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Roof slope definitions and illustrations (C) Carson Dunlop AssociatesRoof Slope or Pitch: How to Find
by using a framing square

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How to use a framing square to find the slope or pitch of a roof.

Roof measurement methods: these articles explain various methods for measuring all roof data: roof slope or pitch, rise, run, area, and other features. This article series gives clear examples just about every possible way to figure out any or all roof dimensions and measurements expressing the roof area, width, length, slope, rise, run, and unit rise in inches per foot.

Page top sketch of various roof slopes is provided courtesy of Carson Dunlop Associates, Toronto.

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- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

How to Obtain Roof Pitch or Slope by Using the Rafter Tables on a Framing Square

Roof rafter tables on a framing square (C) Daniel FriedmanMeasuring the roof slope, pitch, or rise and run is easy if you've got a tape measure and more accurate still if you've also got a level on hand.

But did you know that simple data already printed on your framing square can give key roof slope and rafter data?

[Click to enlarge any image]

A long-standing use of the framing square that is missed by many new carpenters is the rafter table imprinted right on the framing square itself. This data will tell us the required rafter length for a roof of a given pitch or slope.

The rafter table is found on the front of the framing square blade - its longer arm.

See FRAMING SQUARE USER'S GUIDE for a complete dictionary of all of the parts, tables, and uses of a framing square.

The rafter table on a typical framing square gives rafter length for roof slopes with a rise anywhere between 2" to 18" per foot of run.

How to find Roof Slope & Rafter Length on a framing square

How would we use our framing square to measure or to calculate the slope and rafter lengths of this historic Herefordshire U.K. home?

Find the unit rise (say inches per foot) of the roof on the top line in the framing square rafter table, e.g. a 6-inch rise per foot.

Read this number along the inches scale at the top of the framing square.

Directly below the 6 (inch) mark on the framing square read the required rafter length (13.42) per foot of run for a 6-inch rise (6 in 12) roof.

Historic building, Herefordshire, U.K. (C) Daniel Friedman

To calculate the required rafter length multiply the total run length (half the building width plus the width that the roof is to overhang the house wall) by this number (13.42).

For example if the building is 30 feet wide, half the building width is 15 feet. If we want a 2 foot roof overhang from the front of the building wall (ignoring the thickness of wall siding and trim), we add 15 + 2 = 17 feet of total run length.

17 ft. run x 13.42 inches (per foot for a 6 in 12 roof) = 228.14 inches of rafter length, or 19 feet.

Depending on your allowance for making the ridge board and fascia board plumb cuts, you can see that you should be able to cut these rafters out of a 20 ft. 2x piece of lumber.

Use the framing square to get roof slope

We can reverse the usual use of a framing square (finding the rafter length) to figure out the roof dimensions (or slope). Basically, for any roof dimension or slope calculation, if we know some roof dimensions or slope we can calculate the unknown.

As we explain in detail at ROOF SLOPE CALCULATIONS, the basics of plane geometry tell us about the relationship of the lengths of sides of a right triangle:

a2 = b2 +c2

the square of the length of the hypotenuse (a) equals the squares of the lengths of the opposite sides of a right triangle (b) and (c) - which is a mathematical way of saying that if we know any two roof slope measurement numbers we can compute the third.

Given the total rafter length (measure from ridge to lower roof edge) and rafter run (half the building width plus the roof overhang past the building wall) we can figure the roof rise.

If we know the roof run and rafter length, we find the roof slope by calculation or we use the framing square table (just below) and then easily find the roof rise.

Roof rafter tables on a framing square (C) Daniel Friedman

For example, using the numbers from our roof framing square table example just above:

Measured rafter run RR = 17 ft. or 17 x 12 = 204 in.

Measured rafter length RL = 19 ft. or 19 x 12 = 228 in.

RR x (number on framing square) = RL

The trick here is understanding the number on the framing square is the unit-rise per foot of run.

204 x (number on framing square) = 228

(number on framing square) = 228/204 x 12in

228 / 204 x 12 = 13.412

13.412 is the unit rise in inches per foot of our example roof run. By "run" we mean the level or horizontal distance covered by the roof.

Now find the closest number to this on the framing square top row and read the inches just above. In our photo you'll see that 13. 42 is under the 6 on the inches scale. That's our roof slope in inches per foot, or 6 in 12!

Now if we still need to calculate the total roof rise, just multiply the unit rise (6) by the number of feet of run (17) and we've got the total rise in inches (6 x 17 = 102 in.) or (102 / 12) = 8.5 ft. (the total rise is 8 ft. 6 inches).

Of course if you go to ROOF MEASUREMENTS you'll see lots of other ways to figure the total roof rise, run, or slope.


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Continue reading at ROOF SLOPE CALCULATIONS, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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