How to Render an External Wall

Tools and Materials

Before You Start

• Inspect the wall for loose render, cracks, and structural movement — repair before rendering
• Mask off windows, doors, and any surfaces you do not want render on
• Dampen the wall with clean water the day before and again before applying render
• Apply SBR bonding agent to smooth or painted surfaces to improve adhesion
• Fix all beads (bellcast, corner, stop) before starting the render
• Check the weather forecast for a window of dry, mild weather (5-20°C)

Step-by-Step Application

1. Step 1 — Fix render beads. Fix bellcast beads at the base (DPC level), corner beads on all external corners, and stop beads where the render terminates (around windows, doors, and at adjoining surfaces). Bed beads on dabs of render mix and check plumb.
2. Step 2 — Dampen the wall. Spray the wall with clean water using a garden sprayer or hose. The surface should be damp but not running with water. This prevents the wall sucking moisture from the render too quickly.
3. Step 3 — Apply the scratch coat. Mix the scratch coat (1:5 cement:sand with waterproofer and plasticiser). Apply firmly to the wall at 10-12 mm thickness, working between the beads. Use a straight edge to rule the surface flat. Do not overwork.
4. Step 4 — Scratch the surface. When the scratch coat has stiffened slightly (within 1-2 hours), scratch horizontal lines across the surface using a nail float or comb scratcher. These scratches provide a mechanical key for the top coat.
5. Step 5 — Cure the scratch coat. Allow 3-7 days for the scratch coat to cure. Keep it damp by misting with water in dry or warm weather. Do not allow it to dry too quickly or it will crack.
6. Step 6 — Apply the top coat. Dampen the scratch coat. Mix the top coat (1:6 cement:sand with waterproofer and plasticiser). Apply at 6-8 mm thickness, ruling flat with a straight edge. Work in manageable sections.
7. Step 7 — Finish the surface. For a smooth finish, use a sponge float in circular motions when the render has stiffened. For a textured finish, use a plastic float or stipple with a brush. Work the surface evenly for a consistent finish.
8. Step 8 — Cure the top coat. Protect the finished render from direct sun, wind, and rain for at least 48 hours. Mist with water in dry conditions. Allow a minimum of 7 days before painting.

Common Mistakes

• Making the top coat stronger than the scratch coat — causes cracking and delamination
• Applying too thick in one coat — render slumps off the wall; maximum 15 mm per coat
• Not dampening the wall — render dries too fast and cracks
• Rendering in frost or rain — ruins the render; wait for suitable weather
• Skipping bellcast beads — water tracks down and stains the wall below
• Overworking the top coat — brings cement to the surface causing crazing

Cost Estimate (2026)

Related Calculators

Use the Rendering Calculator for sand and cement quantities, or the Monocouche Render Calculator for single-coat through-colour render systems.