How to use the rafter tables on a framing square to measure & lay out rafters or to figure roof pitch, slope, length, and all other roof and angle measaurements.

Also use these tables for stair rise and run and riser heights and stair stringer layout. .

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.

How to Use the Framing Square Rafter Length Tables - the Rafter Tables

Here we describe how to use the rafter tables found on the front blade on a framing square.

[Click to enlarge any image]

The rafter tables allow you to calculate the length of a rafter by choosing a "full scale" number that matches the unit rise (slope or angle) of the roof.

The table will provide full scale

The full scale number will be the length of the rafter in inches for each foot of horizontal run distance.

Our sketch above illustrates the concepts of run (horizontal distance) and rise (vertical distance) along the length of a rafter. Normally roof slopes are expressed as inches of rise per foot of run.

To use the rafter length tables on the framing square blade front you need first to understand the type of rafter you are cutting.

For a common rafter, or for each of the other rafter types, these tables give the length of the rafter per foot of horizontal run.

Above we see the key to the lines in the rafter length table.

This key is at the left end of the front of the framing square blade front.

Here is the

Identification of each line of numbers on the blade front

Holding the framing square with the front of the blade facing you and the tongue pointing down and on your right, you'll see six lines of rafter tables:

• COMMON RAFTER LENGTH PER FOOT OF RUN - this is the top line in the rafter length tables
• HIP OR VALLEY RAFTER LENGTH PER FOOT OF RUN
• DIFF IN LENGTH OF JACKS 16 INCH CENTERS
• DIFF IN LENGTH OF JACKS 24 INCH CENTERS
• SIDE CUT LENGTH OF JACK RAFTERS
• SIDE CUT OF HIP RAFTER OR VALLEY RAFTER

To the right of each of these rafter type line names are columns of numbers under each inch number along the blade's upper edge. Each number gives the unit length in inches of the rafter type for the corresponding particular roof slope also as inches of rise per foot of horizontal run.

Let's look again at the drawing above and review some definitions of basic rafter and roof framing terms.

[click to enlarge any image].

Definition of Rafter Run: You can see that for any rafter, the rafter's RUN is the horizontal distance spanned by the rafter from the center of the ridge to the outer edge of the top plate of the wall.

Definition of Rafter Span: the rafter SPAN is the same as the rafter's run: the total horizontal distance (parallel to the ground) below the rafter.

Definition of Rafter Line Length: Typical carpentry texts refer to the rafter's actual LENGTH from the rafter's plumb-cut face abutting the ridge board to the rafter's bird's mouth plumb cut as the rafter line length.

Definition of Rafter Total Length: When we add the additional rafter length necessary to include the roof overhang I call that the rafter total length. Some texts may also call this the rafter line length so one needs to take care.

Definition of Roof Span: In a symmetrical roof such as the gable roof shown above, the total building width between the outer faces of the walls or wall top plates is the total area spanned by the roof or total horizontal span.

Definition of Rafter Rise or Roof Rise: a rafter's rise or the roof's rise is the total increase in vertical height over the rafter run or rafter span - the vertical distance shown in the center of our sketch above.

Definition of Roof slope: the slope of a roof or rafter is its angle off of dead flat or horizontal, expressed as the number of inches of rise or increase in height per foot or 12" of horizontal run or distance. An 18-in-12 slope roof is one whose height above horizontal increases 18 inches up for every 12 inches of horizontal distance.

Steep Roof Rafter Length Calculation Example 1: 18/12 roof slope rafter lengths

Let's figure the rafter length for an 18 in 12 roof.

On the framing square blade face, using the COMMON RAFTER LENGTH PER FOOT RUN line in the table, look along the blade under the inch number 18 we see 21.63. I marked it in red to make it easier to spot.

[Click to enlarge any image]

That tells us that for a common rafter on an 18/12 sloped roof, the rafter length will be 21.63" per horizontal foot of run.

If in our building the rafter span is 10 feet (the building, would thus be 20 feet wide), we calculate the required rafter length from ridge to the outer face of the wall top plate as follows:

Common Rafter Table No. = 21.63 = inches of rafter length per unit (one foot) of run

Rafter Run Distance = 10 ft or 10 units

Rafter Length = 10 units x 21.63" per unit

Rafter Length = 216.3" or we can divide by 12 to get feet and fractions

Rafter Line Length ridge to top plate outer face = 216.3 / 12 = 18.025 ft.

And to be very precise we could convert the fraction of a foot ( .025) back to inches.

12" /ft. x 0.025 ft = 0.3 " or 3/10 of an inch.

Or if you want to convert this to 8ths 16ths or 32nds or 64ths, multiply 0.3 x 8 (for example to get eights) to get 2.4 eighths - or about 2/8 or about 1/4 " since unlike furniture makers, roof framers won't measure accuracy to more than an eighth.

Rafter Length = 18 ft 1/4"

Add to Rafter Length to Allow for Roof Overhang

Really? No. Not really. We haven't yet considered the roof overhang past the face of the building wall.

Watch out: take a second look at our roof sketch shown earlier.

Rafter Length = 18 ft 1/4" is the length from the ridge centerline to the outer edge of the wall top plate.

You need to increase the run length to account for the amount of roof overhang or eaves out past the wall.

For example on a 10 ft. run, if we want the roof overhang to extend two feet horizontally past the wall front, we need to increase the run length by two feet.

Our new rafter run will be 12 ft. not 10 ft. (before allowing for thickness of siding and trim).

New with overhang Rafter Length = 12 units x 21.63" per unit = 259.56" or, dividing that by 12"/ ft we get 21.63 ft or better, in inches,

12' 5/8" of total rafter length

Subtract 1/2 of Ridge Board Thickness when Laying out the Rafter's Birds' Mouth Cut

Really? is this really the final rafter line length? Well it could be.

If you're not building a precise overhang you don't have to change the total rafter length to account for half the thickness of the ridge board.

Your actual overhang will simply be about 3/4" further out than was called for by your architect. But not to worry, there's not many architects who even know how to use a tape measure while doubled over and scrunched up on a scaffold under the roof eaves.

But you DO need to consider this ridge board thickness when laying out the exact location of the birds' mouth cut at the rafter's lower end.

Watch out: Ignoring this adjustment means your bird's mouth cut for the wall top plate connection will end at the wrong location - the plumb cut part of your birds' mouth cut will be too far out past the wall face. By about 3/4" - and you'll be embarrassed.

When I see a roof framed by a carpenter who simply omits the birds' mouth cuts I figure that she didn't know how to lay out the birds' mouth cut and was just avoiding this predicament.

Details of how to do it as well as photos of hardware that let you omit the birds mouth cut are

at FRAMING SQUARE for BIRDS MOUTH CUT - don't feel bad. In that article you'll see that my cuts are not perfect either.

Henry Page (H.G. Page Lumber, Poughkeepsie NY) used to shout at me when he caught me sorting through my 2x's to pick the straight ones before loading them up on my truck.

You're NOT BUILDING FURNITURE Henry would shout. Just load the goddamn lumber and stop picking over it.

I think Henry Page must have been to some of my job sites.

Example 2: How to Use the Framing Square read all of the types of rafter data for a 6/12 roof slope

Let's look at a 6/12 roof for some rafter lengths

Above my pencil is pointing to the 6-inch mark read along the FRONT of the framing square BLADE (the long arm).

Using the rafter length table below, this signifies that we are looking at rafter lengths for the various types of rafters on a 6/12 slope roof.

That is, on this roof there will be a 6-inch rise in 12-inches of horizontal run.

Now let's put those line names together with the numbers we read under the 6-inch mark on the same blade surface

• A common rafter will be 13.42" long per foot of horizontal run on a 6/12 slope roof.
• A hip or valley rafter will be 18" long per foot of horizontal run on a 6/12 slope roof.
• The difference in length of sequential jack rafters spaced 16" o.c. will be 17.88" long on a 6/12 slope roof.
• The difference in length of sequential jack rafters if spaced 24" o.c. will be 26.81" long on a 6/12 slope roof.
• The side cut length of jack rafters will be 10.75" on a 6/12 slope roof.
• The side cut length of hip or valley rafters will be 11.32" long on a 6/12 slope roof.

Example 3: Find the Common Rafter Length for a 7 in 12 Slope Roof: how to read the COMMON RAFTER LENGTH PER FOOT OF RUN table

Illustrated here is how to figure the rafter length from ridge board face to the edge of the rafter at the face of the outer wall, if the slope is 7" of rise per 12" of horizontal run.

The drawing below is excerpted from ROUGH CARPENTRY ( NAVEDTRA) . I have corrected an error in the original drawing that cited the horizontal run as 6' - wrong, the run is 8' as shown in the example.

[Click to enlarge any image]

Take a look along the FRONT of your framing square BLADE and read the inch-scale at the top edge of the blade. Find the number "7" - (inches). This "7" heads a column of numbers below it, corresponding to the various rafters, hips etc. listed earlier.

Watch out: be sure you are finding the 7-inch number along the TOP of your BLADE FRONT - not the 7" marked on the bottom of the blade front. You know you're reading correctly when the whole inch numbers (like the red 7 in my example) is centered over the column of rafter length numbers below.

The first or highest line under the inches scale is COMMON RAFTER LENGTH PER FOOT OF RUN

There at the top of the data column you will see the value 13.89 - that is the number of inches of rafter length for a rafter whose slope is 7 inches of rise per 12 inches of horizontal run.

The drawing, one we've corrected from NAVEDTRA and cited and provided in PDF form at the end of this page, shows that for each horizontal foot (12") of run, the sloped rafter length will be 13.89".

So what's our total rafter length from center point of the ridge to outer face of the wall?

8 feet of horizontal run (in our example) will give us 8 ft x 13.89" = 111" = 9' 3" of rafter length.

Final Adjustments to Total Rafter Length

Watch out: there are two details yet to consider in the actual total rafter length

1. Ridge board thickness: this rafter length is from the "center of the ridge" so if you are using a 2x as a ridge board and your 2x is thus 1 1/2" thick, your rafter is pushed out by half of that ridge board thickness or 3/4"
   
   - if you don't take this into consideration your bird's mouth cut will be "off" by 3/4".
2. Roof overhang distance: anybody with any sense who is framing a roof (naturally this will excuse some architects), wants some roof overhang or eaves.
   
   If I want my roof overhang to be two feet then my adjusted horizontal run will increase by two feet, so my 8 ft. "total run" changes to 10 ft total run and my rafter length changes to 10 ft. x 13.89" / ft or 138.9" = 11' 6.9" or if you prefer, 11' 6 7/8" total length.
   
   If I were building this roof I'd cut my rafters to 11' 6" and realize that the extra 3/4" from adjustment #1 is going to bring the effective rafter length to 11' 6 3/4" - that's just 1/8" shy of the two foot overhang and good enough for government work.

Example 4: Find the Rafter Length for a Low Slope 3/12 Roof

Finally let's look at a common rafter length for a very low slope roof

Here is another example of common rafter length for a very low slope roof.

In the top line of the rafter table shown above on the framing square BLADE FRONT, near the HEEL of a framing square, look under the 3-inch mark where you find the 3" on the top edge of the framing square blade.

There in the top of the column of numbers you'll see 12.37.

This tells us that the length of a common rafter for each 12" of run will be 12.37" long if the roof rise is 3" in 12.

So if our rafter run or span is 10 feet we calculate the rafter length from ridge to outer wall face as

12.37" / foot of span x 10' of span = 123.7" of total rafter line length (before adding for roof overhang and before subtracting half the thickness of the ridge board.)

Roof Framing Example 5: Location of the Bird's Mouth Cutout in the Rafter

This discussion now has its own page

at FRAMING SQUARE for BIRDS MOUTH CUT

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

This discussion got promoted to its very own web page

at FRAMING SQUARE DETERMINES ROOF SLOPE

...

Continue reading at FRAMING SQUARE for BIRDS MOUTH CUT or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Building & Roof Measurement Articles

• 6-8-10 RULE to SQUARE UP ANY STRUCTURE
• ANGLE FINDER LEVEL
• ARCTANGENT CALCULATES ROOF / STAIR ANGLE
• CALCULATE RISE & RUN from SLOPE
• DECK LEVEL PLUMB SQUARE-UP 6-8-10 RULE
• FOLDING RULER ROOF SLOPE CALIBRATION
• FRAMING SQUARE USER'S GUIDE
  • FRAMING SQUARE PARTS DEFINED
  • FRAMING SQUARE TABLES LIST
  • FRAMING SQUARE RAFTER LENGTHS
  • FRAMING SQUARE for BIRDS MOUTH CUT
  • FRAMING SQUARE OCTAGON TABLE
  • FRAMING SQUARE BACK TABLES
  • FRAMING SQUARE BRACE TABLE
  • FRAMING SQUARE N JOISTS RAFTERS N RUN FT-IN
  • FRAMING SQUARE ESSEX BOARD MEASURE
  • FRAMING SQUARE DIVISION WARNINGS
  • FRAMING SQUARE COLOR & MATERIALS
  • FRAMING SQUARE DETERMINES ROOF SLOPE
  • FRAMING SQUARE for STAIR BUILDING
  • FRAMING SQUARE-UP 6-8-10 RULE
  • FRAMING SQUARE TRUING PROCEDURE
  • FRAMING SQUARE HOW-TO MANUALS
• FRAMING TRIANGLES & CALCULATIONS
• FROGS HEAD SLOPE MEASUREMENT
• HOPPUS MEASURER
• ROOF FRAMING TIES & BEAMS
• ROOF MEASUREMENTS
• ROOF SLOPE CALCULATIONS
• ROOF SLOPE TABLE, TYPES, WALKABILITY
• STAIR RISE & RUN CALCULATIONS

Suggested citation for this web page

FRAMING SQUARE RAFTER LENGTHS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING ROOFING

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