The most common rendering mistake I see is people applying the second coat too soon. They put on the scratch coat, come back the next morning and immediately apply the finish coat. The scratch coat has skinned over but the depth has not gone off. The finish coat traps moisture, movement occurs as the scratch coat cures behind it, and the finish coat cracks.
The timing between coats matters as much as the mix ratio. So does everything that happens before you start.
Before you start: what you need
- Sharp sand (coarse, suitable for rendering)
- Portland cement
- Plasticiser
- Clean water
- Scratch coat tools: darby float, hawk, gauging trowel, scratch comb
- Finish coat tools: plastic float (for textured finish) or steel trowel (for smooth)
- Level and straight edges
- Masking tape for windows and reveals
- PVC beads (angle beads for corners, stop beads for edges)
- Bonding agent if required
Use the Rendering Calculator to work out how many bags you need before buying materials.
Step 1: Prepare the substrate
Clean the surface thoroughly. Remove dust, loose material, efflorescence, paint, algae and any contamination. A stiff brush and garden hose is sufficient for lightly soiled surfaces. A wire brush or pressure washer for heavier contamination. Any paint or bonding agent incompatible with cement must come off.
Fill large voids. Cracks over 5mm wide should be filled with a dubbing-out coat (1:3 or 1:4 mix) and allowed to harden before the main render is applied. Large voids can also be packed with a weak mortar.
Attach PVC beads:
- Angle beads at external corners - these give a straight, robust arris edge
- Stop beads at window reveals, door frames and the top of rendered areas
- Fix with small dabs of mortar
Damp the surface. Before applying the scratch coat, dampen the substrate so it does not draw moisture from the fresh render too quickly. On a hot day, damp twice with a hand sprayer. The surface should be uniformly moist but not wet - no standing water.
Step 2: Mix the scratch coat
Mix ratio for scratch coat: 1:4 cement:sharp sand (adjust slightly for substrate - 1:5 for soft brick)
Add a capful of plasticiser per bag. Mix dry first, then add water gradually until you reach a consistency that holds its shape on the hawk but is workable and does not crumble.
The mix should be slightly stiffer for the scratch coat than the finish coat - it needs to be keyed (scratched) while it is firm, not soupy.
Step 3: Apply the scratch coat
Apply the scratch coat using a rendering trowel (or hawk and trowel for smaller areas), working from the bottom up and from the bead edges inward. Aim for 8-12mm depth.
Use a darby float or straight edge to level the surface horizontally. Work to get it as flat and plumb as possible - the scratch coat sets the shape of the wall; the finish coat cannot correct major bumps.
Key the surface. While the scratch coat is still firm but not fully hard (typically 2-4 hours after application), scratch the surface with a horizontal comb pattern. Use a lath with nails driven through it, or a proprietary scratch comb. The scratches give the finish coat mechanical key. Horizontal scratches are better than diagonal - vertical scratches can cause the finish coat to slide.
Allow to cure. Do not apply the finish coat for at least 24 hours - longer if the weather is cold or the coat is thick. Many renderers wait 3-7 days on multi-storey buildings to allow full shrinkage. If the scratch coat is still damp to the touch, wait.
Step 4: Mix and apply the finish coat
Mix ratio for finish coat: 1:5 to 1:6 cement:sharp sand (slightly weaker than scratch coat)
The finish coat should be mixed to a slightly wetter consistency than the scratch coat to help it flow and achieve the desired texture.
Damp the scratch coat lightly before applying the finish coat.
Apply the finish coat at 6-8mm thickness. Work in manageable areas - on a sunny day, do not apply more than you can texture before the surface begins to skin.
Texturing options:
- Textured/roughcast finish: Pat the surface with a plastic float in a circular motion while still wet
- Smooth finish: Steel trowel, working the surface as it firms up - requires experience and timing
- Wood float (fine texture): Circular float motion with a wooden float
Step 5: Protect while curing
Keep fresh render out of direct sun and away from frost for at least 24-48 hours:
- Hessian sacking dampened and loosely draped over the wall
- Polythene sheeting held slightly off the surface (not touching - condensation causes staining)
In hot weather, mist the render surface lightly during the first day to slow the drying. This reduces shrinkage cracks.
Common rendering problems and causes
| Problem | Likely cause |
|---|---|
| Map cracking (crazing) | Finish coat applied too soon, over-trowelling |
| Large cracks | No movement joints, substrate movement |
| Delamination (render falls off) | Poor substrate prep, surface too smooth, no damp before render |
| Staining/efflorescence | Salts migrating through render from substrate |
| Hollow areas | Air pockets, inadequate key, second coat too thick |
| Algae/green staining | No movement joints trapping moisture, render too porous |
My tips on rendering
Work between 5°C and 25°C. Never render in frost or near-freezing conditions. Avoid rendering in direct summer sunshine on a south-facing wall - the surface dries before the depth cures.
Plan the movement joints from the start. External render on a building needs movement joints at door and window reveals, at changes of substrate (e.g. brick to block) and at maximum 5m intervals across large plain areas. Without them, cracking is nearly inevitable.
Use beads everywhere. Corner beads, stop beads and bell-cast beads at the base are not optional on a professional job. They give the work straight lines, protect the vulnerable edges, and help shed water away from the building base.
Make the finish coat weaker than the scratch coat. This is fundamental rendering practice. A strong finish coat on a weaker scratch coat creates a differential that causes the finish to crack and pull away. Each subsequent coat should be slightly weaker than the one below.
Use the Rendering Calculator to work out exactly how many bags of render you need.