Coving and cornice are 3 dimensional decorative strips covering the join between ceiling and wall. Coving is plain & round, cornice also has additional decorative moulding. Cornice is one of the elements creating the style in many characterful historic houses.
This coving gives an excellant finish and is more damage resistant than foam. Its also non-flammable.
Its rigid, so the wall it sits on must be close to flat. Slight variations are taken up with a mix of
- slight bending of the plaster coving
- cutting the coving to enable it to lay against the wall better
- filling the wall to coving gap after fitting
Polystyrene foam coving is easy & quick to work with. But its not robust, the surface quality isn't good, and its tendency to follow wall variations can look poor on occasion. Butting lengths up to premade corners also looks bad.
Paper covered polyurethane is denser and smoother than polystyrene, and a good result can be obtained. Its not as robust as plaster though, and can bend during fitting.
Run in situ
Cornice run in situ is more work than the premoulded types. Its mainly used to replace a missing section of an existing extrudable moulding. It can create more or less any extrudable profile.
Cut the required profile into a piece of wood. Fit a batten to the wall, the profiled wood will run along this. Mix plaster & apply a layer of plaster to the wall and run the profile along it. There's a limit to how much you can apply for each layer, too much will sag or fall off. When the plaster has reached its max depth, voids get left each time the profile is run. Apply blobs of plaster to theese voids & rerun the profile. This needs doing repeatedly to fully build up the final shape.
Run on bench
The same process as run in situ is used, but with lengths created on the bench. Scrim is included to give the added strength needed. Make your lengths longer than needed.
Moulded on site
Cornice can be moulded on site when needed to replace a missing section of cornice with a non-extrudable shape. Its a labour intensive process.
- A section of the existing cornice is coated with mould release (eg diesel)
- a moulding material is applied, such as rubber moulding compound or silicone sealant
- slightly flexible reinforcement is applied to the back of it, such as scrim & thin wood strips
- When set its removed and used to mould new sections of cornice, always using a mould release agent.
- The new cornice is made from layered plaster and hessian or scrim. The hessian reinforces the plaster.
- Once removed, any surface holes in the moulding are filled
Such mouldings can be heavy, and need sufficient fixings.
Some decorative cornice profiles can be made by putting together standard wooden mouldings available from builder's merchants or DIY sheds. Material cost is higher, but labour far less than moulding plaster cornice. Glue the profiles together.
A good router table that can set the tool at any angle increases the range of producable mouldings even further. You'd need to make such a table yourself.
Youtube has vids on making a suitable router table, routing timber arches etc.
To minimise risk of warping, acclimatise the timber before use, and use clear (knot free) wood.
Glass fibre reinforced plastic mouldings are hollow, light and weatherproof. These are often used for exterior decoration on large buildings. Its possible to diy them, but takes a lot of time.
Premoulded corner pieces make a jointless corner if the room corner is 90 degrees. The corner joint is then replaced with 2 nearby butt joints, which look far worse.
Mitred corners make a good job, but rooms often aren't quite 90 degrees, so filling or adjustability is needed. An adjustable mitre makes this quick. A bevel and electric mitre saw also works.
When patterned cornice is put up such as Acanthus, Egg and Dart etc., the pattern position at the joints isn't adjustable. All but one corner joint is set correctly, but this causes the final 'bastard' joint not to match, and its usually put behind the door or such place so as not to be too noticable.
Traditional cornice installation
Two horizontal setting-out lines are drawn around the room, one showing the how far down the wall the cornice comes, and the other for the ceiling. Professionally installed, mitre boxes are not used. Lines are marked out from the corners which split the corner angle equally. The cornice is held in position and markings are made directly on to the cornice where it intersects the setting out lines. It is cut and fixed in place leaving a small gap with the next length. The gap is filled with cotton scrim, plastered over and patterned matched using appropriate tools.
For large heavy mouldings a temporary support batten is nailed to the wall along the setting-out line.
Adding 3d decoration
Plain coving or extrusion shaped decorative cornice can be made more decorative by adding non-extruded shapes such as egg & dart, flowers etc. The add-on shapes are simply stuck onto the cornice, taking care to get spacing and alignment correct. Once painted they look like part of the moulding.
Layers of paint build up and obscure detail over time, turning a crisp clean profile into something not looking as good. This is traditionally avoided by painting cornice with distemper. Distemper looks much like emulsion but can be washed off with hot water at each repaint to avoid buildup.