Hip Rafter Calculator | Length, Angles, Jacks

Once we get to the valleys and hips, though, the roof framing becomes more about thinking in diagonals then straight lines, and this is where precision matters most, not guessing. Because now, instead of squaring up your wall plates and putting your ridge board right down the middle, it’s a different story. That corner has to have some kind of angled main support beam (but what angle?). The hip rafter is going to be at an angle across your drawing, coming out toward the corner, carrying both the slope of the roof plus angling into the corner. It do double duty; therefore, it must have its own set of cuts.

A lot of folks figure this out by drawing a line on the ground diagonally and then guesstimating what the angle is, which won’t work, because it doesn’t take into account the thickness of the boards. A ridge board is a real thing, and has width (usually an inch-and-a-half thick if it’s made out of standard lumber). When you lay out your birdsmouth cut, if you measure your run right down to the middle of that ridge board without accounting for the thickness in your layout, your hip will either be low or ride up on top of it.

Cutting Hip Rafters

The calculator takes care of all this for you; as long as you know the pitch and dimensions of your roof, you don’t have to guess at any conversion numbers. It automatically adjusts for half the thickness of your ridge, meaning that you are measuring to the proper bearing point for your length, rather then some hypothetical center. But then there are the cuts… and they get complicated.

A typical rafter is cut at an angle (a level cut) and at right angles to that (a plumb cut). Then there’s a hip rafter which requires the same two cuts (horizontal and vertical), plus a third, called a cheek cut or backing bevel. This gives the hip a slope from side-to-side so the jack rafters fits, but also makes the top edge sit flush against the ridge board. If you don’t include this bevel in your cutting, your hips will look awful and your valley lines won’t meet.

And that’s where the multiplier comes in again; essentially the hip multiplier is just a scale factor that enables you to figure out how much longer to make your diagonal stock in relation to the straight common rafter. On a basic six over twelve pitch on a square corner, that number is one point five. This means your hip is half as long again than its run along the house.

An additional level of coordination exists in the Jack rafters, those smaller ones which step down from the ridge to the hip. These fill in the spaces between full length commons, and get successively shorter approaching the corner. To maintain both the look and the strength of the frame, getting the spacing of these rafters correct is critical. When measured individually from various reference points, the cumulative error can happen fast. Instead, draw one layout line along the eave and use it as a starting point against which all subsequent jack locations are marked off. The system will give you a schedule indicating just how much the length drops off at each increment. That way you don’t waste time trying to fit things together on the roof deck, then discovering they don’t match up.

Adding a room addition changes the geometry altogether, and there is also irregular hips to deal with where the corner angle isn’t forty-five degrees. In other words, the plan angle changes everything. Including the backing bevel and length… Which will require adjustments to those standard square-roof formulas as well. The way it plays out is in the reference table on the page. You can see how various angles impact the multipliers. A sharper angle means a longer hip and a steeper side cut. It doesn’t sound like much, but it matters when you are cutting good lumber by hand.

But no matter how you calculate it, safety and verification are key. These tools use geometry to give you an idea about layout. However, they don’t take into consideration structural engineering information, such as bending strength by species, snow load, or wind uplift resistance. Always bring in qualified help and reference local building code when framing.

The math is a good start, but nothing beats a test cut for the final fit. First cut a scrap piece. Check the angle to both the plates and the ridge and tweak as needed. An additional five minutes of testing saves you hours of ripping out work later.

Carpentry meets calculus in the field; hip rafters are where patience is rewarded and haste punished. Cut the measurements; take a deep breath. The result is a roof line that’s tight, true and ready to be sheathed.