Sydney is built on and out of Hawkesbury sandstone. It is workable, it is beautiful, and it is soft — which is exactly why so much of it gets destroyed by well-intentioned cleaning.
Unlike most cleaning mistakes, this one has no remedy. A scorched driveway can be re-cleaned. A stripped roof tile can be replaced. Sandstone that has had its face cut off has to be redressed by a stonemason or replaced outright.
What pressure does to the stone face
Sandstone is compressed sand held together by a natural cement. The outer few millimetres — the weathered face — is denser than the material behind it and acts as the stone’s own protective skin.
A pressure washer strong enough to lift black biological staining will cut straight through that face. What is left behind is:
- Pitting. The softer grains erode faster than the harder ones, leaving a pocked, uneven surface that catches dirt more readily than it did before.
- An unnaturally light colour. Freshly cut sandstone is pale and raw. It reads as damaged rather than clean, and it will not weather back to matching the surrounding stone in any useful timeframe.
- Scoured mortar joints. On older work the mortar is lime-based and considerably softer than the stone. Pressure removes it entirely, and the joints do not go back without a mason.
- Accelerated decay. With the protective face gone, the stone absorbs more water and holds it longer, so growth and salt damage return faster.
Why it is worse on heritage work
Nineteenth-century sandstone was laid in lime mortar, which is deliberately softer than the stone so that the joints fail before the blocks do. It is a sound principle that becomes a liability the moment someone points a lance at it.
Repointing a heritage wall is specialist work. A cleaning quote that turns into a repointing bill is a bad afternoon, and it is entirely avoidable.
How it should be done
Low pressure, with a treatment that draws staining and biological growth out from within the stone rather than removing the surface that holds it. The process is slower and far less dramatic to watch. The stone comes back to its own colour rather than to a raw cut face.
This is the same soft wash logic behind our roof cleaning and house washing: kill the organism and let it release, rather than blasting off the surface it is attached to. Full detail is on the sandstone cleaning page.
Where sandstone and concrete meet
A common complication: sandstone flagging used as an entry path or driveway apron, running straight into plain concrete. These two surfaces need completely different pressures on the same job, metres apart.
Anyone treating the whole area at one setting will either leave the concrete dirty or damage the stone. The concrete side of that work sits under driveway cleaning, and the two get quoted as separate line items for exactly this reason.
On sealing
Sealing after cleaning slows reabsorption and makes the next clean easier, particularly on horizontal surfaces and anything under tree canopy. It is genuinely optional. On a vertical wall with good exposure it is often unnecessary, and we would rather say so than add it to every quote.
The question to ask
Before anyone touches sandstone on your property, ask what pressure they intend to use and what they expect the stone to look like afterwards. If the answer involves the words “like new”, be careful. Cleaned sandstone should look like clean old sandstone, not like new sandstone. The second result means the old surface is gone.
Written by the team at The Prestige Standard, exterior cleaning contractors working across Sydney, including harbourside and heritage properties. Fixed written quotes with every surface itemised, $20 million public liability. See our service areas or call 0494 810 609.
Photo by JKalina on Unsplash.
