I am really into the math involved in cutting rafters on the ground, but I have some questions maybe someone can answer. If I’m using a framing square and I need to calculate a common rafter length I look under the tables that are stamped on the side of the square and lets assume I’m building a garage where the run is 7′ and the pitch is 8/12. The ridge is a 2×10 and the walls are 2×4’s with 3/4″ sheathing. If I multiply the run by the dimension listed for a 8/12 pitch on the framing square (14.42) how do you guys in the field do this math?. 7 feet times 14.42?. Assume you are not using a calculator, not stepping off the rafter, and not using a book with rafter lengths in it. How do you convert from decimals to fractions of an inch where you can pull that length with your tape measure? I’m just curious how this is done, I know how to do it with a CM calculator, and a book of tables but lets assume you don’t have those all you have is a piece of scrap 2×10 and a pencil and a framing square (not a speed square)
Craig
Replies
You can guess pretty close- 1/4''=.25, 1/2''=.5.............3/4= .75................
or-
you can multiply 16ths (16) times .428............and see how close that is to one of the above numbers.
or use a drill index card that has all the fractions and corresponding decimals.
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I was taught to mult. the dec by 16 that = how many 16ths
.50 x 16 = 8 8- 16ths = 1/2"
.632 x 16 = 10.112 10- 16ths = 5/8"
Gets you within a fraction of a sixteenth
NAIL IT !!!
excellent reply thank you, I just tried this out and it works great! close enough NAIL IT!!!
Here you go. http://www.forbes.com/scripts/fract_conv.html
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the run is 7' and the pitch is 8/12. The ridge is a 2x10 and the walls are 2x4's with 3/4" sheathing. If I multiply the run by the dimension listed for a 8/12 pitch on the framing square (14.42) how do you guys in the field do this math?. 7 feet times 14.42?. Assume you are not using a calculator, not stepping off the rafter, and not using a book with rafter lengths in it. How do you convert from decimals to fractions of an inch where you can pull that length with your tape measure?
Since you're doing this by hand , the easiest way using less math is to multiply 7 x 14.42 = 100.94". Forget about 1/2 the ridge deduction and just scribe a plumb line and make a mark 3/4" back plumb from that line and that's your rafter length.
I don't know why you gave the thickness of the sheathing unless you plan on notching the birdsmouth over the sheathing for some reason. I don't do that, I use the plate.
100.94" is 100 -15/16" because .94" is roughly 15/16. How you can check that is, 15 ÷ 16 = .9375". That's close enough to .94".
It gets a little more complicated if you had a 7'4-3/4" run for example. There are two ways to handle that. The fastest and easiest way eliminating math is to just use the 7' and multiply that by 14.42 and get your length and then just add on the 4-3/4" plumb to that and that's your rafter length without the 3/4" ridge deduction.
I would just do it in one shot by deducting the 1/2" thickness of the ridge first and adding on just 4".
The second way if you want to do more math is to divide the run in inches by 12.
7' 4-3/4" is 88-3/4". Just change the 3/4" to .75. (3÷4 = .75)
You have 88.75/12 = 7.395833' x 14.42 = 106.6479" or 106-11/16"
Edited 3/7/2008 4:53 pm ET by Framer
"I don't know why you gave the thickness of the sheathing unless you plan on notching the birdsmouth over the sheathing for some reason. I don't do that, I use the plate."
Is that because you are installing the rafters before the walls are sheathed? (I seem to remember you saying you had a separate sheathing crew). If the walls are stood up sheathed, you don't have much choice but to include the sheathing in the birdsmouth.
not if you hold the ply to top of topplate not the double plate
That will also allow a gable wall to be nailed into the double plate if you overhang the gable plywood.
"That will also allow a gable wall to be nailed into the double plate if you overhang the gable plywood."
This sounds interesting, but I'm not sure I get it. Can you rephrase it for me. I'm a little slow tonight.
if you keep the ply down to top of first plate, when you ply. the gable you have 1.5" to nail into top plate.
IN our case we always use a crane for trusses. When it is on the ground, install the plywood, letting the ply, hang down 1.25".
fly the truss in, have someone on the outside nail the ply to top plate.
Is that because you are installing the rafters before the walls are sheathed?
Yes, I nail the rafters on before the sheathing.
If the walls are stood up sheathed, you don't have much choice but to include the sheathing in the birdsmouth.
I guess that's true if you run the sheathing up to the top plate.Joe Carola
I do try and bring the sheathing up over both plates to tie the wall together. Not that I'm really sure it makes any difference.
I always sheath walls before rafters even though I sheath after the wall is raised.
Edited 3/7/2008 8:49 pm ET by shellbuilder
106.6479 I guess converting that to a useful fraction like you did is what is confusing me. How did you arrive at that decimal equivilent of 11/16?
There are a number of methods that work, certainly any of those posted here so far do .
I have the decimal/fraction equivalents memorized down to a 1/16 and can work them backwards down to a 1/64" very quickly.
Some years back I worked with a bunch of young carps.
One day at lunch the bunch of them got into a pizzing contest of seeing who had memorized the most numbers they used, Drivers license, SS, credit card, PIN etc.
They finally asked me and I pulled out my wallet and that is why I carry this . I then asked them what the decimal equivalent of some fractions were, not a single one could answer .
I told them I memorized the numbers that make me money , not those I use to spend it.
Lunch lesson over.
If I multiply the run by the dimension listed for a 8/12 pitch on the framing square (14.42) how do you guys in the field do this math?. 7 feet times 14.42?.
If you insist on doing the math longhand, just make sure you first multiply the unit length (14.42") by the total run (7) before you round it off to the nearest 1/16th.
14.42" is already a rounding of the actual unit diagonal of 14.4221" and if you round first to the nearest 1/16th and then multiply by the run, you might lose some accuracy.
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Converting from decimals of an inch to fractions of an inch is easy.
First, decide how accurate you want your answer to be: in 1/8s? in 1/16s? in 1/32s?
Lets decide that you are a very anal guy and want to convert to 32nds. You take your leftover decimal number and multiply by 32. In your example, you were multipl1ying 7 units of run x 14.42 = 100.94.
Take the .94 and multiply by 32. that equals .94x32=30.08. In the field, we round that 30.08 down to 30 so the answer 100 30/32"
Technically, your exact answer would be 30.08/32" for the conversion.
Normally, I'd convert to 16ths by multiplying .94 by 16. That rounds down to 15/16".
After you've worked enough with these fractions, you just eyeball the number anyways. To me, .94" falls somewhere between .875 and 1.00....so, I'd mark a heavy 7/8" on my tape.
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