof, the centerline of the hip is correct. The hip drop lowers the hip that amount.

Here's another;
Irregular Hip (forestryforum.com)

[Don P]

I usually begin thinking about a roof as a set of center lines, you need to flesh them out here to see how it will bite you in the corner.... let's throw some names in here. The ridgeboard is at 10:00. A pair of commons @1&7. King common off the end of the ridge at 4:00 and a hip coming in at 2:00, another would come in at 6.

Notice if I layout according to centers my hip corners would end up high.


 

Down at the seat cuts, I have vertical lines showing the HAP, in this case about 5-9/16" vertical height above the plate (HAP) at the seat cut and wall line. and notice the same problem with my imaginary roof plane lines, the hip corners are high. I need to drop the hips, or bevel those edges... one or the other, not both!



 

In this case if the level seat cut on the hip rafter were made to lower the hip ~1/4" the hip will drop enough that the edges of the hip will plane in properly.

I'll keep that sketchup on my computer and can keep evolving the pic if its mud.

[Don P]

Here is the hip rafter calc on the forum with the pitch and span from that drawing plugged in. Notice at the end of all that math it says the hip drop is 1/4". Look at the sketch, the hip corners are 1/4" high.


 

Here is the hip at the top with an unadjusted seat cut. The edges of the hip are high;


 

Here is the 1/4" hip drop layed out on the hip seat cut to be removed. The plumb cut is a mess here, that can be improved a couple of ways.



 


And here is what it looks like up top with the hip dropped, the edges are now in plane with the common rafters all the way along the hip... the sheathing will lay flat.



 

[Don P]

I prefer to back hips, its a couple more cuts for full bearing and it looks better, plus, you know you got it right. If it is open inside you will probably need to back the inside as well.
This is a backed, or chamfered hip.


 

(When I say hip you could substitute "valley", they are just inverse of each other)

The calc above says the hip backing angle is 18.43°.
I drew a perpendicular cut through the sketchup model and then used the protractor function to get the backing angle from the drawing. It measured 18.4°. That is closer than my saw's scale so either way is close enough for guv'ment work.



 

I usually frame hips as 2 ply built up framing on a stick framed roof. That way each side is backed fully and opposite the other. The heel plumb cut is angled each way at the bottom plumb cut. It is stiffer, stronger, easier, better.  If its single ply or heavy timber I may clip the corner of the beam below for a square cut hip. That can be done either way.

[Don P]

In the calc above it gives the hip length.
it calls out 142.875".

Here is where that measurement is taken, Top of the point up top to the centerline of the plumb cut at the bottom. sketchup agreed with the calc, Its saying 11'10 7/8" so again either way works.



 

This is the length and locations for measuring a common rafter, it agrees with the calc;


 

If I were calling it over to the guy cutting I would say "106 and one half plumb to plumb". That might help visualize what we are measuring.

Looking at the first of those 2 pics shows what Tom was talking about. Looking at and thinking about the birdsmouth notches. That is a 6/12 roof. For the plumb and seat cuts on the birdsmouth of the common, you would use 6 and 12 on the respective tongues of the framing square. On the hip, notice the bird has a shallower angle because it is running at an angle across the roof. To layout that birdsmouth and upper plumb cuts you would use 6 and 17 on the framing square.

 For the plumb cuts at the ridge, you would use the 6 and 12 on the framing square for the common rafters and use 6 and 17 for the hip rafter upper plumb cut. For a square cornered building the hip plumb cut is beveled with 45 degree cheek cuts.