Exterior cleaning guarantees are often written to sound like a promise about the future of your roof. They cannot be. The biology does not allow it, and an operator who implies otherwise is either confused or selling.
Here is what can honestly be promised, what cannot, and how to read the difference.
Why regrowth cannot be guaranteed away
Algae, moss and lichen do not arrive because a surface was dirty. They arrive as airborne spores, constantly, on every property in Sydney.
Whether they establish depends on conditions nobody controls: how much shade the surface sits in, how long it stays damp after dew, which way it faces, what is overhanging it, how porous it is, and how humid the microclimate is. A south-facing wall under a fig in Bellevue Hill and a north-facing wall in full sun on the same street are different worlds.
So a clean does not immunise a surface. It resets it. The rate at which it loads up again is set by the site. More: what the green film actually is and why moss returns every winter.
What a guarantee can legitimately cover
That the correct method was used. This is the real one. Soft washing where pressure would damage, no acid on carbonate stone, no film-forming sealer on material that has to breathe, no pressure on a roof. That is a commitment about conduct, and it is enforceable.
That the work was completed as scoped. Every gutter run cleared and flow-tested, every elevation washed, the downpipes actually checked rather than assumed.
That the agreed result was achieved. Not perfection — the specific result written on the quote, including the stains that were flagged as unlikely to fully lift.
That damage caused by the work is made good. This is the part that costs money and the part most worth having.
The soft wash exception, which is not a guarantee but is often mistaken for one
A soft-washed roof keeps improving for weeks after the crew leaves. The treatment kills the organism; weather and UV break down and wash away the dead material.
That means the honest commitment on a soft wash is not about month twelve, it is about week four: if the growth has not released as expected, someone returns and deals with it, and that return is part of the original job rather than a new charge.
Heavy lichen in particular is often a two-visit result from the outset, and it should be quoted that way rather than discovered later. More: lichen and the first 48 hours after a soft wash.
Where sealing changes the picture
Sealers are the one place a time-based figure is meaningful, because a sealer is a product with a manufacturer’s performance expectation behind it.
Even there, the number depends on traffic, exposure and the substrate. A penetrating sealer on a sheltered sandstone wall and the same product on a driveway under car tyres will not last the same time. The honest version quotes the manufacturer’s expectation and names the conditions, rather than printing a flat figure.
And a sealer applied over a stain, over damp stone, or in the wrong weather window fails early regardless of what the tin says. More: how long a seal really lasts.
The guarantee you already have, which nobody can write away
Under the Australian Consumer Law, services carry statutory guarantees. They must be supplied with due care and skill, be fit for the purpose you made known, and be delivered in a reasonable time. These apply to every job and cannot be excluded by anything written in a contract.
A clause disclaiming responsibility for damage caused by the operator’s own method does not survive contact with that. If someone pressure washes your render off the wall, the disclaimer in their terms is not the end of the conversation. The ACCC sets out the guarantees plainly.
This is also why a long marketing guarantee from an uninsured operator is close to worthless. A promise is only as good as the entity standing behind it. More: public liability cover.
How to read a guarantee you are offered
Ask what it covers rather than how long it runs. A twelve-month guarantee covering nothing is worse than a four-week commitment to return and finish releasing growth.
Look for the exclusions. A guarantee with none has not been thought about. Reasonable exclusions include new staining from a source nobody controls, a neighbour’s overhanging tree, storm debris, and surfaces that were flagged as already failing.
Check it is written on the quote. A verbal guarantee at the door is not a document.
And check what triggers it. A guarantee requiring you to prove the surface was not walked on, rained on or touched is a guarantee designed not to pay.
What honest actually sounds like
It sounds like a maintenance interval rather than a promise. On a shaded south elevation in a damp pocket, the realistic answer is that it will need doing again sooner than the sunny side, and the useful thing we can offer is telling you when rather than pretending otherwise.
