Rendering Bungaroosh Walls — Specialist Techniques & Pitfalls

Bungaroosh—Brighton’s distinctive mix of flint, brick, chalk and lime—creates façades rich in history but vulnerable to moisture and movement. Applying render to this porous, irregular fabric demands skills beyond standard blockwork methods. This guide explains how to assess, prepare and finish bungaroosh walls safely, and flags the common mistakes that lead to cracking, damp or total render failure.

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1. Why Bungaroosh Is Different

Mixed aggregate sizes: Voids ranging from fingertip-sized to fist-sized mean uneven suction.
High salt content: Sea air and original chalk nodules introduce soluble salts that migrate outward.
Low compressive strength: Hard cement renders can detach or crush soft inclusions.
Irregular planes: Expect bulges, recesses and poorly keyed mortar joints.

2. Pre-Render Assessment Checklist

Probe masonry with a screwdriver—crumbly points indicate loose lime or salt jacking.
Measure surface moisture using a capacitance meter; readings above 20 % require extra drying.
Check previous coatings: cement or bitumen paint must be removed to restore breathability.
Photograph masonry for your heritage statement if the house is listed or in a conservation area.

3. Suitable Render Choices

Three-coat lime putty render – The gold standard; maximum breathability and flexibility.
NHL 2 base with lime finish – Slightly faster set on windy elevations but still vapour-open.
Hot-lime render – Ideal for exposed gables needing early weather resistance.
DO NOT use monocouche, sand-cement or acrylic thin-coats directly on bungaroosh.

4. Wall Preparation Steps

Step 1: Remove cement
• Needle-gun or hammer-and-chisel; avoid angle grinders that over-cut soft stone.

Step 2: Rake and repoint
• Use NHL 2 or hot-lime mortar flush with surrounding surface; leave rough finish for key.

Step 3: Salt treatment
• Brush on poultice paste (clay + paper pulp) and allow to draw salts for 48 h; repeat until efflorescence slows.

Step 4: Dampen wall
• Light mist with clean water just before rendering; prevents lime de-bonding from rapid suction.

5. Application Technique

Scat coat (harling) – Flick on 3–5 mm lime slurry for uniform key.
Scratch coat – 8–10 mm NHL 2 or hot-lime; scratch horizontal lines 3 mm deep.
Float coat – 8 mm lime putty with sand graded 0–3 mm; rule off to approximate plane while following wall undulations (never over-straighten).
Setting coat – 2–3 mm fine lime putty; finish with sponge float for slight open texture.
Curing – Mist-spray twice daily for the first week; protect with hessian if direct sun or wind exceed 20 km/h.

6. Common Pitfalls to Avoid

Using PVA or cement slurry as a bonding agent – Seals pores and traps moisture.
Trowelling coats too flat – Creates internal shear planes causing future delamination.
Adding waterproofers – Prevents lime from breathing; damp resurfaces indoors.
Skipping salt poultice – Residual chlorides crystallise behind new render and push it off within two winters.
Rapid force-drying with heaters – Causes shrink cracks; slow cure is essential.

7. After-Care & Maintenance

Limewash or silicate paint every 5–7 years for water-shedding and UV protection.
Low-pressure hose-down each spring to remove salt spray.
Fill hairline cracks ≤1 mm with fresh lime putty; monitor larger cracks for structural movement.

8. Case Study — Brunswick Road Cottage

Issue: Cement render bulging, internal damp patches.
Solution: Removed cement, salt-draw poultice x3 cycles, applied three-coat hot-lime and limewash.
Outcome: Indoor relative humidity dropped from 78 % to 55 % within six months; no cracking after four winters.

9. Quick Decision Flow

Is the wall bungaroosh or soft lime brick? → Choose lime render.
Is elevation highly exposed to wind-driven rain? → Use hot-lime first coat, lime putty finish.
Tight deadline in winter? → Tent and heat to 8 °C minimum; never swap lime for cement.

10. Frequently Asked Questions

Can I apply monocouche if I mesh the wall first?
No. Even with mesh, monocouche is too rigid and vapour-tight for bungaroosh.

How thick should the total render be?
Usually 18–22 mm across three coats—thicker sections risk slumping on weak areas.

Do I need planning permission?
In a conservation area or on a listed building: yes. Supply render sample panels and lime specs with your application.