What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thinglath
also lath and plaster, riven lath, expanded metal lath, EML, keys
Usually means the historic TIMBER laths and the ceiling they carry. EML is used in current work and is a different material with a different failure mode entirely.
Australia
Close, not identicallath
also metal lath, render lath, lath and plaster
Both senses in use, with the historic timber version far less common given the age of the building stock and the dominance of plasterboard.
United Statesyours
Same word, different thinglath
also metal lath, expanded metal lath, wood lath, rock lath, stucco netting
Almost always means METAL lath, a current product under stucco and plaster. ROCK LATH is a third thing again — a gypsum board base for a plaster skim, a mid-century transitional product.
Canada
Close, not identicallath
also metal lath, wire lath, stucco lath
As the United States, with metal lath standard under stucco and the historic timber sense confined to renovation work.
The governing standard, by market
United Kingdom
BS EN 13914-2 / BS 8000-10
Design, preparation and application of internal plastering, and workmanship on building sites for plastering and rendering.
United States
ASTM C847 / ASTM C1063
Metal lath, and the installation of lathing and furring to receive interior and exterior portland cement-based plaster.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- How does lath and plaster actually hold itself up?
- By KEYS, and understanding that explains every failure of it. Timber laths are nailed across the joists with a deliberate gap of a few millimetres between them. The first plaster coat is pushed hard enough to squeeze through those gaps, and the material that comes out the far side slumps over and sets, forming a hook over the back of each lath. Those hooks — the keys — are the entire mechanical support: the plaster is not stuck to the laths, it is hanging on them. So when a ceiling fails, it is because the keys have broken, and they break from vibration, from a leak softening them, from a century of thermal cycling, or from somebody laying insulation on top of them in the loft. The plaster then sits unsupported until something disturbs it, which is why lath-and-plaster ceilings come down in one piece.
- Why is metal lath still used when plasterboard exists?
- Because it does something board cannot: it lets a wet coat be applied to a shape. Expanded metal lath is a sheet slit and stretched into a mesh, and plaster or stucco pushed into it keys through in the same way as the historic timber system — but it bends, so it can follow a curve, wrap a column, form an arch or run over an irregular background. It also provides a base on materials nothing will stick to directly, and reinforces the coat against cracking across a junction between two different backgrounds. That is why stucco in North America is applied over lath as standard and why British render over an awkward background specifies EML. The choice against board is about geometry and continuity, not about tradition.
- Why does a repair to a lath-and-plaster ceiling go wrong so often?
- Because the new work is stiff and the old work moves, and because screwing into it breaks what is holding it up. A common repair is to overboard — fix plasterboard through the old ceiling into the joists — which is sound if the fixings reach the joists and if the extra weight is acceptable, and disastrous if the screws land in laths rather than joists, because they then pull on the very keys that were still working. Patching with modern gypsum plaster onto a lime-plastered ceiling puts a hard, strong material against a soft flexible one, and the junction cracks. And any of it done from below, pushing upward, breaks more keys around the working area. That is why serious lath-and-plaster repair is done by supporting from below and consolidating from ABOVE where access allows.