Truss & bevel cut calculator
The three calculations you keep needing while framing one roof, on one screen. Set the pitch once and all three use the same angle.
Result
Enter values and press Calculate.
What to measure first
- Pitch — rise per 10 of run on Korean sites. If the drawing gives degrees instead, switch to angle mode.
- Member width — the dimension the cut line crosses. A 2×6 is actually 140mm, not 152mm. Tube steel uses its nominal size as-is.
- Span — for a truss, the distance between bearing points. Measure to where the load actually lands, not to the outside of the wall.
- Plate width — the length of the seat the rafter rests on. 89mm for a 2×4 plate.
Marking it out
- 1Draw your reference line at the low (short) edge first.
- 2Measure the calculated offset along the member from that line and mark the high edge.
- 3Join the two marks — that's your cut line. Set the saw to the pitch angle shown in the result.
- 4Cutting several identical pieces? Use the first one as a template rather than re-measuring, so errors don't accumulate.
Common mistakes
Using nominal dimensions
A 2×6 is really 38×140mm. Entering 152 inflates the offset by about 6mm, and across several rafters the roof line visibly skews.
Confusing pitch with degrees
A 5-chi pitch is about 26.57°, not 5°. Typing the drawing's pitch figure straight into the angle field gives you a completely different roof.
Cutting the birdsmouth too deep
Once the notch passes one third of the rafter depth, that point becomes the weak link. A steep pitch on a wide plate crosses the line quickly — watch for the calculator's warning.
Forgetting the kerf
Cutting piece after piece, the blade width (around 3mm) adds up. Re-mark each piece when the exact length matters.
Actual sizes of common members
North American dimensional lumber is milled smaller than its nominal name. Enter the actual sizes below.
| 2×4 | 38 × 89 mm | Plates, studs |
| 2×6 | 38 × 140 mm | Typical rafters, joists |
| 2×8 | 38 × 184 mm | Longer joist spans |
| 2×10 | 38 × 235 mm | Heavily loaded rafters |
| 100×100 tube | 100 × 100 mm | Nominal = actual |
FAQ
Can I use this for steel tube instead of timber?
Yes. Bevel-cut geometry doesn't care about the material. Tube steel's nominal and actual sizes match, so enter the marked size directly. Just allow a bit more for kerf if you're cutting with a plasma torch or grinder.
How many degrees is a 5-chi pitch?
About 26.57°. Converting a pitch to degrees means arctan(rise ÷ run), and this calculator shows the angle whenever you enter a pitch. For reference, Korea's 5/10 and North America's 6/12 are the same slope.
How do I pick a truss type?
Short spans take a king post; longer ones move to a Fink (W) or queen post with web members. But the type and the member sizes have to come from a structural calculation covering dead, snow and wind loads. This calculator gives you lengths and angles once that's decided.
Can I skip the birdsmouth?
Butting two sloped faces together leaves very little contact area, so the load concentrates on a point. The birdsmouth creates a flat seat that spreads the load into the plate, which is why standard timber framing includes it.
Calculators you'll use next
These results are reference figures for cutting and ordering. Final member sizes and truss type must follow a structural review for your actual loads.