Determining Rafter Length By Rise and Run

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Forum topic by RonInOhio posted 12-14-2012 12:37 AM 14254 views 0 times favorited 19 replies Add to Favorites Watch
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721 posts in 3011 days

12-14-2012 12:37 AM

I am currently putting rafters on a shed extension. I understand the formulas. I have read up on how to use the
framing square with stair gauges and pythos theory. I have cut one rafter and its not quite as accurate as I would like. I understand there may be small gaps but its a little more than i am happy with.

i searched an old forum topic regarding this subject and one poster said the Rise; is the center measurement of a joist up to the bottom of the ridge beam. I think my initial measurement was from the top of the ridge, down to top of the ceiling joist.

Can someone clear this up ? There is a 5 1/2 inch difference between the bottom of the ridge beam and the top (2×6 ridge beam), so its definitely imperative to know the difference. I googled it but keep getting calculations for rise which is useless unless you know the correct reference points for rise.

I have a non-standard pitch so I can’t just solve for rise using pitch and run.

Many illustrations show rise goes to the peak of the building, but now I’m not so sure this is the correct measurement.


19 replies so far

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John Ormsby

1288 posts in 3884 days

#1 posted 12-14-2012 12:53 AM

Hello Ron,

It seems you are determined to figure out the rafters using long hand math.

There is an old book that is called the Full length Roof Framer by A.F.J. Riechers

It is the roof framers bible. I have been using one since the 70s. I learned how to lay out roofs with a framing square. However, this little book makes it child’s play.

I really recommend you pick up a copy of the book if you are going to do any roof layouts.

Here is a link to what it is through Amazon. You can probably find a good used one through ebay.

-- Oldworld, Fair Oaks, Ca

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21300 posts in 2830 days

#2 posted 12-14-2012 01:00 AM

Used tobe, each and every speed square came with a little blue book, mainly Swanson Speed squares. Inside it, you will find about every type of measurement needed for framing. I think they even sell just the Blue Book.

-- A Planer? I'M the planer, this is what I use

View RonInOhio's profile


721 posts in 3011 days

#3 posted 12-14-2012 01:08 AM

Well thanks for the reply, but I just wanted to know the correct reference for measuring the rise on a gable roof.

The run is simply 1/2 the width from one wall (outside measure) to the opposite wall . Or simply the width of the building (framing only) divided by two. This is the measurement to the center of the ridge beam or the Run.

I’m thinking my mistake all along has been using an incorrect Rise, because i measured from the top of the ridge beam down to the bottom of the ceiling joist.

Addtionally I don’t have a standard pitch of 3/12 or 4/12. Otherwise I could solve for Rise using the formula.

Anyone else ?

View crank49's profile


4032 posts in 3118 days

#4 posted 12-14-2012 01:26 AM

Rise and run is nothing more than exactly what it says.
If you have a 5/12 pitch, the rise is 5” for every 12” of run.
If the run, the distance from the outside wall to the center of the structure, is 144”, then you divide that number by 12 and multiply the result by 5. The result in this case is 60”.
So, your rafter will rise 60” for a 144” run.
That 60” will be the distance from the top of the wall plates the rafter is sitting on to the bottom of the rafter at its highest point at the ridge. Note: I said the rise is to the bottom of the rafter at the ridge because the ridge beam might be different height. The rafter is doing the rising and running so that’s what you have to measure.

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721 posts in 3011 days

#5 posted 12-14-2012 01:37 AM

And how do you determine the length if you don’t know the pitch ?

Well, just got done completely reading your answer crank. So I’m wrong to assume I can just run a tape down from the bottom or top of ridge beam to get the rise in inches ? That measurement over the run would give me my pitch.

If I didn’t have a non-standard pitch i would just measure the pitch with a square. Thought it would be more accurate and easier to just measure down from the ridge beam . Like the 90 degree side of the triangle and use the formula to find rafter length. Rather than try to use the framing square to determine the pitch.

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721 posts in 3011 days

#6 posted 12-14-2012 01:53 AM

I probably didn’t make it very clear in my orignal question.

Disregarding any formulas or rise over run or pythos formula.

If measuring run is the half the width or span.

Then what is the reference point on the center ridge that I would measure off of to get the rise?

Looking from the floor up to the ridge .

Would you measure from the top of the ridge beam up at the peak , straight down to the bottom of the joist resting on the plates ?

Or from the bottom of the ridge beam ,straight down to the bottom of the joist resting on the wall plates?

In other words.

How would you manually measure the rise from the center point of the span, up to the ridge beam ?

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721 posts in 3011 days

#7 posted 12-14-2012 02:12 AM

I’m going to go with the bottom and say its solved. Sorry , sometimes I get hung up on things that should seem obvious.

View jumbojack's profile


1685 posts in 2771 days

#8 posted 12-14-2012 02:19 AM

This will solve it:

-- Made in America, with American made tools....Shopsmith

View RonInOhio's profile


721 posts in 3011 days

#9 posted 12-14-2012 02:33 AM

Don’t need a calculator or math. Need to know what the reference of measure is.

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717 posts in 2445 days

#10 posted 12-14-2012 02:45 AM


-- Stevo, work in tha city woodshop in the country

View RonInOhio's profile


721 posts in 3011 days

#11 posted 12-14-2012 08:28 AM

For those that are fairly new to roof framing, using a roof framing square for the first time can be a little daunting.

To add to the confusion, there are any number of ways one can arrive at finding the length of the common rafter,or
the run/rise ratio, or slope angle.

Making matters worse, some online references mix up the glossary of roof components. Sometimes calling a joist a rafter for example. Or being vague in describing exactly how to properly lay out a common rafter with the framing square.

As one experienced construction worker said, “Ask 5 experts how to frame a gable roof with a 4/12 slope and you are likely to 5 different answers.

So I guess patience , trial and error, and perseverance is the name of the game.

I found out the hard way that if you do build a shed, shop, or anything with a sloped roof, its best to use a standard slope. 3/12, 4/12/, 5/12 . Not something in-between these. Like 3.7 in 12 . Maybe that is typical though.
I don’t know.

But anyway, I found the answer to my question of total rise, was really only one measurement away. That measurement is simply to measure the existing slope angle of one of the existing rafters. I seem to remember doing that about a week ago. And I think it was around 17 degrees.

Since I knew the total rise was one of two choices. I simply plugged each one into a calculator.

For an 18 inch rise the graph came out like so…..

For a 12.5 inch rise …..

So pretty evident the 18 inch rise is indeed the total rise.

Not saying I have this framing square down yet, but I think I can at least proceed further without second guessing things so much.

Thanks to everyone for their input.

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929 posts in 2221 days

#12 posted 12-15-2012 04:10 AM

vertical dimension of two points. point one formed at intersection of a vertical line at the outside of the wall framing. Point two formed at intersection of the lines formed by the top of the rafters from both sides.

Lots of things will affect these two points. slope, width of ridge board, birds-mouth,and others. if you are measuring an existing structure use the BOTTOM chord dimension. use half the outside wall FRAMING width minus half the width of the ridge-beam for run. for rise measure from the bottom edge bottom corner of the rafter at the ridge beam to the floor minus the measurement from the top of the top plate to the floor.

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18373 posts in 3823 days

#13 posted 12-15-2012 04:30 AM

If you know how to use a framing square, draw a work line 2” from the edge of your rafter. Set your square on the line using your pitch ensured by length and rise. Repeat the number of times you have feet in the span. The rafter should be the proper length.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View RonInOhio's profile


721 posts in 3011 days

#14 posted 12-15-2012 06:45 PM

I have arrived at the correct length using rise and run in the formula. For some reason, when I try to step off the length using the square step method , it doesn’t come out to the correct length. I have watched several videos of this being done . Its really got me baffled why the step method isn’t agreeing with the calculations.

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2789 posts in 2444 days

#15 posted 12-15-2012 06:56 PM

Can you get up to the eave to just measure it? Take a 12 inch long stick (or 24 or 36), hold it level with one end to the roof and measure down. That is you pitch (divide by 2 for the 24 inch long stick, 3 for the yard stick).

I’ve also seen a roof estimator hold up a gadget that almost looked like a protractor. I think it had a bubble level on it and he sited along the gable end to see what the pitch was so he could figure out how many squares a roof was. You probably could do it with a just big protractor.

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