Applying Render and External Solid Plaster Finishes
Weather, movement and water
External render has to survive rain, sun, movement and salt, so the rules differ from internal work. This domain tests mix design, suction control, movement joints and the causes of external defects.
Core concepts to know
- Mix design: render mixes are usually weaker than the background so movement is taken by the render, not the masonry; too-rich mixes crack and craze. Follow the specification.
- Suction and curing: dampen absorbent backgrounds; do not let render dry too fast in sun or wind - cure it to allow strength gain and reduce shrinkage cracking.
- Coat thickness and keys: build in coats with a scratched key, and keep coats even so shrinkage is uniform.
- Beads and movement joints: fix stop, corner and bellcast beads correctly, and install movement joints where specified and at openings and changes of substrate.
- Weather: avoid application in extremes of heat, wind or frost; protect fresh work.
- Common defects: crazing from rich mixes or rapid drying, cracking at joints and openings, debonding from dusty or smooth backgrounds, and efflorescence from moisture movement through salts.
Traps
Questions often suggest adding more cement to make render stronger - wrong, it increases cracking. Another trap is omitting movement joints over large areas, or applying render to a frozen or saturated wall.
Worked rule of thumb
For external work ask three questions before you start: is the background right and dampened, is the mix no stronger than specified, and will the work be protected from fast drying or frost? If all three are yes, most common render failures are already prevented.