The fitting tolerance is the gap between a frame and the wall (the reveal) needed to align the frame level and plumb. This gap is filled with expanding foam after alignment and sealed with caulk on the outside. Without fitting tolerance, you cannot align a frame in a wall that is not perfectly straight.
How much fitting tolerance?
The standard fitting tolerance is 5-10 mm per side. This means the frame is 5-10 mm smaller on all sides than the structural opening.
| Situation | Fitting tolerance |
|---|---|
| New build (precise brickwork) | 5-7 mm |
| Renovation (older walls) | 7-10 mm |
| Timber-frame construction | 5 mm |
Installing a frame
1. Place the frame in the opening — On plastic packing blocks
2. Check level — Horizontal and vertical
3. Check plumb — With a spirit level against the jambs
4. Adjust with packing blocks and wedges — Until the frame is perfectly aligned
5. Fix in place — With frame screws or plugs
6. Expanding foam — Fill the fitting gap (not too much, foam expands)
7. Seal with caulk — Seal the outside for watertightness
Related terms
- Structural opening
- Rebate dimension
- Frame