Wiki: Cornice

[Tabby]

Input welcomed

NT

Coving and cornice are decorative strips covering the join between ceiling and wall. Coving is plain, cornice also has additional 3d decorative moulding. Cornice is one of the elements creating the style in many characterful historic houses.

==Types== ===Premoulded plaster=== 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

• [[Filler|filling]] the wall to coving gap

The coving is stuck in place with high grab [[adhesive]]. Once set, the cracks & gaps are [[Filler|filled]].

===Foam coving=== Polystyrene foam coving is easy to work with. But its not as robust, and its tendency to follow wall variations can look poor on occasion.

===Run in situ=== Cornice run in situ is more work than the premoulded types. Its good

• on walls that are smooth but not flat

• 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 # Mix plaster # Apply a layer of plaster to the wall and profile it with the wooden tool # Later apply another layer and profile it, repeating to build up the full depth

===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 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 alternating layers of plaster and hessian or scrim. The hessian reinforces the plaster.

Such mouldings can be heavy, and are likely to need [[Adhesive| gluing]] & [[screw]]ing in place.

===Wood=== Some decorative cornice profiles can be made by putting together wood mouldings. Material cost is higher, but labour far less than moulding plaster cornice.

==Adding 3d decoration== Plain coving or extrusion shaped decorative cornice can be made more decorative by adding non-extruded shapes such as egg & dart. The add- on shapes are simply stuck onto the cornice, taking care to get spacing and alignment correct. Once [[paint]]ed they look like part of the moulding.

==Paint== Layers of [[paint]] build up and obscure detail over time, turning a crisp clean profile into something not looking as good. This can be avoided by painting cornice with [[Paint|distemper]]. Distemper looks like emulsion but can be washed off with [[Domestic Hot Water|hot water]] to avoid buildup.

[[Category:Decorating]] [[Category:Period Property]] [[Category:Plastering]]

[Andy Burns]

The polyurethane foam coving with paper covering does/can look as good as the traditional plaster core with paper covering, the polystyrene stuff still looks as naff as ever (especially with the straight butt pre-made corners).

[Tim Watts]

Not all plastic coving is the same these days. Gyproc Lite is a whole better class of non plaster coving. Apart from the curvature issue, it passes itself off just as well as regular Gyproc and you mitre the corners in the usual way and it paints excellently.

[The Medway Handyman]

Don't know if you want to include this, but I would 'highly' reccommend the Magic Mitre.

a mitre jig that adjusts to the actual angle of the individual corner, not a 90 degree angle. Since walls are rarely at 90 degrees this solves a lot of problems. For example, you can cut internal mitres that fit perfectly & avoid the need to scribe them.

Works on coving, skirting, architrave, dado rails etc. Worth watching the videos.

Around £35 delivered. No connection with the company, just a very satisfied customer.

[Dave Plowman (News)]

Doesn't look like it's worth 35 quid. Is it rather better made than the pics suggest?

[The Medway Handyman]

Its worth the money for what it does so well.

Doubt it would take much abuse on a building site, but its quite well made.

[Tim Watts]

I got a cheap plastic one for mine and it was a real pain. Bearable for the few cuts I needed but if I was doing a whole floor's worth, I think

35 quid would be worth it for getting faster and better cuts. Probably pay for itself compared to the wasted coving I had doing the same cuts twice in some cases!

[mark]

When patterned cornice is put up such as Acathus, Egg and Dart etc., the pattern at the joints are mirrored. This causes the final 'bastard' joint not to match and is usually put behind the door or such place soas not to be immediatly noticable.

Installing cornice: Two horizontal setting-out lines are drawn around the room, one representing 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 appropiate tools.

For the larger and heavier mouldings it may be worth considering running a temporary support batten along the setting-out line of the wall.

This site may be useful:

mark

[S Viemeister]

Yes, the paper-covered stuff looks surprisingly good, and is much easier to install.

[S Viemeister]

Just what I need! Thank you.

[Mark.]

all you need is a simple tool to accurately find the angle, then any mitre saw to make the cut.

12ukp if you google, and almost builder proof.

-

[Tabby]

Lots of great feedback, thank you. Latest version follows...

NT

Coving and cornice are decorative strips covering the join between ceiling and wall. Coving is plain, cornice also has additional decorative moulding. Cornice is one of the elements creating the style in many characterful historic houses.

==Types== ===Premoulded plaster=== 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

• [[Filler|filling]] the wall to coving gap

The coving is stuck in place with high grab [[adhesive]]. Once set, the cracks & gaps are [[Filler|filled]].

===Polystyrene Foam=== 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 poor.

===Polyurethane foam=== 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 good

• on walls that are smooth but not flat

• 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 # Mix plaster # Apply a layer of plaster to the wall and profile it with the wooden tool # Later apply another layer and profile it, repeating to build up the full depth

===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 silicone sealant or rubber moulding compound # 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 alternating layers of plaster and hessian or scrim. The hessian reinforces the plaster. # Once removed, any surface holes in the moulding are [[Filler| filled]]

Such mouldings can be heavy, and are likely to need [[Adhesive| gluing]] & [[screw]]ing in place.

===Wood=== 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.

===GRP=== Glass fibre reinforced plastic mouldings are light and weatherproof. These are often used for exterior decoration on large buildings. Its possible to diy them, but takes a lot of time.

==Corners== 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 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 Acathus, Egg and Dart etc., the pattern position at the joints isn't adjustable. 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 representing 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 the larger and heavier mouldings it may be worth considering running a temporary support batten along the setting-out line of the wall.

==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 [[paint]]ed they look like part of the moulding.

==Paint== Layers of [[paint]] build up and obscure detail over time, turning a crisp clean profile into something not looking as good. This can be avoided by painting cornice with [[Paint|distemper]]. Distemper looks much like emulsion but can be washed off with [[Domestic Hot Water|hot water]] to avoid buildup.

==See also==

• Lots of high end cornices]

• [[Special:Allpages|Wiki Contents]]

• [[Special:Categories|Wiki Subject Categories]]

[[Category:Decorating]] [[Category:Period Property]] [[Category:Plastering]]

[The Medway Handyman]

Anna Kettle would have some useful input here I'm sure.

[Tabby]

Good point. I'll see if I can have a word later. Trouble with these wiki articles is its easy to write one when in waiting mode, but dont always have the time to do all the follow up work.

NT