What render mix ratio should I use?

For a standard two-coat render on blockwork, use a 1:5 (cement:sharp sand) mix for the scratch coat and 1:6 for the top coat. Add a waterproofer and plasticiser to both coats. For exposed or coastal areas, use a 1:4 scratch coat. Never make the top coat stronger than the scratch coat — this causes cracking and delamination.

How thick should each render coat be?

The scratch coat should be 10-12 mm thick, scratched with a nail float to provide a key for the top coat. The top coat should be 6-8 mm thick, giving a total render thickness of 16-20 mm. Each coat should be no thicker than 15 mm to prevent slumping. On very uneven walls, apply in additional thin coats rather than one thick coat.

How long between render coats?

Allow the scratch coat to cure for a minimum of 3-7 days before applying the top coat. In cold weather, allow longer. Keep the scratch coat damp during curing by misting with water, especially in hot or windy weather. The scratch coat should feel firm and not indent when pressed with a thumb before applying the top coat.

What is a bellcast bead and do I need one?

A bellcast bead is a PVC or stainless steel profile fitted at the base of the render, typically at DPC level. It creates a drip edge that throws water clear of the wall below and gives a clean bottom edge to the render. Always use a bellcast bead — without one, water runs down the face and stains or damages the wall below.

Can I render in cold or wet weather?

Do not render when the temperature is below 5°C or falling, or when rain is expected within 24 hours. Frost damages uncured render irreparably. In cold months, use frost-proof additives and protect fresh render with hessian sheeting. The ideal rendering temperature is 10-20°C with overcast skies.

How to Render an External Wall

Tools and Materials

Tools Required

Plastering trowel (280 mm)
Hawk
Nail float (for scratching)
Sponge float or plastic float
Straight edge (1800 mm feather edge)
Spirit level
Scaffold tower or ladder (hire: £80-150/week)
Cement mixer (hire: £30-50/day)
Tin snips (for cutting beads)
PPE: gloves, safety glasses, dust mask

Materials Required

Sharp sand (plastering grade)
Cement (OPC)
Plasticiser (liquid or powder)
Waterproofer additive
SBR bonding agent (for smooth/painted backgrounds)
Bellcast beads
Corner beads (stainless steel or PVC)
Stop beads
Stainless steel render mesh (if needed)

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

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.
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.
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.
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.
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.
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.
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.
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)

Item	Typical Cost
Sharp sand (bulk bag)	£50-70
Cement (25 kg bag)	£5-7
Render beads (per 2.5 m length)	£2-5
Waterproofer and plasticiser	£10-20
Scaffold tower hire (per week)	£80-150
Total (30 m² wall, DIY materials)	£150-350

Related Calculators

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