How to Fit Coving & Cornice

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

Plain gypsum plaster cove costs far less than fibrous plaster: Elliotts lists Gyproc’s 127 mm cove at £12.88 for a 3.6 m length, about £3.60 per metre.¹

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 recommended by the coving maker (for example Gyproc Covafix for Gyproc plaster cove)
• 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, checking the length of the product you choose, since it varies by material and maker (Gyproc’s 127 mm plaster cove, for example, comes in 3.6 m lengths).¹ 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 the exact drop on the wall and projection across the ceiling for that profile, so mark both. 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:

• Cut each piece at 45° so that its bottom (wall) edge is longer than its top (ceiling) edge: the wall edges run right into the corner, while the ceiling edges stop short of it

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):

• Cut each piece at 45° so that its top (ceiling) edge is longer than its bottom (wall) edge: the ceiling edges run past the corner of the wall to meet each other

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. Paint in an older house may contain lead: HSE notes that lead pigments were widely used in household paints until the 1960s, so do not dry-sand old paint.² Coving adhesive will not key adequately to heavily textured surfaces, so an old Artex or textured ceiling needs the texture removed or skimmed over in the contact zone. Do not sand or grind an old textured coating: HSE lists textured decorative coatings, sold under trade names such as Artex, among materials that can contain asbestos, so have it tested before disturbing it.³ HSE treats removing textured coating from a small area, up to around 1 m², with steam or gel as lower-risk non-licensed work, which still needs the right precautions.³

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 (check the adhesive maker’s instructions for timing). 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 finished with rigid filler at the wall and ceiling lines is prone to cracking as the building moves. Flexible sealant accommodates this movement invisibly.
• Painting over bare polystyrene with oil-based paint. Solvent in oil-based paints dissolves expanded polystyrene.⁴

Costs Summary

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.