Equal-Pitch Roof w/ Various Wall Heights
I am framing a house right now that has one span of 40′-3 1/2″ from an 8′ (nominal) back wall to a 9′ (nominal) front porch beam. Pitch is 7/12.
I used Will Holladay’s A Roof Cutter’s Secrets (pp. 240-241) to find the common lengths for both sides. The back side measured 291 11/16″, and the front measured 267 15/16″, giving me a difference of 23 3/4″.
I then performed the calculations on a different span (same pitch) and again found the difference in common lengths was 23 3/4″. So is it safe to assume that for this situation the difference in length of commons will always be 23 3/4″?
Also, I am assuming that the hip jacks that sit on the 9′ beam will also need to be measured using the shorter, front commoon pattern. Is this correct?
EDIT: I also assumed when I laid out all my plates that the ridge would be centered using 1/2 the span (20′ 1 3/4″). When I scaled the plans (using an achitect’s rule) it seems the ridge is biased toward the front; the scaled spans appear to be very close to the “adjusted” spans as described in the book, above. Have I royally screwed the pooch?
Thanks in advance!
Jason Pharez Construction
Framing Contractor
Edited 4/30/2006 5:13 pm ET by JasonPharez
Replies
After a long time of going through your measurements and the Roof Cutters book I came up with numbers fairly close to yours within a 1/16 of an inch. The ridge should be offset 20 1/2 inches to the front, if I got your situation right. 20 1/2 inches on a scale ruler equates to just under 1/2" on the 1/4"=1' scale.
My Mommy says I'm special.
Stilletto, thanks, and you're exactly right.
As per Holladay's calculations, it takes an extra 20 1/2" of horizontal run to have rafters from 8' meet rafters from 9'....i.e., a valley on a 9' wall has to be offset 20 1/2" to plane in with adjoining rafters bearing on an 8' wall, or as you described, the ridge would be 20 1/2" closer to the front.
BTW I ended up going out to the shed and tracing a scale model on a sheet of OSB, then figuring the layout for the walls from the plan.
I also found out that even though the "actual" run of the rafters is different for the front and back, the ridge length is still the same as if both sides came off an 8' wall--2' 11 1/4" in this case (length of bldg= 43'- 1 1/4").
I hate when these little problems stump me, but I'm oh-so-happy when I do the legwork and figure out for myself what the answer is.
I guess tomorrow AM I'll have my lead man re-layout the plates for the "new" runs...when I marked it out on my shed floor I found there's gonna be a slight problem with some of the rafters hitting on the joists, but I can compensate for that.
Thanks again!Jason Pharez Construction
Framing Contractor
No problem man, I run into these situations once and a while framing houses up here.
BTW Roof Cutters Secrets bailed me out this week as well, the Pop Out Window on pg. 295 works very good. I had never built one before without floor joists underneath to support it.My Mommy says I'm special.
I get a 20 9/16" ridge offset, with a difference in commons of 23 13/16".
With an offset height of 12", and a pitch of 7/12, this difference would be constant.
"Citius, Altius, Fortius"
Jason,
Your rafter lengths are different because you have a different run for the front and the back because of your 8' back wall and 9' beam height. Therefore that will move your ridge from the center of the 40' 3-1/2" span.
Since the beam height is 1' taller then the 8' wall height that gives you a triangle with a 7/12 pitch with a 1' rise and a run of 20-9/16" and a rafter length of 23-13/16"
Add that 20-9/16" run to your 40' 3-1/2" span and get 42' 1/16" (Working Span)
Divide 42' 1/16" by 2 = 21' 1/32" (Back Rafter Run and Ridge Placement)
21' 1/32" Run gives you a rafter length of (291-3/4") for the back rafters.
Front Rafter.
40' 3-1/2" - 21' 1/32" = 19' 3-15/32" (Front Rafter Run)
19' 3-15/32" Run gives you a rafter length of (268") for the front rafters.
You said, "So is it safe to assume that, given a similar situation, the difference in common lengths will always be 23 3/4"? Is this difference correct?"
That is true if the height difference for the same 7/12 pitch is always 1'.
Joe, I replied to yours in the same thread on JLC...thanks again.
And thanks again to everyone who "bailed" me out on this....this is yet again another situation where due to my age and inexperience, I have to spend some "hard time" figuring things out!
And all in pouring rain LOL.Jason Pharez Construction
Framing Contractor
Jason,Here's a drawing just in case you want to see where the triangle is and the rafter difference.Joe Carola
Hey, Joe, did you do the free download of Google Sketchup yet?
It's no good for 2D drawings like the one you posted here for Jason, but for 3D visuals, it is terrific.
Gene,I did download it the other day. This should be fun.Joe Carola
Thanks for the drawing, Joe, and that's basically what I did yesterday to figure out how everything went together.
BTW, we set all the ridges, hips, valleys, and stacked all but one of the hips today...after I had my lead correct our layout, everything went really smooth. And everything planed in and worked out correctly. I'm kinda proud of this roof.
We also had to set a 9' height, 31' long, dbl. 2x12 front porch beam...went rather smoothly, too, other than some grunt work!Jason Pharez Construction
Framing Contractor