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