Rough opening
A rough opening is the framed hole in a wall that a window or door is installed into. It is built slightly larger than the unit, so the installer has room to level, shim, insulate and seal the gap around the frame.
On a drawing, the rough opening is the hole in the framing, not the window. Width is measured between the trimmer (jack) studs. Height is measured from the rough sill to the underside of the header. The window or door unit is always smaller than the opening. The gap left around it is where shims, insulation and sealant go, and it is also where most installation problems start.
How much clearance to leave
The right gap depends on how the unit is mounted, so decide the mounting method before anyone frames.
| Mounting method | Typical clearance | What the gap is for |
|---|---|---|
| US wood-frame window with a nailing fin | about 1/4-3/8 in. of play | Leveling. The fin laps over the sheathing. |
| Bracket-mounted European unit | about 1/2-3/4 in. per side | Leveling, shimming and a continuous insulation gap |
| Lift-and-slide or entrance door | Check the sill separately | European sills sit lower and shallower than a US threshold |
Why European orders need extra care
European windows are sized in millimeters, in round modular steps. A 1200 mm unit is 47-1/4 in., not 48 in. A framer who rounds to the nearest familiar inch size builds an opening that is slightly wrong. It is the most common mistake I see on a first European order.
The fix is simple: frame to the factory shop drawing, in millimeters, with the inch conversion listed as a reference only. Before production is confirmed, I check every opening on the shop drawing. The as-built opening on site belongs to the framer or GC, and it is verified before the unit ships, not after it arrives. The full conversion table is in European window rough opening sizes.
To get a budget number, enter the window sizes from your schedule in the estimator. It prices by unit area, so an estimate made from rough-opening sizes will come out a little high.
Common questions
- How do you measure a rough opening for a window?
- Measure the width between the trimmer studs, and the height from the top of the rough sill to the underside of the header. Measure at three points in each direction and use the smallest. Check the diagonals too - if they differ noticeably, the opening is out of square and needs fixing before the window arrives.
- What is the rough opening for a 32 inch door?
- For a standard US prehung door, the usual rule is to add 2 in. to the width and about 2-1/2 in. to the height. That gives roughly 34 in. by 82-1/2 in. for a 32 by 80 door. European doors do not follow that rule - frame to the dimensions on the shop drawing.
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