Alright guys, maybe someone can translate for me… I’m framing an unequal pitch hip roof…six front and back, eight left and right. Twelve inch soffit all around. The best I can decipher from “A Roof Framer’s Secrets” is that if I want equal overhangs all around it will somehow throw my hips out of the actual corners of the house. Is this correct? How do I figure out where to place the hips. In addition how would I figure out my plumb cut for this hips…can’t use 8 and 17 on the square, can’t use 6 and 17 on the square…I just can’t figure out how to make it work although I’ve seen it done a hundred times. Short of the fundamentals I’m a self taught roof cutter, so I don’t have any formal roof cutting or trig background to fall back on….any help? Framed dozens of regular hip, octagons etc but this irregular stuffs got me.
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Replies
Diesel,
You are correct that your hips will be thrown off of the corners in order to have equal overhangs all around. In addition you will have to either deepen one of you seat cuts(the 6/12) or else add to your wall height on the 8/12 side. This is because of the difference in pitches. The 8/12 will be 2 inches lower on a 1 foot overhang so you will probably need to add to the wall height as you won't have the room to take an additional 2 inches out of your seat cut on the 6/12 side. (Unless you are using 2x10's for rafters) As to the cheek cuts on your rafters: on the 6/12 side you need a 53 degree cheek cut for the bevel (6/12 on the plumb cut) on the 8/12 side you need a 37 degree bevel and an 8/12 plumb cut. slope on your hip is going to be 21.8 degrees (4 3/4 inch pitch if you looked at it as a common rafter. ) The cheek cuts at the top and bottom of the hip are the same (37 and 53) The offset on the hip is about 3 inches towards the 8/12 side of the roof. When you figure this roof figure your commons on the 6/12 side and note the run and the rise ht. Then punch the rise ht into your construction master and enter 8 " and hit the pitch button. Now hit run and make a note of it. That will tell you how far in your last common rafter on the 6/12 side will fall.
Example
run on the 6/12 side is 12 feet yields a 6 foot rise
6foot rise on an 8/12 pitch yields a 9 foot run
These 2 points (12 and9 )feet are where the hip will intersect the ridge
as you can see they are not equal which is why you need to offset the hip at the corner in order to match the overhangs. The hip does not run (in planview) at a 45 degree angle to the corner because of the difference in pitches.
Hope this helps you out.
P.s. there is more information on this over at the JLC website
Mark
Mark,
Thanks for the good info. Have an update... while describing to the builder today that the hips would be offset he decided that he wouldn't like how that would look. I tried to tell him it would look correct but he couldn't seem to visualize...make a long story short, he cuts the check and I don't argue. I told him the only other option was to lengthen the soffit (level tail cut) on the six pitch side by 4 1/2 inches which would bring the six pitch soffit/facia cuts in plane with the "naturally" occuring 8 pitch side. Resulting in a 16 1/4" soffit in front and back and 12" soffit on the sides. He liked this better! So now my question is... do I still offset the hips? or can I now cut it as a regular 45 degree hip. Excuse my ignorance regarding the math behind all this. Both my six pitch and my 8 pitch yeilded an 8 3/8 heel stand, so if I cut my lvl hips to the same heel stand will everything remain in plane if I don't offset the hips? I know I'm asking alot here, but your help is greatly appreciated.
Brian
One more if you can stand it... if I were to raise the plates to compensate for equal overhangs, wouldn't that throw off the heel stands and plane of the roof, or do raising the plates and offsetting the hip "cancel out" or compensate for one another?
Get a hold of a copy of Barry Mussels " The Roof Framers Bible" gives you everything you need in an easy to understand reference book.
Brian,
To take them one at a time. First if you are using different size overhangs then it isn't necessary to offset the hip. The reason is that you are compensating for this by in effect, offsetting the overhangs (they are now unequal). You cannot cut it as a regular hip. The cheek cuts will still have to be 37 degrees one side and 53 on the other. You still have an irregular hip in addition to unequal overhangs. The hip does not come down at a 45 degree angle(in planview) to the corners. As to your heel stand you will have to cut the seat cut deeper on the hip just the same as you would on a regular hip. This is in order to bring the edges of the hip down to plane with the jack rafters. The other option is that you can bevel the edges of the hip. The seat cut is a lot easier. As to raising the plate. That would only be necessary if you wanted to equalize the overhangs. Because you have a total drop of 8 inches in 1 foot on the 8/12 side and only 6 inches on the 6 inch side then you would have to either raise the plate on the 8/12 side OR increase the overhang on the 6 inch side in order for your facia and soffits to line up. Since you are doing the latter you can leave the seat cuts alone. Now as to whether the offset of the hip and raising the cancel each other out. I don't know if that is the proper term but here is what is happening in that scenario. Raising the plates raises the tail end of the rafter so that it lines up with the shallower pitch rafter whil maintaining the same overhang. Offseting the hip makes everything plane the same. Hope this helps.
Mark
Mark...
That completely clears things up for me. Thanks so much. I still wish that I had the trig background to understand the "whys" from a mathematical relationship point of view but I have enough experience to understand what is happening...thanks again for your time and concise clarifications. What would I do without this site? Probably waste alot more stock and chalk lines!
Brian
Brian,
You are quite welcome. Understand that you don't have to be well versed in trig to frame most roofs. Simply use the pythorean theorum. Simply stated A squared plus B squared = C squared with C being the hypotnuse of the triangle. This only works for right triangles but most roofs are composed of right triangles. If you have a construction master you can dispense with having to plug in numbers and find square roots because it "thinks" in feet and inches. As such all you have to do is think in rise and run and you can figure any right triangle. Sometimes it helps to think in plan view. That is how I found the run of hip in relation to the commons.
Someone else suggested that you pick up a copy of the roof framers bible. I would highly recommend that you do as it will better explain the relationships of roof triangles to one another. Once you can visualize how they relate it is not hard to figure them. Good luck.
Mark
diesel..... marks way is faster... but i never use the trig on those instances..
the hips will offset to the steep pitch sides.. frame the common steep rafters first...
then pull some string lines with the plane of the steep pitch.. measuring down from the plane will tell you where the intersection will occur. and wether you want to increase the width of the rafter, decrease the heel cut , or raise the plate height....
Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike,
I didn't use trig either. Just simple pythagorean therom and a construction master. Took me about 5 minutes.
mark
Dieselpig,
I agree with the Prof about heading over to the JLC forums. There is a ton of info on roof framing to be had. Here is the link. http://www.jlconline.com/cgi-bin/jlconline.storefront/EN/Catalog/1139
I would also go to http://www.josephfusco.org/ubbthreads/ubbthreads.php