# Bird's mouth joint (rafter seat)

> How deep is a bird's mouth cut?

- Category: Joinery › Scarf & splice joints
- Kind: Hardware
- Basis: Xem Studio design — geometry computed in the generator from traditional proportions
- Page: https://xemstudio.com/objects/birds-mouth-joint/ · JSON: https://xemstudio.com/api/geometry?object=birds-mouth-joint · Plain text: https://xemstudio.com/objects/birds-mouth-joint/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files Xem Studio Design License (free to use and sell from, not to redistribute as files: https://xemstudio.com/license/)
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

## Overall (mm)

| length | width | height |
|---|---|---|
| 153.6 | 80.0 | 106.0 |

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- Woodworking joint drawn for 3D printing: the parts lock together by shape, so a print can be joined without glue or screws. Drawn at true size with the parts line to line (zero gap) in their assembled position — add your printer's fit allowance to the mating faces (your AI or slicer can do this).
- The V notch that seats a sloping rafter on a wall plate: a level seat cut and a plumb heel cut.

## Sizes

| dimension | value |
|---|---|
| stock | 20 × 20 plate, 15 × 30 rafter |
| parts | plate, rafter |
| pitch | 30° |
| notch | seat cut and heel cut, about a third of the rafter depth |
| assembles | the rafter sits on the plate |
| holds | the rafter from sliding off the plate |

## Where the files are

- Xem Studio: [Joinery design (geometry computed in the generator from traditional proportions)](https://xemstudio.com/) — web — Xem Studio design

## Free STEP/STL (3 parts, Xem Studio Design License, Xem Studio)

Index: https://xemstudio.com/files/hardware/birds-mouth-joint/index.json · license: https://xemstudio.com/files/hardware/birds-mouth-joint/LICENSE.md

- birds-mouth-joint-plate: [STEP](https://xemstudio.com/files/hardware/birds-mouth-joint/birds-mouth-joint-plate.step) · [STL](https://xemstudio.com/files/hardware/birds-mouth-joint/birds-mouth-joint-plate.stl) — Wall plate (20 × 20)
- birds-mouth-joint-rafter: [STEP](https://xemstudio.com/files/hardware/birds-mouth-joint/birds-mouth-joint-rafter.step) · [STL](https://xemstudio.com/files/hardware/birds-mouth-joint/birds-mouth-joint-rafter.stl) — Rafter 15 × 30 at 30° with the bird's mouth notch
- birds-mouth-joint-assembled: [STEP](https://xemstudio.com/files/hardware/birds-mouth-joint/birds-mouth-joint-assembled.step) · [STL](https://xemstudio.com/files/hardware/birds-mouth-joint/birds-mouth-joint-assembled.stl) — All parts in their assembled position (each part its own solid)

## Questions

**How deep can a bird's mouth be?**

No more than a third of the rafter depth.

## Fits with

- [Half lap joint (corner)](https://xemstudio.com/objects/half-lap-joint/)
- [Watari ago (cogged cross joint)](https://xemstudio.com/objects/watari-ago-cogged-joint/)