Cornice Sizes, Lengths and How Much to Order

The standard cornice size in Australia is 55mm cove, sold by its face measurement across the cove. The common stocked sizes are 55mm, 75mm and 90mm, in lengths from 3000mm up to 4800mm. To work out how much you need, measure the room perimeter in linear metres, add 10 to 15 per cent for mitres and waste, then divide by the length you are buying.

Choosing a cornice profile and ordering the right quantity of it are two different jobs. If you are still deciding between cove, square set and a decorative profile, our post on choosing the right cornice profile covers that decision. This one deals with what happens after: what the sizes actually mean, what lengths come off the rack, and how to get the quantity right the first time.

What is the standard cornice size in Australia?

When a plan or a client says "standard cornice", they almost always mean 55mm cove. It is the most widely used profile in Australian residential construction and the size most people picture when they think of a plain cornice.

The number refers to the face measurement: the width of the visible curved surface between where the cornice meets the wall and where it meets the ceiling. It is not the height it sits up the wall, and it is not the projection out from the wall. On a 90 degree internal corner with a symmetrical cove, the cornice sits roughly 39mm up the wall and 39mm across the ceiling for a 55mm face, because the face runs on the diagonal. That matters when you are checking clearance against a window head or a door architrave.

Calling 55mm "standard" is a convention, not a rule. It suits ceilings in the 2400mm to 2700mm range, which covers most South East Queensland housing. It does not follow that 55mm is correct for every room, and in a raised ceiling it often looks undersized.

Which cornice size suits which ceiling height?

Cornice scale reads relative to the wall it sits on. The taller the room, the larger the cornice needs to be before the eye registers it as a deliberate detail rather than an afterthought. The pattern below is what gets specified most often, and it is a starting point rather than a constraint.

Two practical warnings on sizing up. First, a larger cornice needs more clearance above windows and doors, and in a room where the window head runs close to the ceiling a 90mm cove will land on the architrave. Check the tightest opening in the room before committing. Second, mixing sizes between adjoining rooms is visible at the doorway where the two meet, so if you are running a larger cornice in living areas plan how the transition is handled at each opening.

What lengths does cornice come in?

Plaster cornice is supplied in fixed lengths, typically from 3000mm through to 4800mm, with 4200mm and 4800mm being the workhorse lengths for residential work. Longer lengths mean fewer butt joints along a wall, and a butt joint in a cornice run is both extra labour and a visible line if it is not finished carefully.

The trade-off is handling. Cornice is long, comparatively fragile and easy to chip on the arris, and a 4800mm length needs a clear carry path and somewhere flat to store it on site. Damaged corners are the single most common reason for a return, and almost all of that damage happens between the truck and the room rather than during the fixing.

The right length to order is the one that covers your longest wall in a single piece where you can manage it. Running the longest walls jointless and using the offcuts on short returns is far more efficient than buying one length and cutting everything from it.

How do you calculate how much cornice to order?

Cornice is bought by the linear metre, so the calculation is a perimeter measurement rather than an area one. The method:

1. Measure the perimeter of each room. Add the lengths of all four walls. For an L-shaped or irregular room, walk the full outline including every return.
2. Do not deduct for doorways. Cornice runs across the top of a door opening, so a doorway does not reduce the run.
3. Deduct only for genuine breaks. A full-height opening between rooms with no wall above it, or a wall where a bulkhead replaces the cornice, comes out of the total.
4. Add 10 to 15 per cent. This covers mitre cuts at internal and external corners, the offcut left at the end of each run, and breakage. Rooms with many corners sit at the top of that range, simple rectangular rooms at the bottom.
5. Divide by the length you are buying, then round up to a whole number of lengths.

A worked example. A 4.2m by 3.6m bedroom has a perimeter of 15.6 linear metres. Add 12 per cent for mitres and waste and you are at 17.5 metres. In 4200mm lengths that is 4.16, so you order 5 lengths, which gives 21 metres and a comfortable margin. Ordering 4 lengths gets you 16.8 metres and leaves you short the moment a mitre goes wrong.

How much cornice cement do you need?

Cornice is bedded in a purpose-made cornice cement, a fast-setting adhesive compound applied to both flanges before the length is pressed into the junction. It is not the same product as a general jointing compound, and substituting a topping compound for cornice cement is a reliable way to have cornice let go months later.

Coverage depends on the cornice size and how heavily the installer beds it, since a larger cove has wider flanges and takes more product per metre. Because coverage varies with technique as much as with product, the sensible approach is to tell the counter your total linear metres and the cornice size and let us work back to the number of bags, rather than estimating it yourself and finding out at the end of the run.

Allow for the finishing coat as well. After the cornice is fixed, the joints, mitres and any gaps against uneven walls are filled and sanded, and that is typically a separate jointing or topping product. Our post on jointing and finishing levels covers which compounds belong at each stage.

Ordering cornice across South East Queensland

Bayside carries cove cornice in 55mm, 75mm and 90mm along with decorative and ornamental profiles, and the cornice cement and jointing compounds that go with them, at all four branches: Morningside, Redlands, the Gold Coast and Caboolture. Standard 55mm and 75mm in the common lengths are held at every yard. Decorative profiles and the longest lengths are worth a phone call first.

If you are running cornice across a whole house, send us the room dimensions rather than a length count and we will do the linear metre calculation, the waste allowance and the cement quantity in one go. Because cornice damages easily in transit, we will also talk through whether it goes on the same drop as your board or comes separately. Call the trade counter on (07) 3245 4488 or arrange delivery or click and collect.