The rough opening is the measurement of the opening in a wall where a frame, door or window will be installed. It is the distance from one wall edge (reveal) to the other, measured on the inside of the opening. The rough opening is always larger than the frame itself — the difference is the shimming space.
Rough opening vs. frame size
| Term | Meaning |
|---|---|
| Rough opening | The opening in the wall |
| Frame size | The outer dimension of the frame |
| Shimming space | The difference (5-10 mm per side) |
| Rebate size | The size of the glass within the frame |
Rough opening = frame size + 2x shimming space
Example: a frame 1000 mm wide + 2x 7 mm shimming space = rough opening of 1014 mm.
How do you measure the rough opening?
Always measure at three points:
- Top — From reveal to reveal
- Middle — From reveal to reveal
- Bottom — From reveal to reveal
Use the smallest measurement as the rough opening. Walls are rarely perfectly straight.
Also measure the height at three points (left, centre, right).
Related terms
- Rebate size
- Window frame
- Shimming space
- Lintel