Cornice | Chalkline

TL;DR

Cornice hides the wall-to-ceiling junction and, when done right, costs almost nothing to do well. The wrong fixing method (construction adhesive instead of cornice cement), wrong substrate prep, or reactive soil movement are the top three causes of the gaps and cracks that come back at PCI. Square-set is the modern alternative: cleaner lines but more labour, higher crack risk on reactive soils, and it exposes every imperfection.

What it is

Cornice is the decorative or plain trim fitted at the internal junction between wall and ceiling. Its practical job is to conceal the plasterboard sheet edges and the inevitable small gap where ceiling and wall linings meet, and to give painters a clean termination line.

Also known as: cove cornice, coving, architrave cornice, moulding.

Category: Internal linings trim.

Types / variants

By profile

Profile	Description	Typical use
Cove (55mm)	Simple concave curve, 55mm face width	Standard residential, new builds, lower ceiling heights
Cove (75mm)	Same profile, wider	Higher ceilings, more visual weight
Cove (90mm)	Same profile, broadest standard width	Feature rooms, high ceilings
Square-set	No cornice. Wall and ceiling sheets meet at a crisp 90-degree internal angle, set and finished flush	Contemporary/modern residential
Shadowline	A small reveal or shadow gap at the junction, formed by a proprietary bead or rebated plasterboard edge	Contemporary, lower profile than square-set, forgiving on reactive soils
Decorative / ornamental	Stepped, OG, egg-and-dart, acanthus, dentil, and other period profiles	Heritage restoration, Federation, Victorian, Art Deco buildings

By material

Material	Characteristics	Best for
Gypsum (plaster)	Dense, heavy, paintable, matches plasterboard; may crack on reactive sites	Standard residential new build and renovation
Fibrous plaster	Reinforced with hessian/fibre, used for large ornamental runs	Heritage restoration, ornamental profiles
EPS (expanded polystyrene)	Lightweight, low cost, moisture-resistant; lower detail resolution	DIY, humid rooms (bathrooms, kitchens where plaster grip is an issue)
XPS (extruded polystyrene)	Denser than EPS, smoother finish, moisture-resistant	Better durability than EPS, still lightweight
Polyurethane	Rigid, dense foam, sharp detail reproduction	High-detail decorative profiles where weight is a concern

Note on EPS/XPS cornice and fire: Standard residential cove cornice (55-90mm) is internal trim, not a wall or ceiling lining. NCC 2022 Volume Two (Class 1 buildings) does not impose the same combustibility requirements on small internal trim elements as it does on wall claddings or structural linings. However, if a project requires a fire-resistance-level (FRL) tested wall or ceiling system, confirm that any EPS trim is outside the tested assembly and does not penetrate a fire-rated lining. For fire-rated systems, use gypsum-based cornice only (verified 2026-05-10 via ABCB NCC 2022 Volume Two Part H3D2).

Properties

Cove cornice dimensions (Gyprock as reference, verified 2026-05-10)

Size	Face width	Standard lengths
Cove 55	55mm per face (wall + ceiling)	3000mm, 3600mm, 4800mm, 5400mm
Cove 75	75mm per face	3000mm, 3600mm, 4800mm, 5400mm
Cove 90	90mm per face	3000mm, 3600mm, 4800mm, 5400mm

Source: Gyprock Cove Cornice product page (verified 2026-05-10).

Weight: gypsum cove cornice is heavier than EPS. Two-person handling is standard for long lengths; one person can manage cut pieces.

Where to use

Standard cove cornice at every wall-to-ceiling junction in timber or steel framed new construction
Square-set or shadowline on contemporary projects where a cornice-free line is specified
Decorative profiles in heritage or period restoration work (profile-match the existing run)
EPS/XPS cove in high-humidity rooms where plaster bond is marginal, or where weight is a constraint

Where NOT to use

Gypsum cornice directly to masonry or brick without correct surface prep; bond failure is common
EPS or XPS cornice in fire-rated tested wall or ceiling assemblies without confirming it’s outside the tested system
Square-set on reactive clay soils (Class M, H1, H2 sites) without accepting the crack risk; engineers in Melbourne and other reactive-soil cities advise against it on movement-prone foundations
Decorative plaster cornices across long unsupported runs without mechanical backup fixings; weight and adhesive alone will not hold over time

Fixing / installation

Method 1: Cornice cement (standard for gypsum cove on plasterboard)

This is the correct method for Gyprock and Knauf gypsum cove cornice to plasterboard substrates.

Cornice cement is a setting-type compound (not a drying-type), providing chemical-set strength. Gyprock Cornice Cement is available in three working-life variants: 45 minutes, 60 minutes, and 90 minutes. Choose based on the run length and your working pace (verified 2026-05-10 via Gyprock Cornice Cement product page).

Substrate prep:

Plasterboard surfaces: no prep needed beyond confirming sheets are fixed and dry.
Painted surfaces: lightly abrade with 120-grit sandpaper or score with a utility knife to improve bond.
Masonry or fibre-cement: apply bondcrete or similar bonding agent before cornice cement.

Mixing: Mix cornice cement with clean water to a creamy/butter-like consistency. Follow the manufacturer’s ratio (approx. 2 kg powder to 1.1 L water for Gyprock brand).

Application:

Mark the cornice setout on wall and ceiling: for 55mm cove, mark 55mm down from ceiling on the wall, and 55mm out from the wall on the ceiling.
Cut mitres at internal and external corners using a mitre box or a Gyprock Mitremasta. For internal corners, measure to the long point; for external, to the short point. Mark the back of each piece before cutting to avoid reversing the mitre.
Apply cornice cement generously along both contact flanges (wall face and ceiling face) of the cornice.
Press cornice firmly into position, aligning with the setout marks. Clean excess cement immediately with a broad knife and damp sponge.
Secure with temporary screws (e.g., 8G x 30mm countersunk screws) along the length to hold the cornice while cement sets. Do not overtighten: the screw head should sit just proud of the cornice face to be filled later.
Leave for at least 2 hours before removing temporary fixings.
Fill screw holes and any small gaps at mitres with fresh cornice cement. Sand lightly when dry, wipe clean.

Source: Bunnings cornice installation guide (verified 2026-05-10); Gyprock Cornice Cement product documentation (verified 2026-05-10).

Method 2: Adhesive (for EPS/XPS and polyurethane cornice)

Proprietary water-based adhesive (sometimes sold as cornice adhesive or construction adhesive compatible with foam) is used for lightweight EPS, XPS, and polyurethane cornices.

Apply adhesive around the back perimeter of the cornice piece.
Press into position on the wall and ceiling. Hold for a few minutes.
Secure with 2 to 3 screws into ceiling framing if lengths are long or if the ceiling provides a grip point.
Wipe excess adhesive immediately.
Coverage: approximately 6 metres per cartridge tube.

Do not use standard cornice cement on EPS or XPS: cornice cement contains sulphates that can react with some foam types and does not bond reliably to foam surfaces.

Do not use standard construction adhesive (stud adhesive) on gypsum cornice: it takes 24+ hours to cure, requires the cornice to be pinned throughout, and is difficult to clean.

Source: Allplasta fixing instructions (verified 2026-05-10); Localsearch: masonry adhesive vs cornice cement (verified 2026-05-10).

Method 3: Square-set (no cornice)

Square-set is not a cornice product: it is the elimination of cornice. The wall sheet is brought up to meet the ceiling sheet at a tight 90-degree internal angle with a maximum 10mm gap. The junction is set and finished with jointing compound to a Level 5 finish per AS/NZS 2589:2017. No physical moulding is installed.

Labour and cost: square-set is more labour-intensive than cove cornice due to the Level 5 finish requirement. Expect a price premium from the plasterer. If any tape, crack, or blemish shows through the paint, it is exposed with no moulding to hide it.

Risk note: do not tape wall and ceiling sheets together at a square-set junction. If the building moves (settlement, seasonal moisture), taped junctions crack and the crack is visible. Set the junction but leave it free to flex slightly.

Method 4: Fibrous plaster cornice (heritage/ornamental)

Large ornamental runs and heritage profiles are installed with cornice adhesive and mechanical backup fixings (screws into ceiling joists or battens, and propping from the floor until adhesive cures). The specific mortar/adhesive mix and fixing pattern depend on the profile weight. Heritage plastering work of this kind should be done by a specialist heritage plasterer.

Tolerances and acceptance

Numerical tolerances for cornice line are set by the HIA Guide to Materials and Workmanship, which is paywalled pending HIA membership.

Cornice line straightness: per HIA Guide. Maximum permitted misalignment from a true straight line is [HIA-149].
Cornice gap at wall/ceiling faces: per HIA Guide. Maximum allowable gap at the contact flanges is [HIA-150].

Visual acceptance (general):

Mitres should be tight: no visible open gap along the mating cut faces.
Cornice line should read straight from normal viewing distance with standard indoor lighting. Obvious dips or rises visible from across the room are a defect.
No visible screw holes after filling and painting.
No cornice cement ridges or smears on wall or ceiling face beyond the cornice.
Where cornice meets a door architrave or window reveal, the junction should be neat with no gaps.

Square-set acceptance:

Junction should read as a crisp, clean 90-degree angle with no visible tape ridging under paint.
Cracks at the junction after painting indicate either frame movement or taped joint; both need investigation.

Working with other trades

Plasterer: cornice is typically plasterer’s scope. Builder confirms the profile spec (size, material, heritage or standard), and that the framing centres allow adequate substrate behind the cornice flanges.
Painter: primer before topcoat. Gypsum cornice (and EPS) must be sealed with a sealer undercoat before any paint is applied, or the porous substrate will show differential sheen.
Chippy: if second-fix architrave and door frames are installed before cornice, the cornice butts to them. If cornice goes first, the architrave butts to the cornice. Agree the sequence with the plasterer at scheduling stage.
Sparky/plumber: no interface at cornice except for penetrations (e.g., split-system pipework through an external wall near the ceiling). Penetrations near the cornice zone need to be confirmed before the cornice run is laid out.

Health and safety

Manual handling: long gypsum cornice lengths are awkward and brittle. Carry horizontally with two people. Do not flex mid-length; gypsum cracks.
Dust: cutting gypsum cornice generates gypsum dust. Wear P2 mask. Cut in a ventilated area.
EPS/XPS cutting: a heated wire cutter or fine-tooth saw is used for EPS. Melting EPS releases styrene vapour. Use outside or in very well-ventilated areas. Do not use a grinder or abrasive disc on foam cornice.
Adhesive and cement: cornice cement is a skin and eye irritant. Standard PPE (gloves, safety glasses).
Ladder safety: most cornice work is done from a ladder or hop-up. Secure the ladder and do not overreach.

Suppliers

CSR Gyprock: standard residential cove cornice, 55mm / 75mm / 90mm. Cornice Cement range (45, 60, 90 min working life). Widely stocked at plasterboard merchants.
Knauf Australia: cove cornice range and jointing compounds.
USG Boral: now part of Knauf.
Siniat (Etex): cornice and plasterboard range.
Bailey Interiors: largest manufacturer of decorative and ornamental plaster cornice in Australia. Federation, Victorian, Art Deco, Colonial, Georgian profiles. (baileysplaster.com.au)
Allplasta: ornamental cornice and heritage profiles. (allplasta.com.au)
Cornice Pro (Gold Coast): XPS cornice profiles.

Trade supply: building merchants, Bunnings Trade, plasterboard wholesalers. Decorative/heritage: direct from specialist suppliers.

[Sponsor / preferred installer slot. ACCC disclosure required.]

What can go wrong

Gaps opening at wall or ceiling face: substrate not flat, wet framing, or building movement. Most common in the first summer after handover on reactive clay sites.
Cornice falling: insufficient adhesive coverage, wrong adhesive type for the material (e.g., construction adhesive on EPS, or cornice cement on XPS that rejects gypsum-based compounds), or painted surface not abraded before re-fix. Old plaster buildings: lime substrate too friable.
Cracking at mitres: building movement, or the mitre cut was not tight enough before cement cured.
Cornice cement ridges showing through paint: excess cement not cleaned immediately. Must be cleaned wet; dry cement is difficult to remove without damaging the adjacent lining.
Square-set cracks: frame movement transmitted to the set junction. More common on reactive clay sites (Class M, H1, H2). Seasonal; often returns each dry season.
Paint refusing to stick: EPS or gypsum cornice not primed with sealer undercoat before topcoat. The porous surface soaks topcoat and the result is flat-patchy. Always prime first.
Mitre cut reversed: easy to do when the piece has been rotated. Mark the back of the cornice (wall face and ceiling face) before cutting so the direction of cut is unambiguous.
Heritage profile not matching: run samples against the existing profile before ordering the whole cornice. Profile matching needs a physical mock-up; relying on a photo is not accurate enough.

References

AS/NZS 2589:2017 (incorporating Amendment Nos 1 and 2), Gypsum linings: Application and finishing (Standards Australia) (verified 2026-05-10 via Standards Australia store)
NCC 2022 Volume Two, Class 1 and 10 buildings, Part H3: Fire safety, ABCB (verified 2026-05-10)
HIA Guide to Materials and Workmanship (national), Housing Industry Association (tolerance values pending HIA membership: HIA-149, HIA-150)
CSR Gyprock: Cove Cornice product page and Cornice Cement product page, gyprock.com.au (verified 2026-05-10)
Bailey Interiors: Australian architectural cornice style periods, baileysplaster.com.au (verified 2026-05-10)
Arbiter Group (structural engineers): cornice cracking causes, arbitergroup.com.au (verified 2026-05-10)