There are 3 types of decorative cornice commonly used in today’s homes. The first type is roving, a relative of sheet plasterboard (gypsum hoard), which consists of a concave hollow-hacked plaster core sheathed in a strong paper envelope. It is fixed in place with adhesive. The second is molded cornice; this is made either from traditional fibrous plaster or from modern foamed plastics to imitate the ornate decorative cornices often found in older buildings, and comes in a range of profiles. Plaster types must generally be secured in place with screws because of their weight, but plastic types can simply be stuck in position with adhesive. The third type is a machined wooden trim with a similar profile to plasterboard cornice, and is either nailed direct to the wall framing or to a nailing strip or barren (furring strip) in the angle of the wall and ceiling.
Apart from its decorative appearance in framing the ceiling, a cornice can also help to conceal unsightly cracks. These often open up around the ceiling perimeter as the ceiling expands and contracts with changes in temperature and humidity, or as the building settles.
FITTING A CORNICE (CROWN MOLDING)
- Hold a length of cornice squarely in the wall/ceiling angle and draw 2 guidelines on the wall and ceiling surfaces. Cut any mitred edges.
- Remove any old wall coverings from between the guidelines by dry scraping them. Cross hatch painted or bare plaster to key the surface.
- Either mix up powder adhesive or use already mixed type. Using a flat scraper ‘butter’ the adhesive on to both edges of the rear of the cornice.
- Press the length into place between the guidelines, supporting it if necessary with partly driven masonry nails. Remove the nails (if used) once the adhesive has set.
- Fit the adjacent corner piece next. Here, the next section also incorporates an external mitre; measure and cut this carefully before fitting the length.
- Complete the external comer with a further length of cornice, hurting the cut ends closely together and ensuring that the length fits between the lines.
- Fill any slight gaps at external and internal angles with a little cellulose filler(spackle), applied with a filling knife (putty knife) to leave a crisp, clean joint, sand the filler smooth once it has hardened.
- Before the adhesive hardens, use a damp sponge to remove any excess from wall and ceiling surfaces and also to smooth over the filled joints.
CUTTING A CORNICE (CROWN MOLDING)
- Make up a large mitre block big enough to hold the cornice, and use this and a tenon saw to make accurate 45° cuts for internal and external corners.
- Some cornice manufacturers supply a paper template that enables cutting lines to be marked accurately for internal and external corners.
- When using cut pieces to complete a wall, mark off the length required directly, square a line across the cornice with a pencil and cut it to length.