Framing Square Rafter Tables: Complete Reference
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Framing Square Rafter Tables: Complete Reference
Every steel framing square has rafter tables stamped into the face. They're small, they're hard to read on a bright jobsite, and they get scratched up after a few months of use. But the information they contain is some of the most powerful math in carpentry — all the rafter length calculations pre-solved, laid out in five rows of numbers that tell you everything you need to know about commons, hips, valleys, and jacks at any pitch.
This page gives you those same tables in a format you can actually read, along with worked examples showing exactly how to use each line. Bookmark it. Pull it up on your phone when the sun's making the numbers on your square impossible to read.
Complete Rafter Table Reference
These tables provide the values stamped on standard framing squares. Use them when your square is hard to read, for unusual pitches, or to verify calculations.
Line 1: Common Rafter Length Per Foot of Run
This is the length of a common rafter for each 12" of horizontal run.
| Pitch | Length/Foot | Decimal | Degrees |
|---|---|---|---|
| 3/12 | 12-3/8" | 12.369" | 14.04° |
| 4/12 | 12-5/8" | 12.649" | 18.43° |
| 5/12 | 13" | 13.000" | 22.62° |
| 6/12 | 13-7/16" | 13.416" | 26.57° |
| 7/12 | 13-7/8" | 13.892" | 30.26° |
| 8/12 | 14-7/16" | 14.422" | 33.69° |
| 9/12 | 15" | 15.000" | 36.87° |
| 10/12 | 15-5/8" | 15.620" | 39.81° |
| 11/12 | 16-5/16" | 16.279" | 42.51° |
| 12/12 | 16-15/16" | 16.971" | 45.00° |
Formula: Length = √(12² + rise²)
Example: For 8/12 pitch with 14' run:
- 14.422" × 14 = 201.9" = 16' 9-7/8"
- Add overhang, subtract for ridge
Line 2: Hip/Valley Rafter Length Per Foot of Run
Hip and valley rafters run at 45° to the ridge, making them longer than common rafters.
| Pitch | Hip Length/Foot | Decimal |
|---|---|---|
| 3/12 | 17-1/4" | 17.233" |
| 4/12 | 17-7/16" | 17.436" |
| 5/12 | 17-11/16" | 17.692" |
| 6/12 | 18" | 18.000" |
| 7/12 | 18-3/8" | 18.358" |
| 8/12 | 18-3/4" | 18.762" |
| 9/12 | 19-3/16" | 19.209" |
| 10/12 | 19-11/16" | 19.698" |
| 11/12 | 20-3/16" | 20.224" |
| 12/12 | 20-3/4" | 20.785" |
Formula: Length = √(16.97² + rise²) — because hip run is 16.97" per 12" common run
Example: For 6/12 pitch with 14' common run:
- 18.000" × 14 = 252" = 21' 0"
Line 3: Jack Rafter Difference
Jack rafters step down in length from ridge to plate. This table gives the length change for each jack.
At 16" On-Center:
| Pitch | Difference |
|---|---|
| 3/12 | 16-1/2" |
| 4/12 | 16-7/8" |
| 5/12 | 17-5/16" |
| 6/12 | 17-7/8" |
| 7/12 | 18-1/2" |
| 8/12 | 19-1/4" |
| 9/12 | 20" |
| 10/12 | 20-13/16" |
| 11/12 | 21-11/16" |
| 12/12 | 22-5/8" |
At 24" On-Center:
| Pitch | Difference |
|---|---|
| 3/12 | 24-3/4" |
| 4/12 | 25-5/16" |
| 5/12 | 26" |
| 6/12 | 26-13/16" |
| 7/12 | 27-3/4" |
| 8/12 | 28-7/8" |
| 9/12 | 30" |
| 10/12 | 31-1/4" |
| 11/12 | 32-1/2" |
| 12/12 | 33-15/16" |
How to use: Starting with longest jack, subtract this value for each successive jack rafter.
Lines 4 & 5: Side Cut Angles
These angles are for the miter cuts where rafters meet hips, valleys, or ridges.
Jack Rafter Side Cut (Line 4):
| Pitch | Ratio (with 12") | Degrees |
|---|---|---|
| 3/12 | 12 & 2-3/8" | 11.3° |
| 4/12 | 12 & 3-1/8" | 14.6° |
| 5/12 | 12 & 3-7/8" | 17.9° |
| 6/12 | 12 & 4-1/2" | 20.6° |
| 7/12 | 12 & 5-1/8" | 23.2° |
| 8/12 | 12 & 5-5/8" | 25.1° |
| 9/12 | 12 & 6" | 26.6° |
| 10/12 | 12 & 6-3/8" | 28.0° |
| 11/12 | 12 & 6-5/8" | 28.9° |
| 12/12 | 12 & 6-7/8" | 29.8° |
Hip/Valley Side Cut (Line 5):
| Pitch | Ratio (with 12") | Degrees |
|---|---|---|
| 3/12 | 12 & 10-7/8" | 42.1° |
| 4/12 | 12 & 10-5/8" | 41.5° |
| 5/12 | 12 & 10-1/4" | 40.5° |
| 6/12 | 12 & 9-7/8" | 39.5° |
| 7/12 | 12 & 9-3/8" | 38.0° |
| 8/12 | 12 & 8-7/8" | 36.5° |
| 9/12 | 12 & 8-1/2" | 35.3° |
| 10/12 | 12 & 8" | 33.7° |
| 11/12 | 12 & 7-5/8" | 32.4° |
| 12/12 | 12 & 7-1/4" | 31.1° |
Using the ratio: Place the two numbers on your square against the board edge. Mark along the 12" side for the cut line.
Modern method: Set miter saw to degree value directly.
How to Read the Table on Your Square
If you've ever looked at a framing square and thought "what are all these numbers?" — this section is for you. Once you understand the layout, you can pull a rafter length in seconds without touching a calculator.
Location
The rafter table is stamped on the face of the blade (the wider, longer arm). The tongue (narrow arm) usually has the Essex Board Measure or Octagon Scale.
Reading the Columns
The numbers along the top edge of the blade (1", 2", 3"... up to 18" or higher) represent the roof pitch. Find your pitch, then read down the column.
Example layout on most squares:
| Row | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|
| Common | 12⅜ | 12⅝ | 13 | 13⁷⁄₁₆ | 13⅞ | 14⁷⁄₁₆ |
| Hip | 17¼ | 17⁷⁄₁₆ | 17¹¹⁄₁₆ | 18 | 18⅜ | 18¾ |
| Jack 16" | 16½ | 16⅞ | 17⁵⁄₁₆ | 17⅞ | 18½ | 19¼ |
| Jack 24" | 24¾ | 25⁵⁄₁₆ | 26 | 26¹³⁄₁₆ | 27¾ | 28⅞ |
Converting Fractions
Square tables use fractions. For calculator work, convert:
| Fraction | Decimal |
|---|---|
| 1/16" | 0.0625" |
| 1/8" | 0.125" |
| 3/16" | 0.1875" |
| 1/4" | 0.25" |
| 5/16" | 0.3125" |
| 3/8" | 0.375" |
| 7/16" | 0.4375" |
| 1/2" | 0.5" |
| 9/16" | 0.5625" |
| 5/8" | 0.625" |
| 11/16" | 0.6875" |
| 3/4" | 0.75" |
| 13/16" | 0.8125" |
| 7/8" | 0.875" |
| 15/16" | 0.9375" |
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Worked Examples
Numbers in a table are only useful if you know how to apply them. Here are three real-world scenarios showing how to pull lengths from the rafter table and turn them into actual cut dimensions.
Example 1: Common Rafter for Gable Roof
Given: 8/12 pitch, 12' run, 16" overhang, 1-1/2" ridge board
Step 1: Rafter body
- Table Line 1 shows 14-7/16" (14.422") for 8/12
- 14.422" × 12 = 173.06" = 14' 5-1/16"
Step 2: Overhang
- 16" run × (14.422 ÷ 12) = 19.23" = 1' 7-1/4"
Step 3: Ridge deduction
- Deduct 3/4" (half of 1-1/2" ridge) measured horizontally
- At 8/12, 3/4" horizontal = ~15/16" along rafter
Step 4: Total
- 14' 5" + 1' 7-1/4" − 15/16" = 16' 0-5/16" (cut from 18' stock)
Example 2: Hip Rafter Length
Given: 6/12 pitch, 10' common run
Step 1: Hip length
- Table Line 2 shows 18" for 6/12
- 18" × 10 = 180" = 15' 0"
Step 2: Deductions
- Ridge: deduct for 45° fit at ridge board
- Fascia: deduct or add for fascia alignment
Example 3: Jack Rafter Layout
Given: 6/12 pitch, 16" OC, hip runs corner to ridge
Step 1: Longest jack
- Measure from hip to plate at first jack location
- Let's say this is 8' 0" = 96"
Step 2: Calculate remaining jacks
- Table shows 17-7/8" difference at 6/12 pitch, 16" OC
- Jack 1: 96"
- Jack 2: 96" − 17.875" = 78.125" (78-1/8")
- Jack 3: 78.125" − 17.875" = 60.25" (60-1/4")
- Jack 4: 60.25" − 17.875" = 42.375" (42-3/8")
- Jack 5: 42.375" − 17.875" = 24.5" (24-1/2")
- Jack 6: 24.5" − 17.875" = 6.625" (6-5/8")
Step 3: Side cut angle
- Table Line 4 shows 12 & 4-1/2 (or 20.6°)
- Set miter saw to 20.6° for cheek cuts
When Tables Fail: Irregular Pitches
Rafter tables cover whole-number pitches. For unusual pitches:
Interpolation
For 5.5/12 pitch:
- 5/12 common = 13.000"
- 6/12 common = 13.416"
- 5.5/12 = (13.000 + 13.416) ÷ 2 = 13.208"
Calculator Method
For any pitch:
- Common = √(12² + rise²)
- Hip = √(16.97² + rise²)
- Or use construction calculator with direct pitch entry
The Math Behind the Tables
Common Rafter Formula
Length per foot = √(12² + P²)
Where P = pitch rise (e.g., 6 for 6/12)
Hip/Valley Formula
Hip run is 16.97" for every 12" of common run (diagonal of 12" square).
Length per foot = √(16.97² + P²)
Jack Difference Formula
Difference = Common Length × (Spacing ÷ 12)
For 6/12 at 16" OC: 13.416 × (16 ÷ 12) = 17.888" ✓
Quality Check Your Layout
Before cutting production rafters, verify:
- Calculated length matches rafter table
- Test rafter fits snugly at ridge
- Birdsmouth seats fully on plate
- Overhang matches plans
- Plumb cuts are truly plumb
- All marks transfer to both faces (use Rapid Rafter)