Mouldings are profiled, continuous lengths of trim that bound, cover or enrich a building element. A moulding always has a cross-section profile that is identical along its whole length — that is what separates it from an ornament, which stands on its own.
You find mouldings everywhere: as a cornice above a facade, as a skirting along a floor, as an architrave around a door, as a cove under a ceiling, and as decorative bead on a panelled door.
Why they are there
Mouldings are rarely just decoration. They usually solve a practical problem:
- They cover a joint. Where two materials or surfaces meet, an irregular line appears. A skirting, an architrave or a ceiling cove turns that into a crisp, controlled transition that also copes with shrinkage and movement.
- They protect. A skirting takes the knocks from vacuum cleaners and shoes; a nosing protects the vulnerable front edge of a stair tread.
- They shed water. Outdoors, nearly every moulding has a drip mould on its underside: a groove that stops rainwater tracking back along the soffit to the masonry. That is a genuine building detail, not a decorative one.
- They create scale. A horizontal band breaks up a tall facade or wall and gives it human proportion.
The basic profiles
Almost every moulding profile is assembled from a handful of elementary shapes:
- Ovolo — a convex quarter circle.
- Cavetto — a concave quarter circle.
- Ogee (cyma) — an S-shaped profile running from concave to convex; the classic among cornice profiles.
- Astragal (bead) — a small semicircular bead, often separating two larger surfaces.
- Cyma recta / reversa — the same S-curve with the convex and concave halves reversed.
- Fillet — a flat band separating the profiles from each other.
Classical mouldings are in fact compositions of these components, in a set sequence and with set proportions to one another.
Materials
- Timber. The traditional choice inside and out. Outdoors it needs sound paint protection and a working drip mould.
- Plaster / gypsum. For ceiling coves and roses indoors. Run with a template over wet lime or plaster, or precast in fibrous plaster.
- Natural stone. On monumental facades, in string courses and weatherings.
- MDF. The common choice for internal trim today: stable, smooth and affordable — but not moisture resistant, so not in bathrooms and not outside.
- Polyurethane or polystyrene. Lightweight synthetic mouldings fixed with construction adhesive. Convenient, light and rot-free, but vulnerable to knocks.
Practical tips
- Cut the mitre to the actual angle, not to 45 degrees. Walls are rarely truly square. Measure the angle with an angle finder and cut the half. On an internal corner in profiled trim, a scribed (coped) joint gives a crisper and more durable result than a mitre, because the joint does not open up as the timber moves.
- Let timber acclimatise first. At least a few days in the room it is going into, before cutting. Otherwise the mitres open up.
- Prime the end grain before fitting; that is where moisture gets in fastest.
- Work from long to short. Cut the longest pieces first; a mistake in a long piece is still usable as a short piece later, but not the other way round.
- Glue and pin. Construction adhesive on the back, secured with a nail gun or fine pins; adhesive alone is not enough on an uneven wall.
Related terms
- Cornice
- Skirting
- Architrave
- Ogee
- String course
- Weathering
- Plasterwork
- Foliate ornament
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.