Old-Timers And Mathematicians . . . Help

Journeyman T

I think most of us have either seen or heard of someone who could calculate rafter lengths in their head. I want to crack the mystery behind it, so I’m calling on you old-timers and mathematicians for help. Are there any tricks to calculating common rafter lengths IN YOUR HEAD (with no framing square either) besides memorizing the “lengths per foot of run,” or is it just plain number crunching after that?

For example, suppose you have a building you measure at 33’. From this point you put your tape measure down and do the rest in your head with no aid of a square.

33’ (span) ÷ 2 (half the span) = 16’-6” (unadjusted run). In this instance I’ll use the unadjusted run for the sake of simplicity and figure on subtracting half the thickness of the ridge board after the rafter is laid out. I’m also going to avoid a building width that has a run including inch fractions so that I don’t complicate this too much right now.

Say you want to frame the often used pitch of 4:12. The “common rafter length per foot of run” is 12.65: 16 (4 squared) + 144 (12 squared) = 160 √160 = 12.64911 or 12.65. If someone already knew the “length per foot of run” for a 4:12 pitch, he could skip the whole A2 + B2 = C2 thing and start with 12.65.

Now the problem becomes a matter of multiplication and conversion: 12.65 x 16.5. For most people it’s kind of difficult to multiply 12.65 by 16.5 and be sure of the answer (208.725) without writing it down.

Of course, the decimal inch fraction .725 must also be converted into an inch fraction of 11/16 (.725 x 16 = 11.6 or approximately 11/16). Answer: 208 11/16” (17’-4 11/16”).

Are there any ways to either BYPASS or SIMPLIFY these last two multiplication and conversion steps so as to make it easier to calculate in your head?

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A W Smith

memorize the results of run for 4 foot 10 foot and 5 foot and 6 foot spans for common rafter rises and then you might be able to do it on your head.

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Al Smith
http://www.awsmith.com

Cole

My questions is why?

Why would you want to do it in your head?

I guarantee I can beat you with my calculator.

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Celtic

That must a pretty big calculator!

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neolitic

The "Full Length Rafter Framer" fits in my pocket.It has ALL the answers.
I've got enough crap floating around inside my head.

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(Sorry....it seems there really are dumb questions)

metalmark

Construction master caculator. It will do everything except cut it for you.

deckman22

I framed for 15 years & never knew of someone who could nor had the desire to calculate rafter lengths in my head.

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Deckman
www.alscustomdecks.com

Ahren

Bingo. That book is the bible of roofs (just remember to subtract 1/2 the ridge!). Why do you want to do that crap in your head anyway? Tables and calculators don't make you lazy, they make you more productive.

Bone Saw

you may need to talk to Ray Babbit, yeah, 15 minutes till wapner, yeah

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Ahren

Definitely 247 toothpicks, definitely. Gotta go to Kmart and get some underwear.

Bone Saw

how much does a candybar cost

$100

how much does a new kitchen cost

$100


holy crap, most Ho's are autistic

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TempestV

the only way I know of to calculate the diagonal rafter length would be the Pythagorean formula
((.5*width of bottom cord in feet))*roof pitch= total rise of roof in inches
1/2*width of bottom cord in inches= run= a, total rise= b, c= diagonal length, a^2+b^2=c^2
33 foot wide building, 4/12 pitch, 16.5'= 198" run, 16.5*4=66" rise
198^2+66^2=C^2=39204+4356=43560, c=208.7"= 17', 4 11/16"

but seriously, the cm5 is your friend- tell it the rise and the run, and push a button and it does all that for you.

Joe Carola

Forgetting about the CM calculator for now, what about the table on your framing square that tells you the "Common rafter length per foot run" measurement under every pitch?

Under the 4 it says 12.65.

12.65 x 16.5 = 208.725" (Rafter Length)

Pythagorean Theorem works but it's an extra step in math and a waste of time to figure a rafter length. You have to figure the rise also to get the length.

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Joe Carola

TempestV

My post was about how you would have to do it without a cm or a square, like the original poster asked.

Joe Carola

Sorry about that.

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Joe Carola

joasis

There will come a day when the old framing squares will be history, and you will only read about the tables. I wish I still had my Grandfather's square...it was well used, and the tables were barely legible.

FYI, I use a Swanson speed square.

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skylands

I'm a fouth generation carpenter. My father and grandfather did everything off the framing square and stick rule. There's no mystery. We all know the rafter is "about" 16 feet (or whatever). After that the framing square did all the work. That's why you find all our math work on the framing inside the walls when you demo homes.

My dad died in 1992. He never owned a tape measure.

I have his stick rule, framing square and scribes that I use today. On my desk sits "The Building Estimator's Reference Book" authored by Frank Walker. Copyright 1950. There are 1780 pages in this book. Plywood is not referenced in this book.

"Starting newel, 8"x8" base, 6"x6" shaft, 54" long....each $7.00.

......those were the days.

K2

You could probably memorize 5 or 6 of the numbers Joe mentioned and do 99% of the older roofs out there in your head. For 20 years i did 4/12,5/12 8/12 and 12/12's. I never bothered to memorize the numbers because my square was right there.

oldfrt

Fifty years from now,a tape measure will be obsolete.Probably plywood too.

K2

I suspect the old timers were peeking at their square anyway.