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Backing the Hip

ziggle's picture

Hi folks,


I have been reading the book "Roof Framing" by Gross and I am curious about when one would use the technique of "backing the hip." In the book, the author states that "backing the hip" is "seldom done" and adds "There is another way (not always the best way) called dropping the hip."


I understand both methods. My question is under what circumstances is "backing the hip" preferred over dropping the hip. Also, what is the best way to perform backing cut. It appears to require two saw passes to cut it. Is that correct? Is there a better way?


Thanks!

(post #104888, reply #1 of 9)

ziggle,

You will hardly ever need to go through the process of backing the hip. Backing and dropping are to allow the hip to "plane in" with the common rafters on each side.

Dropping is far easier and a hell of a lot faster. If you want to see more about it visit: http://www.josephfusco.org/Articles/Roof_Cutting/Basic_Roof_Framing/Documents/raftercutting.htm

and browse around a bit.

(post #104888, reply #2 of 9)

Be careful when reading his backing angles because they're wrong. He says that the backing angle is the same as the hip plumbcut angle, and that's wrong.

He uses an 8/12 pitch in his example. The hip plumbcut angle is 25.24°. He tells you to set the saw just past 25° which is incorrect. The backing angle is 23.09°.

You would back the hip if it was a big timber, or back the bottom of the hip if you had a cathedral ceiling.

Joe Carola
Joe Carola

(post #104888, reply #3 of 9)

Backing the hip would be used in an application like timber framing, where the benefits of backing and the drawbacks of dropping the hip would be magnified.  The wider the hip framing member, the greater the void created just below the mitered sheathing boards that join on top of the hip, and the greater the drop required.  Entiendes?

 “Good work costs much more than poor imitation or factory product” Charles Greene
CaliforniaRemodelingContractor.com

(post #104888, reply #4 of 9)

Construction of the Backing Angle ... links to diagrams detailing how to derive the better known formulas (there are dozens of other backing bevel formulas) from the geometry.


Development of the Backing Angle ... the proportions in plan to each side of the line of the Hip run govern the Hip rafter offset. Offsetting the Hip produces equal shoulder heights on either a backed Hip or dropped Hip.


Joe Bartok
Joe Bartok

(post #104888, reply #5 of 9)

Development of the Plumb Backing Angle ... which is no longer a true backing angle or blade bevel, this is the Backing angle as viewed in plumb section. The same offset ratios occur in plan view but note the length being rotated to plumb. There's the Hip drop.


Revolution of the plane of the Backing Angle ... hope you can make heads or tails of this one. Someday I'll get around to making better graphics, these kind of suck.


Joe Bartok

Edited 10/5/2007 9:48 am ET by JoeBartok


Edited 10/5/2007 9:48 am ET by JoeBartok

Joe Bartok

(post #104888, reply #6 of 9)

Log Valley Rafter and Purlin Details ... another example of why backing the Hip-Valley might be preferrable. This is a regular roof, two intersecting 10/12 slopes so the trough line is centered. We have done irregular or bastard roofs where the Valley was offset to produce equal arcs on each side of the rafter. (Same idea as the equal shoulder heights on a timber Hip rafter).


I can't find the thread but fairly recently in this forum "dropping" the Valley was mentioned. At the time I didn't understand the question. Since then I've had a converstion with a log builder and he told me that many log builders, rather than backing, actually do "drop" the Valley and cut the jack rafters or purlins so that they lay over the Valley rafter.


Joe Bartok


Edited 10/5/2007 10:15 am ET by JoeBartok

Joe Bartok

(post #104888, reply #8 of 9)

Lots to comment on:


When to back the hip?


 

"Backing the hip would be used in an application like timber framing, where the benefits of backing and the drawbacks of dropping the hip would be magnified.  The wider the hip framing member, the greater the void created just below the mitered sheathing boards that join on top of the hip, and the greater the drop required."


This is exactly the kind of comment I was looking for.


Gross Book:


I have confirmed the error in his backing angle calculation. I have also picked up the Holladay book.


Mathematical References:


I have gone through Joe's material in detail (this took some time - there is quite a bit there). I believe I understand his work and have even written my own spreadsheet and have duplicated his results.


Thanks to all for the help.


Ziggle


Edited 10/8/2007 1:31 pm ET by ziggle


Edited 10/8/2007 2:09 pm ET by ziggle

(post #104888, reply #9 of 9)









 



 


This mockup, made from a scrap of 4x4, contains all the angles needed to cut a hip roof with a square-cut fascia.  Angle A represents the pitch of the roof.  Line 2 is drawn at 90 degrees to line 1, from the corner.  It represents the run of the roof.  And the green side of line 1 represents the rise of the roof. 


Line 3 is drawn at the same length as line 2.  Lines 4 and 5 are drawn once the length of line 3 is determined.  Line 5 represents the hip rafter, as line 6 represents the common rafter.  If you cut away the dark brown portion, the piece remaining contains all the pertinent angles, in 3D.


(re-posted, with a new pic, to try to clarify some things - hope it helps)


 

 “Good work costs much more than poor imitation or factory product” Charles Greene
CaliforniaRemodelingContractor.com

(post #104888, reply #7 of 9)

About the only time I back hips is in irregular situations, and only on the steep side. 


I sometimes do it with valleys, but that is once in a blue moon.


 


Edit:  Buy Will Holladay's book and ditch the Gross book.  It is much easier to understand imo.


Edited 10/5/2007 9:01 pm ET by Timuhler

www.pioneerbuildersonline.com From Lot 30 Muirkirk

http://picasaweb.google.com/TimothyUhler