Trim and millwork dimensions

Every trim question in a house has the same input and most people never write it down: the distance from the finished floor to the ceiling. Trim is a set of ratios of that one number, and the reason a 5 in baseboard looks generous in one room and mean in the next is that the wall changed, not the moulding. Decide the wall height first, then the baseboard, then the horizontals, then the casings, and last the crown, because each of those has less room to move than the one before it.

The datum is the finished floor, not the subfloor#

Trim heights are measured from the finished floor: the top of the tile, the top of the board, the top of the carpet pile. This matters because trim is almost always ordered while the subfloor is still bare. A 16 mm (5/8 in) engineered floor over 2 mm of underlay takes 18 mm off every height set from the slab, which is a third of an inch of baseboard lost and, more awkwardly, a third of an inch of reveal gained at the bottom of the door casing.

Mark the finished floor level on the wall in pencil before anything is measured. The same discipline runs through how to measure a room properly and ceiling heights and datum lines, and the build-up you have to allow for is in underlay and subfloor thickness.

Baseboard first, because everything lands on it#

The baseboard is the largest piece of trim in most rooms and it sets the scale the others are read against. The working rule is about 7 per cent of the wall height, adjusted for period.

Two things follow immediately. Baseboard height is bounded above by casing width, since a base taller than the casing is wide has nowhere to die into and needs a plinth block. And baseboard projection must be greater than casing projection by at least 1/8 in (3 mm), or the base returns onto nothing. The sizes and the plinth-block case are worked through in baseboard and skirting heights and door and window casing widths and reveals.

The horizontals sit at a third, and only one of them can#

A wall takes one strong horizontal. Chair rail at 32 to 36 in (81 to 91 cm) is roughly a third of an 8 ft wall and roughly the height of a dining chair back, which is where the name and the function meet. Full-height wainscot goes the other way, to two thirds, at 54 to 60 in (137 to 152 cm). Picture rail takes the top of the wall, 12 to 18 in (30 to 46 cm) down from the ceiling.

Pick one. Two horizontals on an 8 ft wall leave three bands of about 32, 24 and 30 in, which reads as stripes rather than proportion. On a 10 ft wall there is room for a dado and a picture rail together, and that is exactly why Victorian rooms carry both. The heights and the panel arithmetic are in chair rail height and proportion, wainscot and wall panelling heights and picture rail height and hanging from it, and where the rail collides with what goes above it is dealt with in art hanging height.

Casings, reveals and the order of the joint#

Casing is sized off the baseboard, at roughly half its height, and it is set with a reveal of 1/4 in (6 mm) of jamb left showing. The reveal is not decoration. It is a tolerance: a casing run flush to the jamb edge advertises every millimetre the frame is out of true, and the reveal absorbs that.

Casing width also has a hard limit that has nothing to do with proportion. Two doors in a corner, or a door beside a window, need the sum of the two casing widths plus a return, so a 3.5 in casing needs 7 in (18 cm) of wall between openings before it can be mitred at all. Rough opening positions are fixed long before trim is chosen, which is why the sizes in interior door sizes and rough openings belong in the trim conversation.

Ceilings, beams and the depth budget#

Anything applied to a ceiling is subtracted from headroom, and the IRC habitable room minimum of 7 ft (2,134 mm) is the floor beneath which the subtraction stops being allowed. That applies to the cornice as much as to a beam, and the profiles and face heights that fit each band are in crown moulding sizes and proportions. A coffered ceiling with 8 in (20 cm) beam drops needs a 9 ft ceiling to end up at 8 ft 4 in inside the coffer, which is why coffers in 8 ft rooms are always shallower than the drawings suggest. The beam sizes, spacings and the coffer grid are in ceiling beam and coffer dimensions, and the proportional consequences of the ceiling itself in ceiling height and proportion.

The same depth budget governs the fixed joinery that reads as trim: an alcove shelf at 10 in (25 cm) deep with a 1 in (25 mm) face, a mantel shelf at 54 in (137 cm) with a 6 in (15 cm) projection. Those are in built-in niche and alcove shelf proportions and fireplace mantel and surround dimensions.

What to do when the ratios disagree#

Old houses and new ones give different answers, and the honest position is that both are right for their own wall. A 1900 terrace with a 9 ft 6 in ceiling can carry a 9 in base, a dado at 36 in and a picture rail, because the wall is 114 in tall and there is 34 in above the door head to spend. A new-build at 7 ft 9 in (2,362 mm) has 13 in above the head and can carry a 4.5 in base, no rail and no crown at all, and it will look better without them. When the ratio and the catalogue disagree, the wall wins, and the broader rules for that judgement are set out in scale and proportion rules.