Rafter & Stair Calculator

Raftly is a framing calculator for roofs and stairs. Give it the span and the roof pitch, and it returns the rafter length and the cut angle. Give it a floor-to-floor height, and it returns the stair risers, the tread depth and the stringer length.

Numbers You Can Actually Cut To

Every length comes out in the units you work in: millimeters, or inches as a fraction (1/4, 1/8 or 1/16, your choice), never a decimal you cannot find on a tape. Metric and imperial convert live, so a span typed in millimeters still hands you a rafter length in feet and inches if that is what your saw reads.

The Roof: A Rafter Calculator for Six Shapes

Gable, hip, shed, gambrel, unequal pitches, and a lean-to deck on a ledger. Enter a span and a pitch, as rise-per-12, degrees or a percentage; all three convert live into each other, and Raftly gives the rafter length, the plumb cut at the top, and the seat and heel cuts of the birdsmouth. It accounts for the ridge board's own thickness and any overhang, so the length is what you cut, not a number you still have to adjust. A gambrel breaks each slope partway up and gives you two rafters instead of one; an unequal roof puts the ridge off center, with a different pitch on each side.

The Birdsmouth, Marked Out

Raftly draws the birdsmouth close up: the seat, the heel, and what is left above the plate. Under the drawing there is a numbered list: start with a board this long, mark the plumb line at this angle, measure down this much for the heel, square across this much for the seat.

The Stairs: Risers That Add Up

Straight, quarter-turn and half-turn, each with the landing sized for you, plus winders for a corner without one. Give a total rise, and Raftly works out how many risers it takes to keep every one of them equal, the thing that makes a stair safe to walk. Set your own maximum riser height, or the number of risers you want, and the tread depth follows from the run you have available. The stringer drop is worked out for you, and the first and the last riser are marked separately, because that is where a stair is most often built wrong.

A Drawing for Every Job

Every job gets a full set of drawings, with every dimension marked. An elevation of the whole roof or stair. The rafter laid flat, with both saw settings on it, and where it lands on the wall plate. Tap a dimension to change it, and the drawing updates. Send the plan as a PDF or a picture, print it, or save it to this phone; add the client and the site address and they appear in the title block.

What Raftly Does Not Do

Raftly gives you geometry: lengths and angles. It does not size the timber, and it does not tell you how far apart to space the rafters; that is your local code and your own judgment. It does not know any building code, in any country. Where a limit actually matters, like a maximum riser height, you set it yourself, from the rules that apply where you build, and Raftly checks your numbers against it.

Built to Work On Site

No account, no sign-up, and no internet connection needed. Raftly works with no signal, on a roofing job or in a stairwell. There are no ads. Every job stays on this phone, and nothing is sent anywhere. The interface reads in eight languages.

Free Trial

Raftly opens with full access for 7 days. No card is asked for. If you decide not to subscribe afterward, nothing is deleted: every job stays on your phone, stays readable, and export to JSON or CSV stays free forever, whether or not you ever pay.

Made by Arkadia Labs.