Door and window trim is a narrow moulding that covers the joint between a frame and the adjoining wall or plaster. In British practice it is usually called an architrave; in North America it is known as casing. It is set against the reveal of the frame and laps over the wall finish, so that a shrinkage crack, an uneven plaster edge or a gap stays out of sight.
Why it is needed
There is almost always movement between a frame and a wall. Timber moves with humidity, plaster shrinks as it dries, and the building itself expands and contracts with the seasons. A plaster edge finished tight against a frame will therefore nearly always crack open. Trim does not follow that movement — it conceals it. It is fixed to one substrate and slides invisibly over the other.
Sizes and materials
- Profile — usually a rectangular section with a rounded or chamfered visible edge. Common sizes range roughly from 12 x 45 mm to 18 x 70 mm; wider sections are used where the joint is large or irregular.
- Material — softwood or meranti for painted work, hardwood or MDF for interiors, and foil-wrapped PVC where no painting is wanted. MDF is stable and smooth but unsuitable for bathrooms and external use.
- Preparation — prime timber and MDF on all faces, including the end grain, before fitting. Untreated end grain absorbs moisture and is the first place paint fails.
Practical tips
- Glue rather than pin. Construction adhesive bonds to plaster and concrete as well and avoids visible nail holes. Hold the piece with a few pins or tape until it sets.
- Measure the mitre, don’t assume it. Corners in older houses are rarely square. Measure the actual angle and halve it instead of cutting a standard 45 degrees.
- Fix one side only. Attach the trim either to the frame or to the wall, not both. Fixed on both sides, it will crack anyway.
- Seal only the visible joint. A thin bead of decorator’s caulk along the top edge looks crisp; sealing the whole thing serves no purpose and collects dirt.
Related terms
- Window frame
- Reveal
- Cover moulding
- Skirting board
- Plaster
- Stop bead
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