# Calculating Rafter Length?



## Uncle Bob (Dec 11, 2009)

Ok, I'm old; and it has been a few decades; and I don't have a framing square handy; and forgot how to use it anyway.   

How do you determine the rafter length?

New Building:

Building span = 16 feet.

Overhang = 2 feet.

Pitch = 4 in 12.

No hurry; as soon as you quit laughing and can compose yourselve will be fine.   

Uncle Bob


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## conarb (Dec 11, 2009)

Re: Calculating Rafter Length?

Uncle Bob:

That's not quite enough information, we need ridge thickness and bird's mouth depth, but ignoring those factors it's 10' 5 3/16".

Here is a simple calculator, and here is Joe Fusco's  more complex calculator.


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## Uncle Bob (Dec 11, 2009)

Re: Calculating Rafter Length?

ConArb,

Since I'd need 12' rafters anyway; cost wise, it doesn't matter whether the pitch is 4/12 (10' 6") or 6/12 (11' 3"); same money?

Uncle Bob


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## TJacobs (Dec 11, 2009)

Re: Calculating Rafter Length?



			
				Uncle Bob said:
			
		

> Ok, I'm old; and it has been a few decades; and I don't have a framing square handy; and forgot how to use it anyway.    How do you determine the rafter length?
> 
> New Building:
> 
> ...


 :lol:  :lol:  :lol:  :lol:

Method #1: Buy "Rafter Master" by Nix Jacobsen and look it up on Page A-9. (12.65" per foot of run or 12 5/8")

Method #2: Buy "Simplified Roof Framing" by Wilson and Werner and look it up in Chapter VII Table II Page 125. (1/6 pitch = 12 5/8" length in inches, common)

Method #3: Buy Construction Master Pro calculator...read manual...

Method #4: Mathematical method = hypotenuse of right triangle (a² + b² = c²):

1.  span of roof ÷ 2 = total run (16' ÷ 2 = 8' or 96")

2.  total rise for 4/12 pitch = total rise (96" x 4" = 384" ÷ 12" = 32")

3.  run squared = 9216"   rise squared = 1024"  run + rise = 9216 + 1024 = 10240"

4.  square root of 10240" = 101.19 or 101 3/16" or 8'-5 3/16" calculated length to birdsmouth without deducting for ridge

5.  add 25 1/4" for overhang (12 5/8" x 2) = 10'-6 7/16" length from ridge cut (1/2 of ridge not deducted) to fascia cut


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## mtlogcabin (Dec 11, 2009)

Re: Calculating Rafter Length?

Or hire a contractor they always do it right


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## Pcinspector1 (Dec 11, 2009)

Re: Calculating Rafter Length?

UB,

Depending on the rafter width, you take a ruler 1/4" scale, mark 16' spand on a piece of paper, find the center, mark it at 8', measure up 4' for pitch. draw line from top of pitch to the 16' mark (wow it's a triangle)with your 1/4" scale, count the little marks on the 1/4" scale, (which represent feet) then add you desired overhang. In short order 12' long material or use your friends on this board!


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## Pcinspector1 (Dec 11, 2009)

Re: Calculating Rafter Length?

Conarb,

Where were you with these internet calculators 20 years ago?


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## Uncle Bob (Dec 11, 2009)

Re: Calculating Rafter Length?

Thanks, Ya'll

Uncle Bob


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## jim baird (Dec 12, 2009)

Re: Calculating Rafter Length?

I always liked to step 'em off with a square and cut the ends to fit this or that.  A kind of laying on of hands.


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## peach (Dec 13, 2009)

Re: Calculating Rafter Length?

I'd like to know why a plumber has a framing square.


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## RJJ (Dec 13, 2009)

Re: Calculating Rafter Length?

Hire a Professional!


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## Uncle Bob (Dec 13, 2009)

Re: Calculating Rafter Length?

Except for ConArb or Hurricane; all the carpenters are either dead or retired.  I haven't seen a framing square on job site in over 20 years.  It's a lost art.

Besides, a "professional" would spend more on designing the damn thing than I'm going to spend building it.    :lol:    :lol:    :lol:

It's just been a few decades since I was a carpenter; and my alzheimers isn't full blown yet,

Uncle Bob


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## RJJ (Dec 13, 2009)

Re: Calculating Rafter Length?

A failure to plan is a plan to fail.

Short of that, make your own lay out square. It is 4&12 not rocket science. Plumb cut 4/12

level cut 12/4  step it off! That doesn't mean walk on the rafter! :lol:

There are a few wood beaters left!


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## pyrguy (Dec 13, 2009)

Re: Calculating Rafter Length?



			
				Uncle Bob said:
			
		

> Except for ConArb or Hurricane; all the carpenters are either dead or retired.  I haven't seen a framing square on job site in over 20 years.  It's a lost art.......It's just been a few decades since I was a carpenter; and my alzheimers isn't full blown yet,
> 
> Uncle Bob


Mine isn't kicked in too bad..... what was I talking about....

 I'm still kicking but not swinging a hammer anymore except for the repair here and there.

A lost art is right I had a guy come in to do a few things that I could not (ok would not) do a few years ago. He could not cut baseboard and make it fit right the fifth or sixth time much less the first.


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## jar546 (Dec 13, 2009)

Re: Calculating Rafter Length?



			
				Uncle Bob said:
			
		

> ......... I haven't seen a framing square on job site in over 20 years.  It's a lost art.......


I still have my same 2.

1 old rusty/crusty one for framing & general work.

1 newer aluminum one for stringers and rafters with my clip locks.

Don't see how you can build a house without one.


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## peach (Dec 13, 2009)

Re: Calculating Rafter Length?

Oh, Jeff..

Just use pre-manufactured components..

Hire framers..not carpenters..


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## jar546 (Dec 13, 2009)

Re: Calculating Rafter Length?



			
				peach said:
			
		

> Oh, Jeff..Just use pre-manufactured components..
> 
> Hire framers..not carpenters..


Oh yeah, Toll Brothers!


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## packsaddle (Dec 29, 2009)

Re: Calculating Rafter Length?

Don't need framing squares anymore.

Just need one of these.


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## jim baird (Dec 29, 2009)

Re: Calculating Rafter Length?

Y conoces tus derechos, Pack?

Pythagoras must be turning in his tomb.

Incidentally, I read that he was so shy he only lectured from behind a curtain.  Those he knew and trusted were allowed to sit and listen behind the veil with him, thus the derivation of the term, "esoteric", which means "inside the cloth".


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## peach (Dec 29, 2009)

Re: Calculating Rafter Length?

WHEN, just when.. did any of us see a real carpenter on a job site?

It's a lost art..


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## mtlogcabin (Dec 29, 2009)

Re: Calculating Rafter Length?

Peach



> WHEN, just when.. did any of us see a real carpenter on a job site?


Me personally right before the Mariel boatlift: :lol:


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## beach (Jan 5, 2010)

Re: Calculating Rafter Length?

I've always (and still do...) had a "Reichers Rafter Manual" in my bags...where are all you '70's framers?????? Doesn't anybody remember the little blue book that fits in your  bag or is it a Calif, thing....?


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## Rider Rick (Jan 5, 2010)

Re: Calculating Rafter Length?

Beach,

I was there So. Bay Cailf. 70's framer fun in the sun and paid.

No one up here cuts rafters they use trusses.

When I tell the framers up here we cut all the roof rafters on the ground they look at me funny.

The glory days!

Rick


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## beach (Jan 5, 2010)

Re: Calculating Rafter Length?

Glory days is right!!! I miss those days......Gratifying physical work, year 'round tan and you got paid for the work you actually did (if you pieced out)     



> When I tell the framers up here we cut all the roof rafters on the ground they look at me funny.


They look at you funny because they can't understand a word you say!  :mrgreen:


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## conarb (Jan 5, 2010)

Re: Calculating Rafter Length?



			
				Beach said:
			
		

> I've always (and still do...) had a "Reichers Rafter Manual" in my bags.


Me too, except not on bags, but in the bib of my carpenters' overalls.  When I learned the trade the old carpenters made me learn to "step off" with the framing square, but I noticed that thye all had Reicher's book in their bibs, when I asked to =use the book they said no, I had to learn to step off with the square first.  Eventually I found out that to buy the book I had to drive all the way to San Francisco's Stacey's Bookstore, and I went over and bought the book.  When they saw it they still made me keep it in my bib. I brought a tape to the job and they laughed at me telling me to put it away, a carpenter had to use a carpenter's wood folding rule. 




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## TJacobs (Jan 5, 2010)

Re: Calculating Rafter Length?



			
				conarb said:
			
		

> Beach said:
> 
> 
> 
> > I've always (and still do...) had a "Reichers Rafter Manual" in my bags.


Me too, except not on bags, but in the bib of my carpenters' overalls.  When I learned the trade the old carpenters made me learn to "step off" with the framing square, but I noticed that thye all had Reicher's book in their bibs, when I asked to =use the book they said no, I had to learn to step off with the square first.  Eventually I found out that to buy the book I had to drive all the way to San Francisco's Stacey's Bookstore, and I went over and bought the book.  When they saw it they still made me keep it in my bib. I brought a tape to the job and they laughed at me telling me to put it away, a carpenter had to use a carpenter's wood folding rule. 






That must have been a LONG time ago... :lol:


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## beach (Jan 5, 2010)

Re: Calculating Rafter Length?

Most of the buildings Conarb learned on were lost in the great San Francisco earthquake of 1906!   

Conarb, I'm glad to see somebody here knows what a Reichers book is! That was like going from an abacus to a calculator :mrgreen:


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