The gap around an inset door depends on whether the door and its opening move with humidity, how far they move, and the humidity of the room the door will live in [1][2][3]. A dimensionally stable door in a stable opening needs only a fitting tolerance, typically 1/16" per side [1][2]. A solid-wood slab door in a solid-wood face frame needs a gap sized from a movement calculation [3].
Stable doors in stable openings
A frame-and-panel door holds its width through the seasons because its panel floats in the frame's grooves and the narrow frame members move very little [2][3].
A door with a plywood panel, hung in a plywood carcase with no solid-wood face frame, involves almost no seasonal movement on either side of the gap [3].
For a stable door in a stable opening, the gap is a production tolerance. It covers fit variation, slight racking and ordinary shop imprecision. It is not a movement allowance. A standard gap is 1/16" per side [1][2].
Solid slab doors in solid-wood face frames
A solid-wood slab door expands across its width as humidity rises [4].
The stiles of a solid-wood face frame also expand across their width as humidity rises, which narrows the door opening [3].
The slab door widening and the opening narrowing happen in the same season, independently of each other. The gap around a slab door in a solid-wood face frame must cover the sum of both movements [3].
A slab door fitted to a gap copied from a plan, without a movement calculation, can bind in the humid season [2].
Species
Black walnut and black cherry have lower tangential shrinkage than white oak and hard maple, so a walnut or cherry slab door moves less across its width than a white oak or hard maple door of the same width [4].
Wide doors in white oak or hard maple leave two legitimate choices: frame-and-panel construction, or a solid slab with a gap that visibly changes with the seasons [2][3]. A plan should state which choice it made.
The movement formula for a slab door
Seasonal movement of a solid slab = width in inches × the species' tangential dimensional change coefficient (a decimal per 1% change in moisture content, from Table 13-5 of the Wood Handbook) × the moisture content swing in percentage points. The coefficients hold for moisture contents between 6 and 14% [5].
The coefficient in that formula is the tabulated value, white oak flat-sawn 0.00376 per 1% moisture content. It is not the total green-to-oven-dry shrinkage percentage, and it is not that percentage divided by the fiber saturation point; the Wood Handbook's coefficient carries a correction for the dimension at 10% moisture content that the simple division leaves out [5].
Fitting sequence
Measure the opening at its smallest point, not at the middle or the top.
Check the opening's diagonals. An out-of-square opening means the door must be fitted to the opening's actual geometry, and that has to be known before the door is cut.
Rough-cut the door with a margin. For a solid slab, calculate the movement, add a small fitting margin, and stop. Do not plane to final size before the first hang [2].
Hang the door before final fitting. The first hang shows whether the hinge placement is right, whether the gap falls where it was calculated, and whether the door sits square in its opening [2].
Adjust after the first hang, before removing more material. Wood planed off a door cannot be put back.
Plane to the final gap after the first hang. Work the door, not the opening; the opening is already set [2].
Three questions that set the gap
Door construction: a frame-and-panel door or a plywood-panel door needs only a production tolerance of about 1/16" per side. A solid slab needs a movement calculation [1][2].
Opening material: a plywood carcase gives a stable opening. A solid-wood face frame adds the stiles' expansion to the door's movement budget [3].
Fitting season versus service season: a door fitted in a dry winter shop is fitted near its narrowest. Its gap will be at its smallest in the humid season, so the fitting season should be compared with the conditions where the piece will live [2].
What plans leave out
Most plans give one gap for inset doors. The better ones say "fit to opening," which at least acknowledges a fitting process.
Few plans explain that the printed gap assumes a specific construction type. Few mention the face frame's contribution. Few say anything about the service environment.
Sources
- Hewn heuristic data, HEU-R1226 (construction protocol table, 4% MC swing column: 1/16" reveal each edge), Reed Additions, provisional and unreviewed.
- Hewn heuristic seed data, Frame and Panel Doors, 2026-03-05, rows FH-01 (gap allowance for inset doors), FH-02 (seasonal gap planning) and the fitting-sequence rows, unreviewed.
- Hewn heuristic data, HEU-R1257 (seasonal reveal management for inset doors in solid-wood face frames), Reed Additions, provisional and unreviewed.
- USDA Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material (FPL-GTR-190), Chapter 4, Table 4-3; Skaar, C. (1988), Wood-Water Relations, Springer-Verlag.
- USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282), Chapter 13, Equation 13-2 and Table 13-5, dimensional change coefficients, valid 6 to 14% moisture content (as listed in Hewn's notes, entries HEU-R735 and HEU-R739).