Coving — the decorative moulding that bridges the junction between wall and ceiling — is one of the most effective ways to add a period character or a clean, finished look to a room, and it is well within the reach of a methodical DIYer. The maths is simple, the materials are forgiving (polystyrene in particular), and the tools required are minimal. The one skill that makes or breaks the job is cutting accurate mitres at corners.
This guide covers the full process from measuring through to painting, whether you are fitting inexpensive polystyrene coving, mid-range polyurethane cornice or traditional plaster.
Choosing Your Coving
| Type | Cost (per metre) | Lifespan | Best For |
|---|---|---|---|
| Polystyrene | £1–£3 | 20–30 years | Budget DIY; light, easy to cut; not suitable for steam rooms |
| Polyurethane / resin | £5–£20 | 30–50+ years | Period-look at moderate cost; sharper detail than polystyrene |
| Plaster (fibrous) | £15–£40 | Indefinite | Authentic period restoration; professional installation recommended |
| MDF / timber | £8–£25 | 20+ years | Contemporary profiles; paint-grade only |
For a typical Victorian or Edwardian terrace, a 127 mm or 150 mm cove-profile polyurethane cornice at £8–£15/m gives excellent results. Match the profile to the scale of the room: a 90 mm cove in a double-height Victorian hallway looks mean; a 200 mm egg-and-dart cornice in a small box room looks overwhelming.
Tools and Materials
- Mitre box or electric mitre saw (for polyurethane/MDF; polystyrene can use a dedicated plastic mitre box)
- Pencil and tape measure
- Coving adhesive (Gyproc Covafix or equivalent for plasterboard; PVA-based for solid plaster)
- 50 mm masonry nails or countersunk screws (for timber/MDF in solid walls)
- Filler (fine surface filler for joints; flexible sealant for the wall/ceiling interfaces)
- Paintbrush and primer
- Damp sponge and bucket
Measuring and Calculating Lengths
Measure the perimeter of the room and add 10–15% for waste from mitre cuts. Order in full lengths — typically 2 m (polystyrene) or 2.4 m (polyurethane/plaster). In a 4 m × 3.5 m room, the perimeter is 15 m; order 17 m to account for waste.
Mark the position of the coving on both the wall and the ceiling using a pencil line. Most coving instructions give a specific dimension — typically 90 mm up the wall and 90 mm across the ceiling for a 127 mm cove. Use a chalk line or long spirit level to keep this line straight.
Cutting Internal Corners
This is where most first-time fitters go wrong. The rule for a 90° internal corner:
- The left-hand piece is cut with the left end angled 45° to the right (so the face is longer than the back)
- The right-hand piece is cut with the right end angled 45° to the left
In practice: place the coving in the mitre box upside down (ceiling face down on the base of the mitre box), and cut in the correct direction. Most mitre boxes designed for coving are marked LEFT and RIGHT for internal and external corners — follow those markings first before trying to work it out geometrically.
If using an electric mitre saw, set the angle to 45° and position the coving at the same angle as it sits on the wall/ceiling. Mark each piece clearly before cutting.
Cutting External Corners
For external corners (bays, chimney-breast returns, projecting columns):
- The left-hand piece is cut with the left end angled 45° to the left (face is shorter than the back)
- The right-hand piece is cut with the right end angled 45° to the right
Test-fit every mitre dry before applying adhesive. Adjust with sandpaper or a sharp knife — polyurethane trims cleanly. A 1–2 mm gap at an external corner can be filled once the adhesive has set, but a large gap at an internal corner is very obvious and difficult to hide.
Fitting: Step-by-Step
1. Prepare the surfaces. Remove loose paint, grease or dust from the wall/ceiling junction. Old Artex or textured ceilings need to be ground back or skimmed in the contact zone — coving adhesive will not key adequately to heavily textured surfaces.
2. Mix and apply adhesive. Apply adhesive to both the wall and ceiling contact faces in a continuous bead. For polystyrene, use a toothed spreader to apply a consistent layer. For polyurethane, a 6 mm bead of coving adhesive is sufficient.
3. Press and slide. Press the piece firmly into position, sliding it slightly sideways to spread the adhesive. On long straight runs, use masonry nails or temporary 25 mm pins as props until the adhesive grabs (typically 15–20 minutes). Wipe off excess adhesive with a damp sponge before it skins.
4. Fill and finish joints. Apply fine-surface filler to all mitre joints and any small gaps. When dry, sand smooth with 120-grit paper. Fill the top and bottom edges where coving meets ceiling and wall with a thin bead of flexible decorator’s caulk — this accommodates seasonal movement and prevents cracking.
5. Prime and paint. Polyurethane and polystyrene coving must be primed before top-coating — use a water-based primer to avoid the solvent in oil-based paints attacking the foam. Apply two coats of emulsion in the room’s ceiling colour. Plaster cornice can be finished in distemper (traditional) or modern emulsion.
Common Mistakes
- Not marking the position lines first. Without pencil guide lines, it is almost impossible to keep a long run straight as you go round the room.
- Cutting the wrong way. Always mark each piece LEFT or RIGHT before cutting, and check the cut on a scrap piece first.
- Applying too much adhesive. Excess adhesive squeezes out, takes longer to set and is harder to clean up. A steady bead on each face is sufficient.
- Skipping flexible sealant at the interfaces. Coving fixed with rigid filler at the wall and ceiling lines will almost always crack within a year due to building movement. Flexible sealant accommodates this movement invisibly.
- Painting over bare polystyrene with oil-based paint. Solvent in oil-based paints dissolves expanded polystyrene.
Costs Summary
| Scenario | Materials | Day Rate / Labour | Total |
|---|---|---|---|
| Average room (15 m, polystyrene) | £25–£50 | DIY: 0 | £25–£50 |
| Average room (15 m, polyurethane) | £120–£300 | DIY: 0 | £120–£300 |
| Average room (15 m, polyurethane, tradesperson) | £120–£300 | £180–£280/day | £300–£580 |
| Period property (fibrous plaster) | £250–£600 | £350–£500/day | £600–£1,100+ |
A competent DIYer can fit polystyrene or polyurethane coving in an average living room in four to six hours, making this one of the most cost-effective decorative improvements available.
