A roof frame has a small cast of members, and the two you will meet most often are the common rafter and the jack rafter. The common rafter is the regular one: it runs from the wall plate to the ridge, it is the same length as every other common rafter on the roof, and it carries the roof load straight down to the wall. The jack rafter is the short one: it runs from the wall plate to the hip rafter or the valley rafter, it is shorter than the common rafter, and every jack on the same hip is a different length. This guide explains what each one does, how each one is cut, and how the lengths compare on a real roof, so you can tell them apart on the plans and on the job. The lengths themselves come from the rafter calculator, which works out the common rafter length for any span and pitch.
The Common Rafter
The common rafter is the unit of the roof. It runs at right angles to the ridge, from the wall plate up to the ridge board, and it has three cuts: the plumb cut at the ridge, the birdsmouth seat cut at the wall plate, and the tail cut at the eave. Every common rafter on the same roof has the same three cuts at the same angles and the same length, which is why carpenters cut one pattern rafter and use it to mark the rest. On a 28 ft span at 6/12 pitch, the common rafter is 15 ft 7 13/16 in from ridge center to plate, the ridge-to-birdsmouth cut is 15 ft 7 in, and the total board with a 12 in overhang is 16 ft 8 7/16 in. The roof angle is 26.6 degrees, and that angle is the plumb cut angle on every common rafter on the roof.
The common rafter carries the roof load in two directions at once: down the slope to the wall plate, and sideways as thrust against the ridge and the opposite rafter. The ceiling joist ties the two sides together and stops the walls from spreading, which is why a roof without joists or ties needs a structural ridge beam instead of a ridge board. The birdsmouth is the joint that matters most: the seat cut sits flat on the wall plate, the heel sits tight against the plate, and the seat cut must never take more than a third of the rafter depth. On a 2×8, which is 7.25 in deep, the seat must leave at least 4.83 in of wood above it. The how-to post works through the common rafter calculation step by step, including the ridge allowance and the tail.
The Jack Rafter
The jack rafter is a common rafter that stops early. Instead of running to the ridge, it runs to the hip rafter or the valley rafter, and it meets that diagonal member with a compound cut: the plumb angle of the pitch plus the side angle of the 45-degree plan. The jack’s length depends on how far it sits from the corner: a jack at a distance d from the corner along the plate has a run of d and a rise of d times the pitch over 12, so its length is the square root of d squared plus the rise squared. On the 28 ft span at 6/12 pitch, a jack 3.5 ft from the corner is 3 ft 10 15/16 in long, a jack 7 ft from the corner is 7 ft 9 15/16 in, and a jack 10.5 ft from the corner is 11 ft 8 7/8 in. The 7 ft jack is exactly half the common rafter, because 7 ft is half the 14 ft run, and the jacks step up by the same increment toward the ridge.
| Member | Where it ends | Length on 28 ft span, 6/12 | Same as the next one? |
|---|---|---|---|
| Common rafter | Ridge board | 15 ft 7 13/16 in | Yes, all the same |
| Jack rafter, 3.5 ft from corner | Hip rafter | 3 ft 10 15/16 in | No, steps up |
| Jack rafter, 7 ft from corner | Hip rafter | 7 ft 9 15/16 in | No, steps up |
| Jack rafter, 10.5 ft from corner | Hip rafter | 11 ft 8 7/8 in | No, steps up |
| Hip rafter | Ridge end | 21 ft | Yes, all four the same |
There are two kinds of jack rafter, and the difference is which diagonal member they meet. A hip jack meets the hip rafter from the side, and its top cut is the compound cut described above. A valley jack meets the valley rafter from the side, and its top cut is the mirror image. A valley jack is sometimes called a cripple jack when it meets both a hip and a valley, as on a dormer, and it is the shortest member on the roof. The hip rafter post works through the hip, valley, and jack lengths together, including the 21 ft hip rafter on the 28 ft span roof.
Which One Carries the Load
Both do, but they pass it on differently. A common rafter passes its load to the wall plate and the ridge board, and the ridge board passes it to the opposite rafter. A jack rafter passes its load to the hip rafter, and the hip rafter passes the combined load of all its jacks to the corner of the building and to the end of the ridge. That is why the hip rafter is often a size up from the common rafters: on a roof framed with 2×8 common rafters, the hips are often 2×10, because one hip carries the load of a dozen jacks. A jack rafter that is cut short, nailed with too few nails, or left out entirely leaves a section of roof deck with no support under it, and the deck dips there within a year or two. The nails matter too: a jack rafter needs the same nailing at the birdsmouth as a common rafter, plus the nails or a hanger at the hip, and the hip connection is the one most often under-nailed.
Dormer Rafters Are Jacks Too
A dormer is a small roof cut into the main roof, and its rafters are jack rafters in the same sense: they meet the main roof instead of running to a ridge of their own. The dormer’s own rafters are short common rafters on the dormer’s span: an 8 ft wide dormer at 6/12 pitch has a 4 ft run and a 2 ft rise, so its rafters are 4 ft 5 11/16 in long. A 6 ft wide dormer at 8/12 pitch has a 3 ft run and a 2 ft rise, so its rafters are 3 ft 7 1/4 in long. Where the dormer roof meets the main roof, the valley jacks step down the same way the hip jacks do, and the valley rafter carries them. Dormer framing is fiddly work, and it is where a pattern rafter and a careful layout pay for themselves twice over, because the dormer has to shed water into the valley without a leak at the joint.
How to Cut Them
Cut the common rafters first, from the pattern rafter, and set them in place. Then measure the hip rafter length from the formula, cut it, and set it. Then cut the jacks one at a time from the hip outward, or cut the longest jack first and use it as the pattern for the step: each jack is shorter than the one before it by the same step, so once the step is known, the whole set follows from one measurement. Test-fit the first jack against the hip before you cut the rest, because the compound cut is easy to get backwards: the plumb angle is the same as the common rafter’s plumb cut, but the side angle runs the other way on the two sides of the hip. Mark the jacks left and right separately, and keep them in separate piles, because a left jack will not fit on the right side. The rafter calculator gives the common rafter length that every jack length is built from, and the mistakes post covers the cuts that go wrong most often, including the birdsmouth depth and the ridge allowance.
Related reading: rafter calculator, plus our guides on hip rafter calculator and rafter length mistakes, and the roofing calculators hub.
Framing a Hip Roof?
Hip roofs mix common rafters, jacks, and hips. We will measure your roof for free and work out every length. Call now for a free inspection in Waukegan and Lake County.
Frequently Asked Questions
What is a common rafter?
A common rafter is the standard rafter on a gable roof: it runs from the wall plate to the ridge board, at right angles to the ridge, and every common rafter on the same roof is the same length. On a 28 ft span at 6/12 pitch, the common rafter is 15 ft 7 13/16 in from ridge center to plate. The common rafter carries the roof load from the deck to the wall plate and the ridge.
What is a jack rafter?
A jack rafter is a short rafter that frames into a hip rafter or a valley rafter instead of running to the ridge. It is shorter than a common rafter on the same roof, and each jack is a different length: the jacks step down toward the corner. On a 28 ft span at 6/12 pitch, a jack 7 ft from the corner is 7 ft 9 15/16 in long, which is exactly half the common rafter.
How do you tell a jack rafter from a common rafter?
By where it ends. A common rafter ends at the ridge board. A jack rafter ends at the hip rafter or the valley rafter, with a compound cut where it meets the diagonal member. A jack rafter is also shorter than the common rafter on the same roof, and the jacks on one hip are all different lengths, while the common rafters are all the same length.
Are jack rafters structural?
Yes. A jack rafter carries the same roof load as a common rafter over the part of the roof it covers, and it passes that load into the hip rafter, which passes it to the corner and the ridge. A jack rafter that is cut short, nailed poorly, or missing leaves a section of roof deck with no support under it, and the deck will dip there. The hip rafter must be sized to carry the jacks as well as its own load, which is why hips are often a size up from the common rafters.
Do I need jack rafters on a gable roof?
No. A gable roof has only common rafters, because there is no hip or valley for a jack to frame into. Jack rafters only exist on hip roofs, valley roofs, and roofs with dormers, where a diagonal member interrupts the run of common rafters. A dormer has jack rafters where its roof meets the main roof, and the dormer rafters themselves are short common rafters on the dormer’s own span.
Zayn Robert is a content specialist focused on roofing, construction, and home improvement topics. He creates easy-to-understand guides that help homeowners learn about roof measurements, materials, maintenance, and repair decisions. His work combines detailed research with practical insights from roofing professionals at Zuniga Roofing Inc.