Jack Rafter Length Calculator

Laying out common rafters is something most framer can do in their sleep. Pull the chalk line. You got your run, you got your rise, you know what to do. It is a piece of basic geometry, made better by practice.

Then there are jack rafter. They’re at the corners of your house where the main plane of the roof meets a hip or valley. They don’t repeat the same exact length more than once. The first one is longer than the last one was; the second one’s shorter. Getting the spacing down on paper separate a good-looking roof from an uneven-looking roof.

How to Measure and Cut Jack Rafters

Here’s the tricky part of math when dealing with jack rafters. You’re going to need two separate measurements. One is the distance across ridgeline or plate, typically 16 inches or 24 inches on center. The other is length on the sloping part of the roof, which is the amount of lumber that runs up the pitch. These two won’t be identical.

To save yourself some time deriving the common difference between each cut by hand, use the calculator above and enter your plan angle and roof pitch. It’ll do all the work for you.

The trick comes down to getting this one number right: the common difference. This means finding out how much each following jack rafter grows or shrinks. Once you know it, then you can skip doing any more manual math. Because if you don’t get this number correct, then every single other rafter after will be slightly off by a fraction, and those fractions add up quickly.

Also consider the geometry of the corner itself. On a square house, the hip rafter is a forty-five degree angle. That’s pretty predictable math. But so many houses these days is built with irregular angles. A complicated wing, a dormer, a bump-out; all of those create some sort of hip or valley that isn’t forty five degrees.

The angle of the plan alters the horizontal run of the jack before any pitch enters the picture. Using the same old sixteen inches between jacks without considering that angle mean your jacks will be crowded or there will be wide gaps around the ridge. It takes only a moment to measure that angle out on the floor plan and adjust accordingly. One little step, no biggie on the roof deck, but it saves a world of headaches.

Where theory becomes saw blade is in the deductions. The end of a jack rafter isn’t a perfect right angle; it must have a plumb cut meeting either the valley or hip, and also a seat cut resting on the plate. To calculate that, we deduct the amount of material required for those cuts from the full slope length.

When entering calculations into the calculator, simply enter the thickness of your framing lumber (which will be used for both cheeks) along with the depth of the seat you want. The tool will then adjust the final cut length to match. Remember to split out the physical cuts (the plumb cut, the cheek cut and the seat deduction) from the geometric slope itself. First figure the slope length, then do the plumb cut, cheek cut and seat deduction. That way you can maintain the common difference as pure, and use one increment for all your jacks.

Material choice matter more than most people realize. The seat depth of a two-by-eight differs from that of a two-by-six jack rafter. That affects the rest of the bearing length. The tables on the page shows the pitch factors for common stock sizes and explain how the actual depth of the lumber impacts the cut.

You may think cutting a deeper rafter makes it stronger, but then again you’re asking for a deeper birdsmouth as well. Cut the lumber too deep and you’ll affect the strength of your beam. As a general rule, keep the seat depth at one-third of the lumber depth. That will still provide enough support to work while keeping its strength.

Laying out the first jack takes some skill. You don’t always get a full 16 inch spacing between the corner and the center of the first rafter. It might be offset by the corner design or by the thickness of the hip rafter. After the first point has been made, the remaining points is based off the common difference.

After you make that first mark, cut a test jack and check its fit against the hip. Is it snug? Check your deduction. Loose? Check your angle. Rafters are more about being consistent than precise. An occasionally imperfect rafter is one thing. A rafter out of sequence (like a jack) throw off the whole plane. That’s where this tool can help you get back on track quickly.

What was once a jumble of angles and slope becomes a simple list of cut lengths. Sure, you’ll need to cut it out and square the lumber. But the guessing part is gone.

The house goes up straight. Nails go in true. The house stays dry.