Coving Installation Guide: Plaster, Polystyrene and Duropolymer — Cutting, Mitring and Fixing

Quick Answer: UK coving comes in three main materials: plaster (heritage / period; sand & cement, gypsum, or lightweight plaster; £8–25 per 2m length), polystyrene (cheap, lightweight, soft profile; £3–8 per length) and duropolymer / hard polyurethane (modern, paintable, crisp profile; £10–30 per length). Standard coving widths are 100mm, 127mm and 165mm. Installation: surfaces clean and dry, coving cut with mitre saw at 45° for internal/external corners, fixed with coving adhesive and (for plaster/heavy duropolymer) supporting screws or nails until set. Plaster coving is the traditional UK finish and the longest-lasting (60+ years); duropolymer is the contemporary equivalent for new build and refurbishment.

Summary

Coving (also called cornice in the trade, though strictly "cornice" implies a more decorative moulded profile) defines the wall-ceiling junction with a soft transition. Without it the junction is a hard pencil line — fine in some modern interiors, but UK period and traditional properties expect coving. The choice between plaster, polystyrene and modern composite materials determines durability, aesthetics, installation cost and reparability.

The biggest installation errors are uneven mitres at corners, gaps at the wall-ceiling line, and the wrong adhesive for the material. Polystyrene coving is fitted with foam adhesive and silicone-style sealant gaps; plaster coving uses proprietary plaster-based coving adhesive and often needs mechanical support during set; duropolymer uses high-grab solvent or hybrid polymer adhesive.

This article covers the installation sequence for each material, the cutting and mitring detail, and where each is appropriate. For period-property restoration of decorative cornice, see period cornice restoration. For repair of artex ceilings before coving installation, see artex removal skim coating.

Key Facts

• Standard coving widths — 100mm, 127mm, 165mm (measured along the diagonal face)
• Plaster coving lengths — 2m or 3m typical; longer reduces joint count
• Polystyrene lengths — 2m typical
• Duropolymer lengths — 2m or 2.4m; some 4m lengths for premium ranges
• Plaster coving weight — heavy: 2m × 100mm length approximately 5–7 kg
• Polystyrene weight — very light: 2m × 100mm length approximately 200–400g
• Duropolymer weight — medium: 2m × 100mm length approximately 1–2 kg
• Mitre angle — 45° for internal corners; 45° opposite for external corners
• Standard adhesive — proprietary plaster-based for plaster coving; foam or polymer for polystyrene; high-grab for duropolymer
• Set time — plaster coving 12–24 hours initial set, 48 hours full; foam adhesive 30 minutes; polymer 1–2 hours
• Fixing in addition to adhesive — plaster coving sometimes supported with galvanised nails or screws through the coving into the wall during set
• Paintable — all three materials are paintable; matt emulsion typical
• Lifetime — plaster 60+ years; duropolymer 30–50 years; polystyrene 15–25 years (more vulnerable to damage and yellowing)
• Fire performance — Plaster coving has good fire resistance; polystyrene is flammable (some grades treated); duropolymer falls between
• Listed Building considerations — replacement of historic coving usually requires matching profile; consult Conservation Officer

Quick Reference Table

Detailed Guidance

Preparation

Before fitting coving:

1. Inspect the wall and ceiling for damp, looseness, or unsound plaster
2. Strip wallpaper and remove any flaking paint from the area where coving will sit
3. Score the wall and ceiling with a knife along the line where the coving back will sit — this helps the adhesive grip and breaks any paint
4. Mark a level line on the wall — coving doesn't follow the ceiling; it follows a horizontal line. For sloping ceilings, draw a guideline level all the way around the room
5. Dust off; do not paint surfaces until coving is fitted

Plaster coving needs dust-free, slightly damp surfaces for plaster-adhesive bond. Polystyrene and duropolymer accept dry surfaces.

Setting out

For a typical 4-wall room:

1. Measure each wall length
2. Plan cuts to minimise joints; use full lengths where possible
3. Identify corner positions
4. Position the longest run first (most visible from doorway)

Cutting and mitring

Coving is fitted at a 45° angle in the corner of wall and ceiling. The mitre cut must reflect this geometry.

For 45° mitre with a mitre block or mitre saw:

• The coving sits in the mitre box on its back (the side that touches the wall and ceiling) flat against the floor of the box
• The 45° cut runs from the upper inner edge to the lower outer edge for an internal corner
• The cut runs the opposite direction for an external corner

Internal corner (most common):

• Left-hand piece: 45° cut, with the long edge along the wall side
• Right-hand piece: 45° cut, mirror image
• When fitted, the two pieces meet at the corner forming a tight joint

External corner:

• Left-hand piece: 45° cut, opposite direction
• Right-hand piece: 45° cut, mirror image
• When fitted, the pieces wrap around the corner with the joint at the corner peak

For complex / non-90° corners:

• Measure the actual angle (most rooms are not exactly 90°)
• Cut each piece at half that angle
• Test fit before adhering

Installing plaster coving

1. Apply adhesive to the back of the coving (wall side and ceiling side) — proprietary plaster-based coving adhesive
2. Press coving into position firmly, holding for 30–60 seconds
3. Wedge with timber offcuts or scrap if needed to hold position during set
4. Optional mechanical support — galvanised nails or screws driven through the coving into the wall studs / ceiling joists, set just below surface; filled and skimmed flush after set
5. Smooth excess adhesive at wall and ceiling junctions with a small spatula and wet finger
6. Allow 24 hours before sanding, filling joints, painting

For internal corner joints, butter both faces with adhesive; press together at the corner; smooth excess.

For external corners, the same; if there are small gaps, fill with the adhesive paste once dry.

Installing polystyrene coving

1. Apply foam adhesive in a thin bead to the back of the coving — wall and ceiling sides
2. Press into position; hold for 30 seconds
3. No mechanical fixings needed (the coving is too soft to take nails / screws)
4. At corners, apply a small amount of foam to the mitre faces; press together
5. Excess foam at the surface — wipe with a damp cloth; do not let it cure on the coving face

Polystyrene coving is the quickest to install but the most easily damaged later. Soft to the touch; cleans easily but breaks on impact (knee, ladder, chair).

Installing duropolymer coving

Duropolymer is denser than polystyrene but lighter than plaster. It paints well, takes crisp profiles, and is the modern default for paint-finish coving:

1. Apply hybrid polymer adhesive (or proprietary brand-specific adhesive) to the back of the coving
2. Press into position, holding for 1–2 minutes
3. Mechanical support rarely needed — the coving is light enough to stay in place
4. At corners, butter the mitre face; press together
5. Smooth excess adhesive with a damp cloth

Quality duropolymer brands (Orac, NMC, Decosa) produce crisp profiles that hold their detail through painting and re-painting cycles.

Joints, gaps and finishing

Even with careful fitting, joints at the corners and between lengths will need filling:

1. For plaster coving — Filler or proprietary plaster repair compound; sand flush when dry
2. For polystyrene — Decorator's caulk; wipe smooth with a damp finger
3. For duropolymer — Decorator's caulk or proprietary brand filler

Gaps at the wall and ceiling lines:

1. For all coving types — Decorator's caulk; gun-applied; wipe smooth with a damp cloth or finger
2. Allow to dry; sand if needed; paint

Painting

All coving materials are paintable:

1. Prime if recommended by manufacturer (some duropolymer needs an undercoat)
2. Two coats matt or satin emulsion — typically the same colour as the ceiling for a unified look, OR a contrasting colour for period feature effect
3. Brush in the gripper / cove direction; finish with light strokes

In traditional UK interiors, coving is painted the same colour as the ceiling (white) so it visually extends the ceiling and softens the wall-ceiling junction. In some contemporary or period schemes, coving is painted to contrast.

Frequently Asked Questions

What's the difference between coving and cornice?

In modern UK use, "coving" usually refers to a plain curved profile at the wall-ceiling junction; "cornice" refers to a more decorative moulded profile, often associated with period properties. Functionally they're the same junction treatment.

Can I fit coving over existing wallpaper?

No — strip the wallpaper back from the area where coving will sit. Adhesive bonds poorly through paper.

What's the largest gap a coving can hide?

A typical 100mm coving covers about 50mm of wall and 50mm of ceiling diagonally. If the wall-ceiling junction has a step or large gap (>15mm), coving will not cover it cleanly — fill with plaster first or use a wider coving.

Can I paint coving before installing?

Better to install first then paint — joints and gaps need filling and would show under pre-painted coving. Paint as the final step.

How long does coving adhesive take to set?

Plaster coving adhesive: 12–24 hours initial set, 48 hours full strength. Foam adhesive (polystyrene): 30 minutes. Polymer adhesive (duropolymer): 1–2 hours.

Can I fit coving on an artex ceiling?

Yes, but the artex must be sound. If it's flaking or loose, remove or skim over first. If the artex contains asbestos (pre-2000 installations), follow the asbestos safety procedure — see artex removal skim coating for asbestos guidance.

How do I match coving in a period property?

Take a photograph or a cast of the existing profile and contact a heritage plasterwork specialist. Standard merchant profiles may not match period decorative profiles — bespoke moulding by a specialist is often needed for Listed Buildings. See period cornice restoration.

Regulations & Standards

• BS EN 13658 — Metal lath and beads for use with rendering or plastering (relevant for plaster coving on metal substrates)

• BS EN 13279 — Gypsum binders and gypsum plasters (for gypsum coving products)

• BS 5492 — Code of practice for internal plastering (historical reference)

• BS EN 13501-1 — Fire classification of construction products (relevant for stairwells, escape routes)

• Approved Document B (fire safety) — Class 1 spread of flame for stairwells; check material fire rating

• Approved Document C (resistance to moisture) — for damp-prone areas

• Listed Building Consent — replacement of historic cornice typically requires LBC

• BSI — BS EN 13279 series — gypsum plaster standards

• Painting and Decorating Association — UK trade body

• Orac Decor — Installation guides — duropolymer manufacturer guidance

• British Gypsum — Plaster coving — gypsum-based coving reference

• SPAB — Society for the Protection of Ancient Buildings — heritage plasterwork guidance

• period cornice restoration — restoration of decorative cornice in period properties

• artex removal skim coating — preparing artex ceilings before coving

• skim coat — ceiling preparation under coving

• hanging wallpaper guide — wallpaper interaction with coving line

• colour schemes for tradespeople — colour selection for coving