Author Topic: Calculating Valley Jack Rafters?  (Read 6613 times)

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Offline sjdehner

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Calculating Valley Jack Rafters?
« on: May 06, 2009, 03:57:23 AM »
Hi,

Hopefully class is in session today...

I'm looking for some assistance with calculating valley rafters & valley jack rafters. Any tutelage or links would be welcome.

Thanks for any thoughts.

Shawn
“Whether we and our politicians know it or not, Nature is party to all our deals and decisions, and she has more votes, a longer memory, and a sterner sense of justice than we do" -Wendell Berry

Offline Don_P

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Re: Calculating Valley Jack Rafters?
« Reply #1 on: May 06, 2009, 04:31:15 AM »
This is mine, I wouldn't necessarily trust the jack lengths because it varies according to how you start layout but the "difference" is correct... how much to deduct from one to the next. If you start with the longest you can reuse a screw up the next jack up.
http://windyhilllogworks.com/Calcs/irreghip.html
There's more here;
http://www.windyhilllogworks.com/CalculatorIndex.htm
There are much more knowledgeable roof cutters and mathmeticians than me out there. Joe Fusco and Joe Bartok both have good calcs online, there is one on the Construction Resource forum too.
Check back in with dimensions, details and a sketch if you get stuck?

Offline JRR

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Re: Calculating Valley Jack Rafters?
« Reply #2 on: May 06, 2009, 05:17:00 AM »
Rafters and stair chords ... places where the "occassional craftsman", DIY's who don't do it often ... perhaps never again ... can waste a lot of lumber thru miscuts/miscalculations.

I've learned to make "test" lumber stock by sticking together scrap pieces of lumber for the experimental cuts. 

For example:  Assume the rafters are to be cut from 2x10's.  Attach two shorter pieces of scrap 2x10 together with lengths of 2x4's ... so that the whole thing is long enough for the application.  Use screws for easy disassembly and re-adjustments.  Make the end cuts and length adjustments until you get the exact fit you want.  Then easily and confidently transfer the final shape to a new piece of full-length 2x10 stock and make the cuts "perfectly" with no waste of stock.  Often you can disassemble and reuse the same test piece over and over for rafters of graduating lengths.

Offline Redoverfarm

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Re: Calculating Valley Jack Rafters?
« Reply #3 on: May 06, 2009, 12:30:41 PM »
You might try www.blocklayer.com.

Offline sjdehner

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Re: Calculating Valley Jack Rafters?
« Reply #4 on: May 06, 2009, 03:06:53 PM »
Hey, I appreciate the links and information. Thanks for taking the time to give out some direction. Shawn.

“Whether we and our politicians know it or not, Nature is party to all our deals and decisions, and she has more votes, a longer memory, and a sterner sense of justice than we do" -Wendell Berry

Offline NM_Shooter

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Re: Calculating Valley Jack Rafters?
« Reply #5 on: May 06, 2009, 03:33:34 PM »

I've learned to make "test" lumber stock by sticking together scrap pieces of lumber for the experimental cuts. 


What a great tip... thanks!  to further this a bit, I wonder if there is a way to put end pieces on two pcs of pipe that telescope and you can adjust them to fit.  I don't know if that would flex too much or not.  This has the potential of saving a ton of wood.
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Offline Don_P

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Re: Calculating Valley Jack Rafters?
« Reply #6 on: May 06, 2009, 05:14:33 PM »
These are the guys calcs I mentioned this morning.

This is Joe Bartok's site, if you scroll down the calcs are lower down on the page.
http://ca.geocities.com/web_sketches/
This is one of his roof calcs;
http://ca.geocities.com/web_sketches/hip_valley_dimensioning/framing_working_points.html

This is Joe Fusco's
http://www.josephfusco.org/Calculators/Simple%20Roof%20Calculator.html

This is the Construction Resource calc
http://www.construction-resource.com/calculators/roof-frame.php

I use JRR's method when I'm using expensive or hard to source materials. It works great and helps avoid making dumb mistakes. If you're using nice exposed beams a miscut can be a week's pay. I've also built sections of roof out of tacked up sticks and strings when I've had visualization problems.

Offline Jens

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Re: Calculating Valley Jack Rafters?
« Reply #7 on: May 06, 2009, 06:45:30 PM »
One thing I do sometimes for valley or hip rafters, is to take a piece of stock about 4' long or so, and make my valley/hip seat cut in it.  Make sure that the vertical measurement from the plate to the top of the rafter matches that of the common rafters, so that the roof will be in line...this usually means a deeper seat cut, to drop it a bit more.  Draw a line going straight from the plumb cut of the seat, and put a nail in the top of the rafter.  Now, I tack the dummy to the layout marks on the corner of the plate.  Take a tape measure, and go from the nail to the center of where the rafter mounts to the ridge, that is the measurement for your valley/hip rafter.  Don't forget to leave extra on the end, so that you can trim the tail to be perfectly in line with the ends of the common rafters, I usually use a string line along the commons, stretching over the long tail, and make a pencil mark, trim it in the air.  I hope that was clear, I am not always good at explaining my methods...just ask PEG ;D

One more note, if you are having trouble figuring out the plumb and seat angles, stretch a string line from ridge to plate.  Make sure that the line is held above the plate enough (in line with the commons, can do this by eye sometimes), and use your speed square at the ridge.  Put the square on the face of the ridge, and the string will cross at the desired angle (if you are sure that the pitch you read is wrong, flip the square around), don't forget that you can read in degrees, or hip scale. 

Hope that helps some.  I have had to work irregular roofs this way, and ended up doing most stuff with stringlines, and scrap rafters.  It takes patience, more than skill.  I even built a hip roof for a bay on a workbench this way, hoisted up in one piece, worked like a charm!
just spent a few days building a website, and didn't know that it could be so physically taxing to sit and do nothing all day!

Offline Jens

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Re: Calculating Valley Jack Rafters?
« Reply #8 on: May 06, 2009, 06:49:47 PM »
Oh yeah, forgot about the jacks.  I usually stretch a tape from three or four commons back, eye it up for square to the rafters, and make sure they are on layout.  Once you are sure they are, 16" (or 24") center to center, just slide it down the rafters until the tape layout lines up with the hip, make your mark.  I wish, at times like these, that I could just make a darn video of the process!  If you take the math out, and pull your tape out instead, things get much more clear most of the time.  Like I said before, I hope this helps (not just confuses!).
just spent a few days building a website, and didn't know that it could be so physically taxing to sit and do nothing all day!

Offline Don_P

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Re: Calculating Valley Jack Rafters?
« Reply #9 on: May 07, 2009, 05:17:24 AM »
Sounds like we could work together Jens  :)
One thing the blocklayer calc does really well is checking the birdsmouth on the hip. It cannot come inside the plate. It might take some trial and error with the common rafter birdsmouth dimensions to make that happen.

big word for the day, iteration. When an engineer has to try several times to get something to work, homing in on the answer, it is referred to as an iterative process.

Offline Jens

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Re: Calculating Valley Jack Rafters?
« Reply #10 on: May 07, 2009, 06:11:27 AM »
I actually never had anybody to teach me how to cut in roofs, and the books that I have read do not talk about any actual calcs for the seat cut on hip/valley rafters.  Had to just kinda figure it out.  Like I said though, patience more than skill.  Common sense comes in very handy too!
just spent a few days building a website, and didn't know that it could be so physically taxing to sit and do nothing all day!