Architrave profiles and door casings

At a glance

Sections 70 mm wide and about 20 mm thick; the names fix the shape, not the size. The right-hand column is our commentary, not a standard.

The profile family

An architrave is the moulding round a door or window opening. In modern construction it hides the joint between the wall or ceiling finish and the timber casings that form the opening; in the United States the same member is called casing. The word also has a classical sense, the lowest part of an entablature.

The eight sections below are drawn in plan at the door jamb, with the wall hatched below each one. Each is our own construction from straight lines and circular arcs. The names follow our moulding profiles sheet, and its warning applies: a name never fixes a size, and “ogee” can mean either S-curve. Specify a section by a drawing.

At the jamb: lining, reveal and what the wall does to it

The architrave is fixed to the face of the wall and to the timber lining that forms the opening, and covers the joint between them. It is usually set back a little from the edge of the lining, so that a narrow strip of lining shows beside it. That set-back is the reveal. We found no source for how much it should be: the 8 mm drawn is an illustration.

An architrave is fixed to surfaces it does not control, so it inherits their errors. The LABC Warranty Technical Manual (Version 11, Section 20) treats reveals as acceptable if they are plumb to within ±3 mm for openings up to 1.5 m high (±5 mm above that), and door frames if they are plumb to within ±5 mm over their total height. Those are warranty acceptance levels for new homes, not design tolerances. They still show the size of the problem: a lining 5 mm out of plumb, with an architrave fixed true, turns an 8 mm reveal at the foot into anything from 3 to 13 mm at the head. Decide which line the architrave follows, the lining or a plumb line, because it cannot follow both; the survey and scribing rules are on our joinery tolerances sheet.

Head and foot: mitres, blocks or nothing

A casing has to turn two corners. At the head, the legs and the head either meet at a mitre or are cut square into corner blocks, sometimes called rosettes. At the foot, the leg either stops on the floor with the skirting butting to it, or stands on a plinth block, which the dictionary defines as a plinth interrupting a door or window architrave at the floor. Neither is a rule: all three details in the drawing work if they are drawn deliberately.

The geometry decides some of it. For skirting to butt to a casing, the casing has to stand proud of the skirting, so the casing is the thicker of the two; for a block to take both, it is thicker than both. Trade guidance seen in search summaries also makes plinth blocks taller than the skirting and a little wider than the casing, which we treat as a convention to confirm. Show the foot in elevation on the drawing, and say whether the floor finish is laid before or after the casing, because it changes where the leg stops.

Sizes: no standard, and nominal is not finished

We found no British Standard or regulation that sets architrave width, thickness or projection. BS 1186-3, the standard for wood trim, was not read. The rules of thumb in circulation for skirting and architrave disagree with one another; they are set out side by side on our skirting and architrave sizes sheet. The 70 x 20 mm used in the drawings here is an illustration, not a recommendation.

What does catch specifiers out is the difference between the size a piece is sold as and the size it ends up. A UK timber merchant’s listing shows it:

That is one listing; other sections lose different amounts, and the amount depends on the supplier and the species. But it shows the trap. If a schedule asks for a [70 x 20] mm architrave and the workshop starts from stock sold by its original size, the finished piece comes out smaller. A workshop making the section to a drawing would have to start from about [75 x 24] mm if it lost the same 5 mm and 4 mm. Write “finished size” in the schedule and let the workshop work backwards.

Fire classification and fire doors

In England, Approved Document B says what does and does not count as a wall or ceiling lining for reaction to fire. For walls it leaves out doors and door frames, and architraves, cover moulds, picture rails, skirtings and similar narrow members (Volume 1 paragraph 4.3, Volume 2 paragraph 6.3). For ceilings it leaves out architraves, cover moulds, picture rails, exposed beams and similar narrow members (Volume 1 paragraph 4.6, Volume 2 paragraph 6.6). The edition is the 2019 one incorporating the 2020, 2022, 2025 and 2026 amendments, as published on GOV.UK in November 2026.

That is not a general exemption. The wall behind the architrave is still a lining and has to meet the classification for its room (Table 4.1 in Volume 1, Table 6.1 in Volume 2). Scotland, Wales and Northern Ireland have their own guidance, which this sheet does not cover.

A fire doorset is a different matter. It is tested and certified as a set with its frame and seals, and its installation instructions say how it is to be fitted. Do not plane, rebate or cut a fire door leaf or its lining to suit an architrave, and fix the architrave only in the way the door’s certificate allows. A fire door specialist and the project fire strategy decide, not this sheet.

Specifying a casing so it is made and fitted as drawn

• Draw the section at 1:1. Width, thickness, radii and chamfers dimensioned, wall side and opening side marked.
• Say finished size. Give the finished width and thickness, and let the workshop allow for planing.
• Fix the reveal. State the set-back from the edge of the lining, and which line the casing follows when the lining is out of plumb.
• Choose the head and the foot. Mitre, corner blocks or neither; block sizes if used; how the skirting meets the leg or block. Draw the foot in elevation.
• Fire doors. Name them on the door schedule and say that the leaf and lining are not planed or cut to suit the casing.
• Mock up one door. Fix one casing with its skirting and floor finish on site before the run is made, and check the reveal along the whole leg.
• Matching an existing casing. Take a template on site with a profile gauge, and have the workshop redraw it.

We coordinate the casing drawing and any mock-up with the partner workshop that will make the trim, before the schedule is fixed. For a flush casing, read shadow gaps, flush skirtings and minimal trim, and for where horizontal rails stop at the casing, dado, chair, picture and plate rails.

Ref [A-01]: architrave, profile as detail [D-13] at 1:1, [70 x 20] mm finished (not nominal), [pencil round, 6 mm radius on both arrises]. Material [ ]. Fixed to the wall and the lining, set back [8] mm from the edge of the lining (reveal), following [the lining / a plumb line]. Head [mitred / with corner blocks]. Finish [ ]. Drawn finished sizes govern; no width, reveal or block size is to be taken from a catalogue or a proportion rule.

Corner blocks [90 x 90] mm and plinth blocks [90 x 200] mm, [25] mm thick, thicker than the architrave and the skirting. Skirting [150] mm high butts to the plinth block. Block and architrave sizes as drawing [number].

No architrave. Lining set flush with the finished plaster face. Shadow gap [10] mm wide by [12] mm deep, formed by [a bead / a rebate], continuous round the opening. Wall within [3] mm of flat on a 2 m straight edge and lining within [3] mm of plumb.

[Fire doorsets are fitted to the certified installation instructions. Architraves are fixed as the door certificate allows. The leaf and the lining are not to be planed, rebated or cut to suit the architrave.]

Standards and sources

• Approved Document B Volumes 1 and 2 (England), 2019 edition with amendments to 2026: Vol 1 paras 4.3, 4.6; Vol 2 paras 6.3, 6.6. Read 5 Nov 2026.
• LABC Warranty Technical Manual Version 11, Section 20 Tolerances (2022/2023): 20.1.3, 20.1.6. Read 5 Nov 2026.
• BS 1186-3, wood trim (named only, not read; no size claimed). Definitions: Designing Buildings wiki, Architrave; Dictionary.com, plinth block

Questions we are asked

What is the difference between an architrave and a casing?

They are the same member. Architrave is the UK word for the moulding round a door or window opening, which hides the joint between the wall finish and the timber lining. Casing is the usual US word. In classical architecture the architrave is the lowest part of the entablature.

How wide should an architrave be?

No standard we found sets it, and the rules of thumb in circulation disagree. Choose a width for the door and the room, put it on a 1:1 drawing, and mock one door up before the run is made. Our skirting and architrave sizes sheet sets the rules side by side.

What is a plinth block?

A block at the foot of an architrave that takes the skirting, so that the two do not have to meet at a mitre. The dictionary defines it as a plinth interrupting a door or window architrave at the floor. Corner blocks do the same job at the head.

What is the difference between nominal and finished architrave size?

The nominal size is the size a piece is sold as, and the finished size is what is left after planing. One UK merchant lists a 19 x 50 mm rounded architrave with a finish size of about 15 x 45 mm, so planing took 4 mm off the thickness and 5 mm off the width. Write “finished” in the schedule.

Do architraves need a fire rating in England?

Approved Document B leaves architraves, cover moulds, picture rails, skirtings and similar narrow members out of what counts as a wall lining (Volume 1 paragraph 4.3, Volume 2 paragraph 6.3; 2019 edition with amendments to 2026). The wall behind is still a lining. Fire doorsets are a separate matter: do not cut a fire door or its lining to fit an architrave.

Can a door have no architrave?

Yes. The lining is set flush with the plaster and a shadow gap runs round the opening. It shows every error in the wall and the lining, so it needs a flat wall and a plumb lining, and a mock-up. See our shadow gaps sheet.