Irregular Hip, Unbacked

UpNorth

Irregular hip, say major (lesser) pitch is A:12, minor pitch is B:12, and hip member is C width.

The hip member will not be backed, and thus the roof planes are to bear on the arrises.

The center of that hip member will fall on a line that is parallel to the hip plan line. How is that offset calculated, and to which side, major or minor, does it shift?

Edit: I am attaching a little visualizer, a .jpeg screencap of a Sketchup model, showing an unbacked 2x4 hip having an 8:12 major and 10:12 minor. You can see the two arrises in plane, and the angle of the shot clearly shows that the member is not centered under the hip line.

Attached Thumbnails

 

cbfx3

you are overthinking this. See the hip and valley thread. dont think of it as shifting... think like this. The 10 side will plane with the 10/12 rafters and the 12 side will plane with the 12 rafters. the edge of the hip planes so dont worry about the center of the hip itself. Just keep the edge right and there is only one place it can be in... the right spot

BillD

I do not know of a formula to calculate the position of the hip, which I think is what you are asking.

When doing a bastard hip I always made an "L" shaped template out of plywood the depth of the overhang. I put a nail in the outside edge of the template where the edge of the hip would be. Then hold the template on the corner with a string going from the nail to the top of the hip location. The line falls over the plate where the hip needs to be located. It always lands on the steeper side of the roof.

Bill

Birch

This is as High Tech as my math expertise gets. I hope you get it, sorry if you don’t. ( let me know…)

(Sqr. rt. (A^+B^) /A ) x C = one sides off the corner layout (offset for B)

(Sqr. rt. (A^+B^) /B ) x C = the other sides off the corner layout (offset for A)

What this does is scales down a triangle at the proportions of the split pitches with the hypotenuse of the triangle equal to the Hip’s thickness (C). The dimensions solved are the right angle legs of the scaled triangle. I usually solve for these when doing my roof calc because I need this info to help calculate the longest jacks, I step them down from there. (I believe the formulas above are the ones I am using in my roof calculator spreadsheet.)

*The formulas will apply to the reciprocal roofs layout as shown. IOW, if you are dividing by A then that is for the layout on the B wall, and vice versa. (/B for A) *(I just use the rule that the larger leg is always for the lesser pitch)

This is easier to do with the CM Calculator;
Enter Pitch A as a Run, enter pitch B as a Rise, press Diag, (save settings, Stor pitch), Enter the hips thickness ass the Diag, press the Rise and Run keys for your dimensions.

Also, you can lay the hip out with a framing square as I described in the Hips and Valleys thread and just measure them.

Timuhler

When I took the eyebrow class in Porland last January, Billy Dillon (Timber Framer-in caps because it is so cool) showed a way to figure out the hip shift.

Of course I can't remember it, but I will try this weekend. I'm attaching a picture that showed him drawing it. It took just a few seconds.




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Birch

Tim,

Thanks for your post.

While working up an edit overlay on you photo I realized I made a mistake in my Offset formulas in my previous post. Sorry for any confusion, I got my numerators and denominators reversed.

They should read like this;

A / (Sqr. rt. (A^+B^)) x C = one side’s off the corner layout (offset for B)


B / (Sqr. rt. (A^+B^)) x C = the other side’s off the corner layout (offset for A)

Birch

Tim, UpNorth, all,

I edited over Tim’s photo of Billy Dillon's drawing to show the geometry of the proportionately placed Irregular Hip, unbacked. Billy’s drawing did not line up perfect with my edits and I did not try to make any perspective corrections. The concept is shown and I hope it is understandable.

Since the lines at the shoulders (arrises ?) can be visualized as the same length as the effective hip line (Center line, prior to cutting) the Hip can be simply calculated from the same effective run as an adjacent common, marked in a similar fashon as a common with the standard HAP, and cut using the plan view bevels.

Also, I gave Billy’s drawing some real proportions to work with, 4 and 8 pitches, since it appeared to be approximately a 1 by 2 rectangle.

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blakeb92

u can also scale the raffter lenghts downafter u figure the lenghts and make the same template on a piece of plywood extending the walls to the edge and useing the scaled down lenghts(only reason to scale down so it willfit on a piece of plywood) swing a arch with a tape useing the scaled down numbers from were the corresponding walls intersect the edge of the ply wood and were the two arch intersect is your plane of the hip from the corners of the facia snap a line from the corner thru the intersection of the two archs and this should give u how much to slide the hipit should always slid the the higher plate and it will also give you the bevels of the jack raffters on each side of the hip. all this hingers on the poper playe hieghts to make the planes line up but thats just math. this saves me time cause can do it 2 diementionally and no a bunch of strings . they dont call is a bastered for nothing

olle

Richard you are right the drawing is not perfect . I did it in about 1 min. and drawing on a white board . It was done only to show how easy it is to shift geometrically. This system is done by most all carpenters in the world except Americans , because they all back their hips and valleys . Simple ,fast and accurate . This shift equals the hight of the backing on the sides of Hip or Valley and looks better when work is exposed.
It is amazing to me how many times we go over this in all the different forums . I do lots of timber stuff when the underside is exposed so I need to be able to have this as a tool in my kit. when I am stick framing I have almost never shifted a Hip or valley. The only times I have done it are when the pitches are radically different
Billy

Birch

Billy,

Your illustration is perfect enough for me to understand the concept behind placing an unbacked irregular hip on the outer 90 deg corner.

I only added my edits to show (add too) that the same geometry is also located at the other end of these Bastard hips/vals. I did this to illustrate how simple it is to use the calculated effective length to mark and cut these rafters. No additional corrective adjustments (drop/shift/shorten) are required if this concept is visualized and understood.

The truth (geometric fact) of the matter is; if the irregular hip is not placed proportionately, as you and I have shown, then it will need to be backed on at least the steep side to plane. An unbacked hip/val rafter will only fit in one place.

I agree that the forums are overwhelmed with this repetitive discussion. But I have yet to read in a reference book on roof cutting about how to simply apply the length and cut these rafters. They almost all discuss some sort of hip drop, shift, or shortening of the rafter. They are (In my view) the standard cause of the confusion.

Regards, Richard