How to read the rafter tables on a framing square
Those six rows of numbers stamped into the steel are a complete rafter calculator, no batteries required. Here is what each row means and how to put them to work.
Pick up a steel framing square, the big L-shaped one with a 24 inch body and a 16 inch tongue, and look at the face of the body. Under the inch marks you will find rows of numbers stamped into the metal. Those are the rafter tables, and they have been riding along on framing squares since the 1800s. They answer the hardest question in roof framing, how long to cut each rafter, for any common pitch, without a calculator and without trigonometry.
The tables work because of one simple convention. Roof pitch in the United States is expressed as inches of rise per 12 inches of horizontal run: a 6/12 roof rises 6 inches for every foot it travels toward the ridge. The inch marks along the body stand in for the rise. Look under the 6 and every number in that column applies to a 6/12 roof. Look under the 8 and you are reading an 8/12 roof. Once you know that, the six rows fall into place.
The six rows, top to bottom
Most squares label the rows along the left end of the body. From the top, they read as follows.
| Row | What it tells you |
|---|---|
| Length common rafters per foot run | Inches of rafter length for every 12 inches of horizontal run, at the pitch under that inch mark |
| Length hip or valley per foot run | The same idea for hips and valleys, which run diagonally and are longer for the same run |
| Difference in length of jacks, 16 inch centers | How much shorter each successive jack rafter is when they land 16 inches on center |
| Difference in length of jacks, 24 inch centers | The same step-down for 24 inch on-center spacing |
| Side cut of jacks | The number to set on the tongue for the angled cheek cut where a jack meets the hip or valley |
| Side cut of hip or valley | The setting for the cheek cut where a hip or valley meets the ridge |
The first row is the one you will use most. It gives the length of a common rafter, measured along its slope, for every 12 inches of run at that pitch. The second row does the same for hips and valleys, which travel at 45 degrees to the commons in plan and therefore need more length to cover the same run. Rows three and four handle jack rafters, the shortened commons that fill in between a hip or valley and the wall plate: because jacks land on a diagonal member, each one is shorter than its neighbor by a fixed amount, and the table gives that amount for 16 inch and 24 inch spacing. The last two rows give side cut, or cheek cut, settings, the angles that let jacks seat flat against a hip or valley and let the hip or valley seat against the ridge.
A worked example: common rafter on a 6/12 roof
Say you are framing a 6/12 roof and the run from the outside of the wall plate to the center of the ridge is 6 feet. Find the 6 inch mark on the body of the square and read the first row beneath it. It says 13.42. That means every foot of run at a 6/12 pitch needs 13.42 inches of rafter.
Multiply by the run: 13.42 × 6 = 80.52 inches, call it 80 1/2 inches. That is the length of the rafter along its measuring line from the plumb cut at the ridge to the heel of the birdsmouth. Two adjustments finish the job. First, the table measures to the centerline of the ridge, so deduct half the thickness of the ridge board, measured square to the plumb cut: 3/4 inch for 2× ridge stock. Second, the table length ends at the birdsmouth heel, so any tail for the eave overhang gets added beyond it. Convert the decimal carefully, 0.52 inches is just over a half inch, and mark the plumb cut, the adjusted length, and the birdsmouth along the measuring line.
The same procedure works in the second row for hips and valleys. Under the 6 you will find 18, meaning a hip or valley on that roof needs 18 inches of length per foot of common run. Multiply by the same 6 foot run and lay out from there.
Stepping off with the body and tongue
The tables are one way to use the square. The older method, stepping off, uses the tool's shape directly. Set the square on the rafter stock with the tongue reading the rise on one edge, 6 inches for our roof, and the body reading 12 inches on the same edge. The tongue now lies along the plumb cut angle and the body along the level, or seat, angle. Scribe the plumb line, then slide the square down the board one full 12-and-6 step for every foot of run, marking as you go. Six steps for a 6 foot run, then the same ridge deduction as before.
Stepping off is satisfying and it works, but every step is a chance for the square to drift a hair, and small errors accumulate over six or eight steps. Most framers use the tables to get the length in one multiplication, then use the square, or a triangular rafter square as covered in our guide to how to use a rafter square, to scribe the actual plumb and seat lines.
What the tables will not do
The rafter tables assume a regular roof: a level ridge, walls at 90 degrees, hips and valleys at 45 degrees in plan. Irregular pitches, bastard hips, and off-angle wings need either the math done longhand or a construction calculator. The tables also stop at the numbers stamped on the square, usually 2/12 through 18/12, and they say nothing about the birdsmouth depth, overhang, or ridge deduction; those come from your framing plan and local code. We cover the layout side of the birdsmouth in our step-by-step birdsmouth guide, and the pitch conventions behind all of this in roof pitch explained.
The tables give you the length. The marks still have to be square.
Here is the part the tables cannot help with. Once you know a rafter is 80 1/2 inches along the measuring line, every one of those marks, the ridge plumb cut, the heel line, the tail cut, has to be carried square across the face of the board, and on gang-cut or double-checked work, across both faces. A framing square only marks one face at a time, and every flip and re-registration is a chance for the lines to disagree.
That is the problem the Rapid Rafterwas built for. It is a patented dual-sided square, U.S. Patent No. 11,654,545 B1, invented by master carpenter Peter Toomey of Liberty Hill, Texas, who has been building since 1984 and founded All Seasons Decks & Gazebos. In the open position it straddles the stock and marks three sides of any 2× material in one motion, both faces and the top edge, and because it registers on the faces, you get a perfect perpendicular line regardless of the quality of the board. Folded flat, it works as a normal everyday square. The Original is made in the USA from a proprietary polymer at $27.99; the machined aluminum Pro is $79.99. Popular Mechanics named it Gear of the Year 2025, it won an LBM Journal 2026 Innovation Award, and it is available at The Home Depot.
Read the table once. Mark all three sides once.
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