I have been away from this site for several weeks now and have just back to working on my roof due to illness. I have my roof framed minus the jack rafters. Can some one explain to me how to find out what my longest jack rafter should be?
Since this is an addition and I am tying into the existing roof I found that the pitch is not exact. It is between 4 and 4 1/2-12 pitch. I tried using rafter tables,pythag. theorem, etc. to get my (common) rafter lenghts and finally just made a template that I clamped to my joists and pulled a measurement all the way to my ridge and got my rafter lengths that way. I ended up finding out that my walls are not perfectly straight on top.
The specs. are 26-5 1/2 span, 4 to 4 1/2-12 pitch hip, 16″ o.c. rafters.
Can someone suggest a method I can use that even a caveman like myself can use to find out jack rafter lengths.
Thanks
Edited 11/4/2008 1:46 pm ET by blownonfuel
Replies
Just subtract 15-9/16" from the run of your last common rafter. The number you get will be your long point.
If your span is 26'5-1/2", your run for the common is 13'2", or 158". If your pitch is 4.25/12, your unit rafter length ratio is 1.06087.
So, your common rafter is 167-5/8" (158*1.06087) and your jack rafter is 151-1/8" (1.06087*142-7/16"). That is from your plumb line to the long point. Bevel at 45° and you're all set.
Jon Blakemore RappahannockINC.com
Fredericksburg, VA
Edited 11/4/2008 2:40 pm ET by JonBlakemore
Thanks Jon. Jon what tells you to use 15-9/16? Do I subtract 15-9/16 from each jack rafter? Is that the jack difference? Also do I use a 12" (4-12) plumb cut or 17"(4-17) on the jack rafter.Thanks
I don't figure them out before hand. Measure from the hip rafter to the outside of the wall, them cut.
I have found that this is the easiest and doesn't require any math>G<
Edited 11/4/2008 4:17 pm ET by frammer52
Thanks Frammer. How do you know how far down the hip rafter to start your layout? I was thinking of doing the same thing but I did not know where on my hip to pull my measurement from. I was afraid that my jacks would not be at 16" o.c..
Easy, start a common rafter at the intersection of the common and hip rafter. Then slide a carpenter square down the common rafter and make the mark, either 16 or 24 and follow the same procedure for the next, until done.
For jacks spaced 16" OC, you subtract 16" off of the run of the previous jack rafter to get the run of the next (shorter) jack.This assumes that your hip is at 45°, which would not be the case if you had a bastard hip. If your pitches are all equal forget what I just said about the bastard hip.Look at the attached drawing. Note the intersection of the ridge, common rafter, and hip rafter. Because the hip rafter meets the common rafter 1-1/16" away from the ridge, you would add that to the spacing between the rafters (14-1/2") to get the number to subtract from the common run.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Okay that makes sense Jon. Thanks. I'm worried my walls might not be as straight at the top plate as they should be also since the pitch is not exact I might end up with jacks either too short or long if I used that math. Is there a way to figure jacks manually?