Cornice, crown moulding and coving: types and sizing

At a glance

General trade practice; a partner plasterer and joiner to confirm each cell.

Cornice, coving, crown: what the words mean

The words overlap, and suppliers use them loosely. A working split that holds on a drawing:

• Coving is a simple concave moulding where wall meets ceiling: one hollow, usually plain.
• Cornice is the projecting moulding at the top of a wall. It can be one element or several stepped elements, and it may be plaster, timber or another material. The word comes from classical architecture, where it is the topmost part of the entablature.
• Crown moulding is the American word for a cornice, and in practice for a timber or MDF moulding set at an angle (sprung) across the corner.

Because the usage differs between suppliers, do not write “coving” or “cornice” alone on a schedule. Draw the section and give its measurements (next section). The names of the individual curves are on our moulding profiles sheet.

Measure it the same way every time

Three numbers describe a cornice that is sprung across the corner: the drop (down the wall from the ceiling to the lower edge), the projection (out along the ceiling from the wall to the top edge) and the spring angle (between the back of the moulding and the ceiling). A cornice that is not sprung, like most run plaster, is drawn the same way: drop and projection, then the section at 1:1.

A face length alone is not enough. A 140 mm face can be 100 mm down by 100 mm out, or 70 mm down by 120 mm out, and the two look different on a wall. For a straight back, the face length measured along the back is the square root of (drop squared plus projection squared): 141 mm for 100 by 100. State finished size, not nominal, and say which edge is the datum.

Carrying a cornice you like to another ceiling

We found no standard, regulation or primary source that gives cornice sizes by ceiling height. One secondary source quotes size bands by period, which we have not verified and do not reproduce. What a designer can do is take a cornice they like in one room and carry it to another. Two ways of doing that give different answers, and the gap between them is the design decision.

• Same apparent size. From a standing viewer the cornice looks the same size if its drop grows with the distance from the eye to the ceiling edge. We assumed an eye height of 1.6 m and a standing distance of 2.0 m from the wall. This asks for only a modest increase.
• Same fraction of the ceiling height. The cornice grows in proportion to the room. This asks for a much larger increase and is the more usual reading of “proportion”.

The table starts from a drop of 100 mm in a 2.4 m room. For another starting size, multiply the figures. The projection grows by the same ratio if the shape is simply scaled.

Treat the two columns as the ends of a range to test on a full-size mock-up in the real room, in the real light. Ratio rules for the other horizontals (skirting, dado, picture rail) are on the moulding proportion guide; they are conventions too.

Corners: the angles worked out

The joint in a corner lies on the diagonal that halves the corner angle. For a plain moulding that is half the corner angle: 45 degrees in a square corner. A sprung moulding needs a compound cut, because it leans. Laid flat on the saw with its back on the table, the settings for a 45 degree spring are below. We derived them from the geometry (spring angle measured from the ceiling, h half the corner angle: miter = arctan(cos h x cos spring / sin h), bevel = arcsin(cos h x sin spring)) and checked them by two independent methods; they reproduce the familiar 35.3 and 30 degree pair for a square corner.

Real corners are rarely square. Measure each corner, expect the settings to move, and cut a test corner before the run. A spring other than 45 degrees gives other values, and suppliers quote spring from the wall or from the ceiling, so state which. Where a corner is out of square or the ceiling is not level, the joiner or plasterer will scribe or adjust on site; the allowance belongs on the drawing (see the joinery tolerances and scribing sheet).

Who makes it, and what to ask

The type decides the trade. A plasterer runs or casts plaster; a joiner cuts timber and MDF; foam mouldings come from suppliers. That changes who is on site, when, and where the joints are. The studio coordinates the choice with its partner workshops and does not state that it supplies any of these itself.

Fibrous plaster has its own standards: BS EN 13815 for the casts and BS EN 15319 for the design of the work (both listed by BSI as current and under review; we did not read the texts). Materials are compared, with the generic fire lists, on the moulding materials sheet.

A cornice that hides light

A cornice or cove can hide an LED strip, but that is a lighting and electrical design question, not a moulding one. The moulding sets the sight line and the access; heat, drivers, wiring and maintenance access come from the luminaire data sheet and a qualified person. Our lighting built into joinery sheet covers the questions to ask. We state no distances or temperatures here.

Fire classification in England

In England, Approved Document B (Volume 2 paragraph 6.6; Volume 1 paragraph 4.6) says a ceiling does not include “architraves, cover moulds, picture rails, exposed beams and similar narrow members”. A cornice is not named in the list. Whether a narrow cornice counts as a similar narrow member, and what class a wide or built-up cornice needs, is for the fire engineer and building control to decide on the project’s fire strategy; the ceiling and wall either side are still linings. Edition read: 2019 edition incorporating the 2020, 2022, 2025 and 2026 amendments, as published on GOV.UK in November 2026. Scotland, Wales and Northern Ireland have their own guidance, which this sheet does not cover. The three generic fire lists and what they cover for each material are on the moulding materials sheet.

Briefing a cornice so it is not guessed

• Draw the section at 1:1 and mark the drop, the projection and (for a sprung moulding) the spring angle.
• Name the type and the trade: run in place, cast, timber, MDF or foam; and who fixes it.
• Show the corners: mitre, cope or cast corner; how an out-of-square corner is handled; where length joints may fall.
• Mock up one corner and one run in the room before the schedule is fixed.
• Ask for the fire evidence the project needs from the fire engineer first, then from the supplier.

To match an existing cornice, see our guide to specifying a custom or matched moulding.

Ref [C-01]: cornice, [stepped, run in place] plaster, section as detail [D-21] at 1:1, drop [100] mm, projection [110] mm. Colour and finish [ ]. Length joints, if any, as shown on [drawing]. Mock-up of one corner and one run in [room] to be approved before production. Classification evidence [required / not required] as confirmed by [fire engineer / building control].

Ref [C-02]: crown moulding, [timber / MDF], drop [100] mm, projection [100] mm, spring angle [45] degrees measured from the ceiling. Corners mitred, settings confirmed on site for each corner. Longest single length [ ] mm. Finish [ ].

Standards and sources

• Approved Document B Volumes 1 and 2 (England), 2019 edition with amendments to 2026: Vol 2 para 6.6; Vol 1 para 4.6. Read Nov 2026.
• BS EN 13815:2006 and BS EN 15319:2007, fibrous plaster: Both Current, Under Review (BSI, 7 Nov 2026). Not read.

Questions we are asked

What is the difference between cornice and coving?

Coving is a plain concave strip; a cornice is a projecting moulding that can have several elements. The words overlap in the trade, so quote the drop, the projection and a 1:1 section rather than the name alone.

What size cornice suits a 2.4 m ceiling?

We found no standard that sets it. Test a full-size mock-up in the room. If you like a cornice at one ceiling height, the table in this sheet shows two ways to carry it to another.

Is plaster or timber better for a cornice?

It depends on the profile and the building. Plaster gives long crisp runs and period ornament but needs a plasterer and is heavy when cast; timber is stainable and repairable but moves at joints and comes in limited lengths. Ask the partner plasterer or joiner about lengths, weight and fixing for the actual profile.

How do I mitre a crown moulding in a corner?

A plain moulding takes half the corner angle. A sprung crown laid flat needs a compound cut: for a 45 degree spring in a square corner, 35.3 degrees miter and 30 degrees bevel. Real corners are rarely square, so cut a test corner.

Does a cornice need a fire class?

In England, Approved Document B does not name cornices in its list of narrow members that a ceiling excludes (Volume 2 paragraph 6.6), so it is a question for the fire engineer or building control. The ceiling beside it is a lining either way.