I have tried to put a Sketchup demo together to illustrate what I consider to be roof cutting basics. I hope this will eliminate the confusion that many roof cutters experience with Hip and Valley rafters, Regular or Irregular. This simple concept is applicable to all flat slope roofs and also works for either 90 degree plan-view corners or dog-leg intersecting angles too, and for any combination of pitches, equal or unequal.
This is based on a simple geometric principle of equal angles and line lengths that create parallelograms. Any two parallel lines that cross two other parallel lines in the same plane will create an equal sided shape known as a parallelogram. Any other lines contained in the plane that are parallel to the sides of the parallelogram will be equal in length to the sides.
Based on that, in lay terms of the framing trade we can assume that “Long to Short†and “Out to In†and “Center to Center†will basically describe the same equal, or proportional, relationships. For a roof cutter this means that a line contained in the interior, or center of a rafter, is equal to one that is found on its sides. This concept is easily understood for common rafters but confusing when applied to Hips and Valleys, yet the same concept of parallel lines and equal lengths hold true for them as well, they are just not square parallelograms (rectangles), but to keep the HAPs equal the opposing reference points for the cheek cut miter angles are located square, or perpendicular, to each other.
This means that the Hip/Valley rafter can be marked on its side just like a Common. Only the point of reference, from center to side, need be changed.
All rafters can be calculated and cut using dimensional info found in plan-view. The typical calculation for determining both the Common and the Hip/Val lengths are to first find the effective run of the common. Span minus the Ridge thickness divided by 2. Whether using the framing square tables, or a reference book with tables, or a Construction calculator the length of both commons and H/V rafters is based on a “Per Foot Runâ€. This is “Per Foot†of the common’s run.
This is kind of similar to the Swanson’s “One Number Methodâ€â€¦ only what they mean is that you only need to use one number on their square to mark either the common or the Hip/Val…one number ,two scales, as opposed to using the framing square where you needed two numbers held on the boards edge to give you either the H/V or Common plumb angles. Commons were marked Pitch on 12†and H/V were marked Pitch on 17â€.
But when you calculate the two different rafter types, you do so based on one number, the effective run of the common.
This concept in roof cutting makes basic roof cutting tasks simple. It eliminates any extra corrective measures that are practiced with other methods that are executed from the center line of the Hip/Valley, such as “Dropping the Hipâ€, “Shifting the Hip†and/or “Backing a Hip†to bring them back into plane. The Hips are placed proportionately with this method and the geometry will automatically locate the plane-in points. And if you still want to back a hip or valley this method allows for this in a simple way too and keeps the exposed sides of exposed rafters equal in height on both sides.
The SU model scenario is for demo purposes only and uses 6/12 for the Major pitch and 8/12 for the Minor pitch. You can use your Page up/down keys on your keyboard to scroll the views.
If you have comments or questions, please post them. Hopefully the concept is clearly demonstrated. (I’m just learning how to use Sketchup)
Replies
Daaaaayyyum..THAT is cool. I didn't get all over the whole roof, but what I did fly around to, looked sweeeet.
THANKS
Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations
"We strive for conversion,we get lost in conversation, and wallow in consternation. "
Me.
can't open
"can't open"
You need to have Sketchup installed to open the drawing.
Mike HennessyPittsburgh, AP
You can download Sketchup for free here.
http://sketchup.google.com/download/
thanks!
Very nice work MrJalapeno! You can't tell me you're "just learning" skecthup because you've done some very excellent advanced work there. A mock up like that would have been very welcome in my younger days when I was trying to learn how to cut a roof.
I noticed that you chose to have your hip in your unequal hip roof land directly over the outside corner of the plates. I know that is very common in some spots but rare in others. I think for your purposes it illustrates the cuts perfectly. I'm just commenting on that one aspect because I started doing irregular hips in the late 80's and I never saw one that landed over the corner like that.
I really like what you've done though and I'm really impressed. If I was still running a crew, I'd borrow your idea and create one that showed my guys how to calculate and figure for the cornice too.
Bob's next test date: 12/10/07
Jim Allen,
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I did the SU demo without tails or cornice or mentioning step-offs for equal overhangs to just get the basic idea across first.
Here is the very first SU model demo I did about a week ago for Tim U. over at JLC. This one is less detailed but shows the pitch relationships reversed with the major pitch being the steeper one and I included tails. Notice the stepped plate height and run that is necessary to equalize the overhangs. You’ll notice that they are not on pitch because I was able to adjust the HAPs (Like on a real job =) and use three 2x4 plates to build up the plate (4-1/2”, on pitch called for 4” rise and a 6" run).
Edited 6/11/2008 12:16 am ET by MrJalapeno
Edited 6/11/2008 12:16 am ET by MrJalapeno
Jim,
Around here, no one seems to care if the overhangs are unequal. It is interesting how things are different around the country. A lot easier to frame unequal overhangs though.
It wouldnt be easier for me because I'm not opposed to altering the heel to accommodate the necessary height change. Bob's next test date: 12/10/07
Tim and Jim,
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I don’t know if you guys have thought about this, when you calculate your overhangs for irregular roofs with level soffit/fascia the overhang width(s) will be set in stone. If you frame for unequal overhangs the proportions are fixed. If you frame for equal overhangs there is only one width that will work for both sides and that is a fixed dimension too. Only regular/equal pitch roofs provide for various widths, but they're always equal (from where the hip/hap lies). =).
Edited 6/11/2008 3:08 pm ET by MrJalapeno
Edited 6/11/2008 3:09 pm ET by MrJalapeno
"If you frame for equal overhangs there is only one width that will work for both sides and that is a fixed dimension too. "I don't agree with that statement unless you add another qualifier. Bob's next test date: 12/10/07
The qualifier is "Irregular Roof/equal overhangs" si?
I'm with Jim Allen on this one - I don't see how you can say that there is "only one width" that will work. You can use a 12" overhang on both side, or a 16" overhang, or whatever. You just have to adjust the heel heights (Or HAP) to make it plane out.
The absent are never without fault, nor the present without excuse.
Boss,
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What I am saying is once you have adjusted your HAP to accommodate a 16” equal overhang, then that is the only width that will work. Kind of like when you send out an irregular trusses hip set for an 8/12 and 12/12, and the 12/12 side has a 4” HAP and an 6" offset adjustment. Only a 12” overhang will work for them.
Same pitches, 5-3/8" HAP and 8" offset adjustment, 16" overhang only.
So I guess another qualifier is that the “one width” overhang is predetermined.
I did hindsight some confussion with that statement after posting it. Sorry about that.
Edited 6/11/2008 4:09 pm ET by MrJalapeno
"I did hindsight some confussion with that statement after posting it."
You hindsighted some confusion with that statement too.
(-:
Life is like a grinding wheel. It will either sharpen you up or grind you down, depending on what you're made of.
Whatever it is, that's what it is. =)
Again, sorry for any confussion.
Jim Allen and Boss, both are right.<!----><!----><!---->
Edited 6/12/2008 3:09 pm ET by MrJalapeno
Thanks for sharing this! :)
I don't generally cut roofs but I like knowing this stuff, cuz ya' never know when I might need it. Also, I just really like this stuff.
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