Can I plaster over lime with gypsum?

No — incompatible. Gypsum traps moisture causing lime substrate to fail. If existing lime plaster sound, repair like-for-like with lime. If failed, strip back to masonry + reapply lime.

How do I know if my walls are lime or gypsum?

Pre-1920 brick wall: almost certainly lime-mortared + lime-plastered originally (may have been over-skimmed with gypsum in 1960s+ refurbishment). 1920s–1950s: transitional, often gypsum on brick. Post-1950s cavity wall: gypsum. Test: chip sample + drop in dilute HCl acid — lime fizzes, gypsum doesn't. Or feel surface: lime warm + soft, gypsum cool + hard.

Is lime plaster better for indoor air quality?

Yes — moisture-buffering + permeability help regulate humidity 40–60% optimal. Lime absorbs + releases vapour reducing condensation risk + mould growth. Synthetic-impermeable surfaces accelerate humidity swings + mould risk.

Lime vs Gypsum Plaster London 2026 | Builderr

Lime Plaster vs Gypsum Plaster for London Renovation — Which Is Better?

Lime plaster (calcium hydroxide-based) breathable + flexible + heritage-compatible but slow-setting (30+ days full cure) and 2× cost of gypsum. Gypsum plaster fast-setting (24hr) + cheaper + harder finish but impermeable + brittle. Lime mandatory for solid-wall period properties, listed buildings, conservation areas. Gypsum standard for modern cavity-wall construction. Cost £25–£45/m² lime vs £12–£25/m² gypsum.

Lime plaster properties + use cases

Composition: hydrated lime + sand + (optional) hair fibre for reinforcement. NHL (Natural Hydraulic Lime) grades 2/3.5/5 by strength. Set time: initial set 24–48hr, full cure 30–60 days (slow CO2 reaction). Breathability: vapour open (μ ~10–20) — allows moisture migration outward through walls. Flexibility: tolerates building movement (settlement, thermal cycling) without cracking. Heritage compatibility: matches Victorian/Edwardian/Georgian original construction. Required for: solid 9-inch brick walls (single-skin), lime-mortar masonry, listed buildings (LBC condition typical), conservation areas where wall fabric retained. Failure mode if mismatched: synthetic/gypsum on solid wall traps moisture → interstitial condensation → rot + damp + paint failure + spalling brick. Cost: NHL 3.5 base coat + finish coat £25–£45/m² supplied + applied by skilled lime plasterer.

Gypsum plaster properties + use cases

Composition: calcium sulphate (gypsum) — bonding coat (Thistle Hardwall, Carlite Bonding) + finish coat (Thistle Multi-Finish, Carlite Finish). Fast set 1–2 hr; full cure 24–48 hr (workable next day). Hard finish: excellent surface for paint, low impact resistance, accepts wallpaper well. Impermeable: water-vapour-resistant — traps moisture in walls. Brittle: cracks easily with building movement. Use cases: modern cavity-wall construction, plasterboard skim, brick + block walls with cavity, internal stud partitions. Standard new-build + standard modern renovation spec. Cost: bonding coat + 3mm finish coat £12–£25/m² supplied + applied.

Mixed approach + heritage detail

Many London renovations use both. Period rooms (front reception, original bedrooms) lime-plastered to walls + ceilings — retains heritage character + breathability + period detail (cornice + ceiling rose moulding take lime well). Modern extensions + bathrooms + kitchens gypsum-plastered for speed + cost. Joining boundary: avoid lime → gypsum direct interface (moisture differential causes failure); separate with permanent threshold (door reveal, downstand beam). Plasterer skills: lime plastering specialist trade — fewer competent operatives in London than gypsum; book 6–12 weeks ahead. Builderr maintains relationships with 3 lime specialists in London (1 SPAB-accredited) for heritage projects. Ceiling detail: lime ceiling on lath substrate (split chestnut or sawn riven) for true Victorian restoration; lime on EML (expanded metal lath) for modern lime application; lime skim on plasterboard for budget compromise (lower breathability).