Shadow gaps, flush skirtings and minimal trim

At a glance

These are the limits a new-home warranty scheme treats as acceptable, not design tolerances and not a standard. Our notes and arithmetic are in the third column. A designer can require tighter figures, and later editions of the manual may differ.

Three ways to meet the floor

A conventional skirting stands proud of the plaster, so the board hides both the line where the wall meets the floor and the unevenness of the wall in its thickness. A flush skirting has its face level with the plaster, and a shadow-gap skirting sits in a recess with a gap beside it. In both the second and the third, the junction between two surfaces is made to read as a clean line, and any difference between the wall and the trim shows up in that line.

• A. Conventional. The skirting is the joint cover. The wall can vary behind it; the board is scribed to the floor.
• B. Flush. The face of the skirting is level with the plaster, with a joint at its head. A wall that is out of flat shows as a step or a changing joint, so it needs a plaster stop and a flat wall.
• C. Shadow gap. The skirting is recessed, with a gap of a set width and depth above it. A wall that is out of flat shows as a gap that changes width.

We found no standard that fixes these details. The sizes drawn are illustrations, and the build-up (a plaster stop bead above a recessed skirting) is a generic construction, not a product.

Where else the same detail turns up

• At the ceiling. A wall with no cornice, and a reveal formed by a bead where it meets the ceiling. The manual expects cracking of up to 2 mm at wall, floor and ceiling junctions, which a flush junction shows and a designed reveal can take.
• At a door lining. No architrave, the lining flush with the plaster and a gap beside it. The details are on our architrave profiles and door casings sheet.
• Between panels. Our own journal says of flat and shadow-gap panelling that the whole detail lives in the gap, which leaves nowhere to hide and demands accurate setting out. See wall panelling set-out.
• Under fitted joinery. A plinth set back so that a unit seems to float, with the recess reading as a shadow. The floor still has to be scribed to.

What an uneven wall does to a gap

The manual treats a wall as acceptable if it is flat to within 3 mm on a 2 m straight edge. Put a straight trim 10 mm from a wall that is out by that much and the gap is not 10 mm: it measures 7 mm where the wall bulges toward the trim and 13 mm where it falls away, in a run of 2 m. The same wall behind a conventional skirting would never be noticed. The drawing exaggerates the scale across the gap about 55 times so that you can see it.

The table works the same sum for other gaps and other walls: the range the gap measures, and the change as a share of the designed gap. It is our arithmetic, assuming a straight trim against a wall that varies by that amount over the run. A narrow gap suffers most: a 5 mm gap against a wall out by 3 mm measures between 2 and 8 mm.

We found no standard for how much variation in a gap is acceptable. The only quantified reveal tolerance we read is in the US Architectural Woodwork Standards (2009, Section 10, 1.5.6.1.2): doors and drawer fronts that leave a 6.4 mm reveal with a countertop edge may vary by 3.2 mm either way, half the reveal. It is a US cabinet clause, not a trim standard, but it shows how large a share of a gap a standards writer will allow as variation.

The floor is the second source. The manual allows a floor to be out of level by 3 mm in each metre up to 6 m. A skirting held level over a floor that falls at that rate gains 9 mm of gap in 3 m. That is why a shadow-gap skirting is scribed to the floor, or the floor is levelled first: ask the flooring contractor for the levels before the skirting is ordered. The survey method is on our joinery tolerances and scribing sheet, and the general survey on site survey and dimension verification.

What goes wrong, and what to write

Most failures come from three places: a substrate that is less flat than the drawing assumed, a floor that is not level, and nobody owning the last finish. The table lists them with the clause that prevents each. It is general practice, not a standard.

Who owns the interface, and in what order

• Survey when the walls and floor are finished, and measure flatness, plumb, corners and floor level, not one reading per room.
• Write the substrate tolerance once, in the plastering or dry-lining scope and in the joinery scope, with the same figure. The plasterer forms the walls and the beads; the joiner fixes the trim; the decorator leaves the gap open.
• Say whether the floor finish goes in before or after the skirting, because it sets where the gap at the foot starts.
• Mock up one wall, with the real wall, floor, light and paint, and approve it before the run is made. The approved mock-up is the standard for the gap. Our sample hierarchy sheet says where a mock-up sits.
• Agree how it is accepted. The wording for what counts as a defect is on snagging and acceptance criteria.

This is typical practice with no primary source, and contracts differ: write the responsibilities into yours. We coordinate the details and the mock-up with the partner workshop that will make the trim, before the schedule is fixed.

Fire classification and gaps in England

In England, Approved Document B leaves skirtings, architraves, cover moulds, picture rails and similar narrow members out of what counts as a wall lining, and architraves and similar narrow members out of what counts as a ceiling (Volume 1 paragraphs 4.3 and 4.6, Volume 2 paragraphs 6.3 and 6.6; 2019 edition incorporating the 2020, 2022, 2025 and 2026 amendments, as published on GOV.UK in November 2026). The wall behind a flush skirting or a shadow gap is still a lining and has to meet the classification for its room.

A gap is also an opening. Where it crosses a fire-resisting or acoustic partition, or the line of a cavity barrier, it is a detail for the fire engineer or the acoustic consultant, not for this sheet. We give no fire-stopping or acoustic figures. Scotland, Wales and Northern Ireland have their own guidance, which this sheet does not cover.

Ref [S-02]: skirting [18] mm thick x [100] mm high, [recessed] with a shadow gap [10] mm wide x [10] mm deep at its head, formed by [a plaster stop bead / a rebated profile], as detail [D-14] at 1:1. Material [ ]. Finish [ ]. Scribed to the finished floor, with the gap at the foot measured from the floor and not from a datum; floor finish [laid before / after] the skirting. Gap width consistent within [ ] mm along each wall, as agreed on the mock-up.

Walls and ceilings receiving shadow-gap or flush trim to be flat within [3] mm on a 2 m straight edge and within [3] mm on a 450 mm straight edge, and plumb within [3] mm up to 2.4 m high. Corners square within [ ] mm in 500 mm. Checked by [party] before joinery is fixed; work outside the limits corrected by [the main contractor] first.

A mock-up of [the skirting / the ceiling junction / a door lining] is to be built in [location] by [party] and approved by [designer] before production; it sets the standard for the gap. The gap is to be left open and painted [colour], not filled with paint, filler or sealant.

Standards and sources

• LABC Warranty Technical Manual Version 11, Section 20 Tolerances (2022/2023): 20.1.3 to 20.1.7. Read 5 Nov 2026.
• Architectural Woodwork Standards, 1st edition (2009), Section 10, 1.5.6.1.2 (US practice): BS 8212, 8000-5, 8000-8 and 1186-3 named only, not read.
• Approved Document B Vols 1 and 2 (England), 2019 edition with amendments to 2026: paras 4.3, 4.6, 6.3, 6.6. Read 5 Nov 2026.

Questions we are asked

How big should a shadow gap be?

We found no standard that sets it. Choose the width against what the wall and floor will vary by: a wall within 3 mm of flat on a 2 m straight edge turns a 10 mm gap into one that measures 7 to 13 mm, and a 5 mm gap into one that measures 2 to 8 mm. The wider the gap, the smaller the share that the error represents.

What is the difference between a flush skirting and a shadow-gap skirting?

A flush skirting has its face level with the plaster, with a joint at its head. A shadow-gap skirting is recessed, with a gap of a set width and depth above or below it. Both show any unevenness in the wall, so both need a flat substrate; the flush skirting shows it as a step, the shadow gap as a change of width.

What flatness does the wall need behind a shadow gap?

The LABC Warranty Technical Manual (Version 11) treats a wall as acceptable within 3 mm on a 2 m straight edge, and board joints within 3 mm on a 450 mm straight edge. That is a new-home warranty limit, not a design tolerance. For a shadow gap, write your own figure into the plastering or dry-lining scope, and confirm it by survey and a mock-up.

Why does my shadow gap look uneven?

Because the wall is not flat, the floor is not level, or the corners are not square, and the gap makes each visible. A straight trim against a wall out by 3 mm can show a gap that varies by 6 mm over 2 m, and a level skirting over a floor falling 3 mm in a metre gains 9 mm in 3 m.

Who is responsible for the flatness of the wall behind a shadow gap?

The contract decides. Usually the plasterer or dry-liner builds the wall to the limit written in the specification and the joiner scribes within it. Write the same figure into both scopes, and say what happens where the survey finds more.

Does a shadow gap affect fire or acoustic performance?

It can, where it crosses a fire-resisting or acoustic partition or the line of a cavity barrier. That junction is a detail for the fire engineer or the acoustic consultant. We give no fire-stopping or acoustic figures.