Plastering and Finishes in London Renovations: What to Expect

The decision whether to patch existing plaster or fully replaster a room is one of the most common judgment calls in a London renovation.

• **When patching is appropriate:**
• •Small areas of localised damage (e.g., where a socket was moved, a chase was cut, or minor impact damage)
• •Where the existing plaster is sound and well-bonded to the substrate throughout the room
• •Where the cost of full replastering is not justified relative to the value of the improvement (e.g., a room that will not be redecorated as part of the main renovation)

• **When full replastering is appropriate:**
• •Where the existing plaster is blown (hollow — tap it with your knuckle and you will hear the difference between sound plaster and plaster that has separated from the substrate)
• •Where there is extensive patching throughout the room (more than 20–30% of the surface) — patches will always show through decorations to some degree
• •Where the chase runs and service installations have left the walls extensively disturbed
• •Where the project brief is for a high-quality finish — a room with full replaster and careful preparation will always produce a better painting result than a patched room

**The 'skim' or 'plasterboard and skim':** In London renovation projects, stud walls and drywall partitions are typically finished with plasterboard followed by a 2–3mm skim coat of finishing plaster over the board — this produces the smooth, even surface required for a high-quality paint finish. 'Dot and dab' (plasterboard applied on adhesive dots over existing brick or blockwork) is a common way to create a smooth surface on existing masonry walls without the depth or cost of a full three-coat plaster system.

Original Victorian plaster is a three-coat lime system: scratch coat (lime and coarse aggregate), floating coat (lime and fine aggregate), and a lime putty finish. This system has excellent vapour permeability — it 'breathes' with the movement of moisture through the wall — and is the ideal complement to the solid brick walls of Victorian properties.

**Why modern gypsum plaster can be problematic on Victorian walls:** Gypsum plaster (modern Thistle plaster and similar) is a non-breathable material. Applied over a solid brick wall in a Victorian property, it can trap moisture and cause the plaster to blow, damage the wall behind, and potentially exacerbate damp issues. This is particularly relevant where the wall is an external wall.

• **When lime plaster is the right choice:**
• •Conservation Areas or listed buildings where traditional materials are required
• •Properties with recurring damp or moisture issues in the walls (the breathability of lime allows moisture to equalise rather than accumulating behind the plaster)
• •Where original Victorian lime plasterwork in good condition is being patched — lime-on-lime creates a better bond and compatible movement characteristics
• •Where ceiling cornices and mouldings are original lime work — new patches should be in lime to avoid differential shrinkage and cracking at the interface

**Sourcing lime plasterers:** Lime plastering is a specialist skill — not all plasterers are experienced in lime work. In London, there is a reasonable supply of lime-experienced plasterers, particularly in the inner London boroughs with high concentrations of period properties, but they should be specifically identified and their experience with lime work confirmed.

• **Cost comparison:**
• •Modern gypsum skim (plasterboard and skim): £15–£25/m²
• •Gypsum replaster (full two-coat on existing substrate): £20–£35/m²
• •Lime plaster (full three-coat): £45–£80/m²

**Sequence within the project:** Plastering must come after first-fix (all services — plumbing, electrical, heating — are installed and tested within the wall and floor structure) and before second-fix (sockets, switches, radiators, sanitary fittings). The sequence is:

1. First-fix plumbing, electrics, and heating 2. Plasterboard (where applicable) 3. Plastering (skim or full replaster) 4. Drying time 5. Second-fix electrics, plumbing, heating 6. Flooring installation (some floor types) 7. Joinery installation (skirtings, architraves, doors) 8. Decoration (priming, painting)

• **Drying time:**
• Gypsum plaster drying is one of the most common causes of project delay in London renovations. Modern gypsum plaster (Thistle) requires approximately:
• •3–5 days before light decoration (priming only)
• •4–6 weeks for full drying of a two-coat plaster application
• •Heated and ventilated rooms dry faster; cold or damp conditions extend drying time significantly

Drying plaster before second-fix and decoration is essential — applying paint or second-fix finishes to wet plaster causes defects (cracking, flaking, staining). In a renovation programme, the plasterer needs clear access to each area before moving on, and drying time must be built into the overall programme.

**Coving and cornices:** Where original coving or cornices are present and the walls are being fully replastered, the cornice should be masked and protected before plastering. New cornice — where existing has been lost — is typically run in position by a specialist or cast from a GRP or plaster moulding matched to the period profile. Run-in-place lime cornice is the most authentic and long-lasting option; GRP cornice is faster and lower cost.