- Measuring tape
- Mitre Box
- A very fine saw
- Chalk line and chalk
- Paint scraper / filling knife
- Caulking gun for applying adhesive if adhesive is in a cartridge
- Drop sheets if installing cornice in a room where tiles or carpets are already installed.
- Sponge or mutton cloth for wiping away excess adhesive.
- Very fine sandpaper
Cutting cornices is a skill that requires a bit of practice. If you are a novice then it is recommended to buy an extra length or two of cornice to practice on.
Measure the linear meters of the walls to which the cornice must be installed. Add 10% to 15% for waste. Ensure that the surface of the walls and ceilings, to which the cornices will be installed to, are free from dust and dirt and is completely dry. You may use a 120 grid or finer sand paper to ensure the installation area is clean but make sure that all dust from sanding is wiped clean with a clean dry cloth.
Familiarise yourself with the cornice size. In the diagram below you will see three measurements:
- Ceiling : Projection – The distance from the wall surface to the front edge of the cornice
- Wall : Drop – The distance from the ceiling surface to the bottom edge of the cornice
- Face – The size of the cornice face
Make sure that the cornice is a suitable size all around the intended installation area before commencing with the installation. For instance, ensure there is sufficient space between the top of cupboards and the ceiling. The cornice in the illustration would require at least a 95mm gap.
Measure out and mark the installation line. To do this, measure the drop of the cornice (B) at each change in wall direction such as corners. Then, use a chalk line to mark a straight line to each measurement mark around the room. This will serve as the installation guideline.
Always start the installation at the most visible corner in relation to the entrance to the room. Below is a diagram that indicates which corner will be the most visible. The reason for this is to minimise any joints in line of sight. Always start with the longest lengths first to minimise waste.
To mitre a corner
Mitre joints are formed in the corner of rooms or where the coving is going to finish part-way along a wall, and the end needs to be finished off with a piece of cornice that returns towards the wall to give a neat finish. The trick to succesful mitres is to ensure that the cornice is placed correctly inside the mitre box. The cornice is placed upside down in the mitre box at the angle to which it is installed, usually 45 degrees. To make sure the angle is correct, place the ceiling bedding edge (Projection – A) against the bottom side of the mitre box and the wall bedding edge (Drop – B) against the side of the mitre box opposite to you. To get the angle correct it is easier to measure the projection size of the cornice (measurement A) from the side of the mitre box opposite to you and make a mark against the bottom side of the mitre box. Now place the ceiling edge (Drop – B) of the cornice at the mark at the bottom side of the mitre box every time to ensure consistency of the angle. When measuring a cornice cut, make sure you measure along the edge that will be placed against the wall. To help you keep track of which is the wall edge scribble along that edge in pencil or write the word wall at various intervals along that edge on the back of the cornice. There are two walls to every corner. A left wall and a right wall. You need one length of cornice for each wall of the corner.
Cutting an inside corner
When cutting the length for the left wall for an inside corner, the saw will point left in the mitre box and everything left of the saw is waste. When cutting the length for the right wall for an inside corner, the saw will point right in the mitre box and everything right of the saw is waste. When you take the cornice out of the mitre box turn the length around so that the left edge is on the right and the right edge is on the left. The corners should now fit perfectly.
Cutting an outside corner
When cutting the length for the left wall for an inside corner, the saw will point right in the mitre box and everything left of the saw is waste. When cutting the length for the right wall for an inside corner, the saw will point left in the mitre box and everything right of the saw is waste. When you take the cornice out of the mitre box turn the length around so that the left edge is on the right and the right edge is on the left. The corners should now fit perfectly.
Apply a bead of adhesive along both long edges of the cornice, as well as the butt end that is being glued to another cornice or wall. Otherwise no adhesive to the butt ends at this point. Don’t be frightened of putting on too much adhesive, you will be able to scrape it off easily using the filling knife and a wet cloth or sponge to clean the wall. You want it to just be squeezing out from the edges as you press the coving into position. Be careful not to squeeze out all the adhesive as the adhesive will lose its ability to bond and cure. Align the cornice to the chalk line on the wall to ensure a consistent straight installation. If you are installing from a corner, then leave just a few mm’s from the wall to allow the other mitre end to fit with the adhesive applied.
Once the first cornice length is installed then apply adhesive to the next length including the mitre joint or butt end that is joined with the first length. Pins can be used to keep joints in place. Excess adhesive must be scraped away or wiped away with a damp sponge or mutton cloth. If adhesive is used from a bucket, then excess adhesive may be returned to the bucket.
Use your filling knife to press adhesive into any gaps, pushing adhesive into any joints that do not quite meet and any undulations in the surface of the wall
Once the installation is complete allow 48 to 72 hours for the adhesive to cure.
Before painting, inspect all the joints and sand away any excess adhesive or fill exposed joints. The adhesive may have receded slightly during the curing process in which case the joints would need some filling. You can use the polystyrene adhesive to fill the joints or alternatively any acrylic filler. Be sure to leave a bit of excess over the joints which should be sanded smooth after 24 hours.
Only use acrylic paints when painting polystyrene cornice.