Render is the most common wall finish on newer Sydney homes and the one most frequently damaged by cleaning. The damage is usually invisible on the day and obvious a year later.
Check the wall first
Two checks, both quick, both routinely skipped.
Tap it. Work across the wall with a knuckle. Sound render is dull and solid. Delaminated render knocks hollow. Drummy render will come off under pressure, and the clean becomes a repair.
Look at the cracks. Hairline crazing is cosmetic. Straight vertical cracks at window corners are movement. Diagonal stepped cracking is structural. Water driven into any of them goes behind the render and spreads the delamination.
The detail is in cracked render: what cleaning will and won’t fix.
Which render you have
Cement render — traditional sand and cement, usually painted. Rigid, cracks more readily, patches well.
Acrylic render — polymer modified, more flexible, less permeable. When it does crack, water behind it has nowhere to go.
Texture coat over foam or lightweight panel — the one to be most careful with. The substrate dents under pressure and water entry into foam is a serious problem. See foam cladding: the wall that dents under a pressure washer.
Full comparison in cement render, acrylic render and texture coat.
The method
Render is a soft wash surface. The growth is killed chemically and rinsed at low pressure — pressure does not kill mould, it removes the visible layer and it returns.
- Protect and pre-soak the garden. Hypochlorite kills foliage on contact.
- Close windows and doors, and identify weep holes, vents and electrical points.
- Wet the wall with plain water first.
- Apply the mix bottom to top, so runs do not streak dry wall below.
- Let it dwell. This is where the work happens.
- Rinse top to bottom at low pressure, wide fan, kept moving.
- Rinse the garden and surrounds.
The chemistry is in what is actually in a soft-wash mix.
What pressure does to it
On painted render, high pressure removes the paint film in patches, leaving a mottled wall that needs repainting rather than cleaning.
On texture coat it flattens the texture, which cannot be restored without recoating.
And on any render it drives water through hairline cracks and into the substrate, which is the mechanism behind delamination later.
Also never aim at the weep holes along the base of the wall. They are drainage, and forcing water in reverses their function.
Common problems
Green and black growth on shaded elevations. The standard case, responds completely — here.
Streaking below windows and sills. Usually a sill with no drip groove, or a blocked one, letting water run back against the wall rather than dripping clear.
Staining below a gutter joint — a leak, and cleaning it is temporary until the gutter is fixed.
White bloom. Efflorescence, meaning water is moving through the wall. Brush off dry and find the water — here.
Rust streaks from a fixing or a lintel behind the render. The cause is inside the wall.
How long it lasts
Two to four years in Sydney. Less on a south elevation under canopy, less again near a lagoon or in a gully.
Extending it means dealing with the conditions: cut back vegetation for light and airflow, fix the gutter or drainage keeping the wall damp, and treat annually once clean.
For a quote, see render cleaning.
