I’m framing an unequal pitch intersecting roof, which was considered to be the holy grail when I went to hammer school. The math comes fairly easy to me, and I had no trouble making a developed drawing and laying out commons, hips and jacks. But the one part that gave me trouble was dropping the hip correctly. I thought you matched the rafterback height of the commons, but that didn’t work in practice – too high. I looked back through the old books but not a lot is said on this particular matter, and I had to revert to the old eyeball/straightedge method to get things lined up right. Now although this has given me a satisfactory result (eventually), trial-and-error is not my style, and those hips are 6×10 fir, 26 ft long. It’s not cool to be doing cut adjustments when one of those beasts is dangling from the crane.
The roof cut is 8.25:12 on one side of the hip and 10:12 on the other. To make it more interesting, the hips don’t run right up to the ridge; a gable roof extends on above this, so they just let into pockets in the wall corners. So no 1st common/end common instersection to conventiently nail into and self-align. At the bottom, it all sits on top of fir post and beam over a patio. Finally, the whole thing is visible from below, so all the posts (10×10), beams (3.5 x 11.5) and rafters (2×10) have to be finish nailed and nicely fitting – no fudging permitted. Even the strapping has to be pretty. Nonetheless, it’s a fine challenge and I feel privileged to be given a crack at it.
Can anyone explain how to do this right the first time? “Fake it ’til you make it” just isn’t me.
Wally
Replies
The problem is that when you have a hipped roof the two sides are comming up at differant slopes. (I know what you are thinking "duh") Follow me for just a minute.
By default, you are still assuming that the edge that would be the "point" of the hip (where both sides plane together) is still directly above the center of the hip. But because the slopes of the sides are differant, the edge is no longer centered.
Think of it this way... you have a ridge and the each side has a differant pitch. Say 9 and 15. Well in the 3/4" to the center of the ridge, the 15 will project far above where the 9 side would project to. Hip and ridges are no differant, except the hip is worse, because you have more than 3/4" to the center because you are not perpendicular.
There are three ways to solve this. (all of which the math is best done with a 3d cad software, so that you can visually verify the correct answer)
You can rip the appropriate angles on to the top of the hip. If you do this, most of your hard work will be done at this point. You will have differant angles on each side, but the point will have to be in the center. Finding the angles is the hard part.
You can calculate how far to move the Hip off-center. By moving the hip alittle down the wall of the shallow pitch side (and at the ridge) you will be off centering the projection of the two sides. The hard part off this is figuring how far.
The last and probably the easiest way is to adjust the height of the "rafterback" (I call it the stand). you probably noticed that the steep side would have a little higher stand than the shallow side. This method is easily done in the field. Get your hip where you want it. Once nailed, take a tape measure and measure straight up the side of the hip, exactly at the point where the side of the hip crosses the outside of the wall. That height is what you will make you stand (rafterback). For best results you can do this on both sides.
Hope that helps. I know i'm not the best at clarifying my ideas, so if you have any questions, just ask.
kyle. do you have a 3d drawing of the 3d method "stand" ?Mike Smith Rhode Island : Design / Build / Repair / Restore
There's alot involved with this roof but there's alot of answers for you. Do you have any measurements? If so post them here. It also sounds like you have a Dutch Hip situation. Do you have a Construction Master Calculator?
Joe Carola
Yes it occurred to me that this is essentially a dutch hip. I should have explained that this is a canopy roof out over a patio area around three sides of the building. So the main buiulding roof framing is complete (that was fun too, TrusJoist system over post and beam, 10:12 with 12 valleys, dormers with raked overhangs) and we are now tying in this architectural roof to it.
No, I don't own a construction master, I just use a regular scientific calculator. We weren't allowed to use 'em in school and I'm still too cheap to buy one. From the few times I've used one, they seem to save a bit of time, but don't do anything I can't figure out from scratch. Neither offers any of the minute details I needed for this situation. I still had to draw the hip and jacks full size to get the shortening and sidecut numbers for each side of the roof.
On the whole, this roof has gone very well, I'm just a perfectionist that expects to better each time. Having to mess around with the hip and resort to empirical methods while it was up in the air was not part of the plan.
Thanks,
Wally
wally... framer has a good point... i own at least 3 scientific calcs...but since i bought my CMIII .. they gather dust..
the conversion from decimal to fractions alone is worth the price
and , i've almost forgotten what the hell SOHCAHTOA ever referred toMike Smith Rhode Island : Design / Build / Repair / Restore
Ok, sorry - that calls for a "tangent"... The Indian legend of Pythagorean theorem:
An Indian chief once had three squaws. He gave the first squaw a deerskin hide to consummate their relationship on, he gave the second squaw a bearskin hide for the same purpose and he gave the third squaw a hippopotamus hide. The deerskin squaw had one son, the bearskin wife had two sons, and the squaw on the hippopotamus hide outdid them both - she had three sons.
The moral of the story:
The son's of the squaw on the hippopotamus hide are equal to the sons of the squaws on the other two hides. Oh yea, the third squaw's name... SOHCAHTOA of course!
As to the original question in the thread - I would do what I've allways done since I forgot everything I learned in math class besides that story... CHEAT WITH 3D AUTOCAD! Sorry, wish I could be more help. :-)>Kevin Halliburton
"I believe that architecture is a pragmatic art. To become art it must be built on a foundation of necessity." - I.M. Pei -
Mike:
Here is a 3d model that I created to prove the method of beveling the top of the hips. To much work to do with 30' hips (which is probably the average that I work with). I was going to post the Cad file but it was to large so here are two pics of what I am talking about.
Did try it on a dog house once. Looked like furniture. Hated to cover it with decking.
Sly_Karma:
I have 3 calculators on my desk. 2 scientific and 1 construction master 4. I use the scientific the most, but I love the CM4 because it does everything in feet and inches.
To All:
I had a hippie for my trig class in college and he said that SOHCAHTOA meant...
Some Old Hippie Caught Another Hippie Trippin On Acid.
He didn't have a indian mating story to go with it, but he looked like he was speaking from a personal experience.
Wally,
You really don’t need 3 calculators or 2 cad programs to figure any of the information for irregular hip roofs. You can do it with a square, bevel square and tape measure as long as you know which lines to draw.
With your roof I’d not go with dropping the hip because with a 6" hip thickness you need to make up 3" and that’s a lot of drop. A quick guess is it’s about an 1" on the 8 side and an 1-1/2" on the 10 side. Back beveling may take a bit more time, but will make for an easier go when putting the jacks in.
Edited 12/20/2003 8:17:36 AM ET by Resurrected
if only it were so easy as backing the hip. But the engineer feels that the 10x6 beam he spec'd is adequately sized for 20 ft clear span, but not excessive, so no backing allowed - too much loss of strength. Believe me, I thought of it right away....
All due respect on the construction master thing, but I've always believed in knowing how to do it from first principles. Then when an oddball comes along - like this one - I can reapply those principles and proceed to desired outcome. I do have a fraction function which is useful at times. Better yet, when working alone I do it all in metric and avoid all that messy feet-inch-fraction keying. But since most of the crew don't speak metric, I am often forced to go imperial so they can cut what I want. But I draw the line at stairs. Just can't bring m'self to leave metric when it comes to stairs. Grew up metric, it's still my mother tongue.
My old school texts are strangely quiet on the minutiae of unequal pich roofs. Anyone point me somewhere?
Wally
Sorry to Resurrected, I forgot to open your pdf attachment before posting another request for mathematical way to calculate hip drop for split pitch roof. Thanks, it was what I was looking for.
Wally
Wally,
I’m glad it came in handy. If you can get the April 2003 issue of The Journal of Light Construction, there is an article I’ve written in it that details how to mechanically layout all aspects of a split pitch roof using just a square, bevel square and tape.
If you like I can give you the complete steps for calculating the layout with either a scientific calculator or a Construction Master IV.
Good luck.View Image
I am from the KISS school. A hip is the intersection of 2 or more planes. Simply frame as much common roof as possible on both sides, intersect the lines with a straight edge or a string line, you have your hip.
Are you planning on shifting the hip towards the 10/12 side to keep equal overhangs? Probably not because then you would have to raise the top plate on the 10/12 side.
I know you said you have 10x10 posts, without knowing your overhang I can't tell you how much to shift it.
If you plan on keeping the hip on the corner with different overhangs and you can't bevel it then you can shift the hip towards the 8.25/12 side a little bit until it planes in with the two different pitches.
Since you have a 26' 6x10 beam as you know there's no room for era so putting in all the commons for both sides and running a 2x across the plumbcuts of your overhangs and nailing them at the intersecting point will be a good idea as "Miles of Trim" pointed out.
The math in figuring isn't a problem in a perfect world so your best bet to get your points and measure exact. Or by a 26' 2x10 or micro and cut a test piece.
This is a thread almost a year ago if you scroll down a little ways you will see where we start talking about shifting the hip that we discussed shifting the hip to plane in without beveling and just adjusting the H.A.P cut on the hip.
http://www.jlconline.com/cgi-bin/jlconline.storefront/3fe440bf00c32cd2271a401e1d290645/Catalog/1139?read=6670
Her's a couple of pictures I drew that went along with the thread.
Joe Carola
Edited 12/20/2003 11:21:28 AM ET by Framer
Wally,
This graphic pretty much detail the action of shifting and sliding the hip into a correct position to allow the two different roof planes to plane into the hip.
Basically you can drop the hip by two methods; one being you lower the hip along its vertical plane and the second is you move it back towards the ridge or shift it in its horizontal plane.
Both accomplish the same thing they cause the outer edges of the hip to align with their respective roof planes.View Image
I haven't checked this in my code book for awhile, but even if you are using the 6x10 beam as a hip, you should be able to bevel each side without reducing any strength.
I could be wrong, but I think you could still bevel both sides. I'll do some checking later. I have to go and beat my brother at raquetball :-)
So tell the engineer that! They often spec well in excess of code and their word is like God's as we all know. No planing of the hip beam permitted.
The hips are all finished now - with their off-centre strapping joint line - and the architect seems happy. Next fun project is getting the tail cuts to work with fascia at the corners. The architect in his wisdom wants all the tails cut square to the roofline - regardless of the pitch. So now I will have the fascia intersecting at the corner in a mitred joint that will not be plumb because the 10:12 side leans out further from the vertical plane than the 8 1/4:12 side. Going to be a lot of planing and tweaking on those hips tails to make this work. At least he went with my fascia look - two piece rough-sawn 1" fir. The smaller 6" wide piece can be adjusted up or down over the 10" piece behind to take out and unevenness in the tails. (No rough fascia, and all visible from below).
Went skiing today and it was sunny and mild with a touch of fresh. Makes the roof framing headaches of the week all float away...
Wally