How long does a roof clean last?

There's no fixed lifespan for a roof clean. A clean removes or kills the moss, lichen and staining on the day. How long the roof then stays clean depends on shade, nearby trees, the roof material and the cleaning method. The claims Sydney roof cleaners publish run from protection "for up to 12 months" to a recommendation to re-wash "every 5-10 years". None of the claims we reviewed came with independent testing, so treat any single number with caution. Have your roof looked at every year or two.

This guide explains what drives regrowth, how the method changes the result, whether roof cleaning is necessary at all and how to make a clean last.

What Sydney roof cleaners claim, and what's missing

Here is what some Sydney operators publish about how long their work lasts:

• A roofing company says its spray-and-wait treatment "gradually restores your roof over 3-5 months and protects it for up to 12 months".
• A soft-wash specialist recommends "scheduling roof soft washing every 5-10 years". It claims soft washing outlasts traditional pressure cleaning "by 30-70%".
• A licensed roofing firm says it finishes every job "with a biocide treatment that slows regrowth".
• A pressure-cleaning company says its biocide results "can take a few weeks to a few months to fully appear".

Notice what's missing. In the pages we reviewed, none of these claims was backed by independent test results, and the "30-70%" figure has no stated source. The honest answer is that longevity varies from roof to roof, and your own roof's conditions matter more than any headline figure.

Why growth comes back faster on some roofs

Moss, lichen and algae need moisture and shelter. Anything that keeps a roof damp for longer shortens the time between cleans.

• Shade. In Sydney, south-facing slopes get the least sun. Roofs under trees or next to taller buildings also stay damp for longer after rain.
• Overhanging trees. Branches drop leaves that hold moisture and organic matter on the roof. They also block the sun.
• The roof surface. Porous surfaces, such as older concrete tiles and unglazed terracotta, hold moisture and give growth something to grip. Metal roofs shed water faster, but shaded sections can still grow mould and lichen.
• Debris left sitting on the roof. Leaves packed into valleys, and gutters that hold water, keep the lower edge of the roof wet.

Two neighbouring houses can need very different schedules. A sunny, steep metal roof may stay clean for years. A shaded tile roof under a big gum may start to show regrowth much sooner.

How the cleaning method affects how long it lasts

There are two main approaches. We compare them in detail in soft washing vs pressure cleaning.

• Soft washing applies a cleaning solution at low pressure. The solution kills the growth, and the dead material washes away with rain over the following weeks and months. Soft-wash operators argue that this lasts longer because the growth is killed, not just blasted off the top.
• High-pressure cleaning gives an instant visible result. On concrete tiles, though, it can wear away the surface coating. Because of that, pressure cleaning is often followed by a restoration and repaint.

Whichever method is used, ask whether the job ends with a treatment to slow regrowth, and what that treatment is. Several Sydney operators finish with a biocide or fungicide for exactly this reason.

Why a roof may not look clean on day one

This catches many people out. After a soft wash, heavy lichen dies straight away but can take weeks of weather to break up and fall away. One Sydney exterior cleaner puts it this way: "the roof keeps getting cleaner after we leave."

So judge the result over time, not on the afternoon of the clean. A good cleaner will explain the timeline up front and send before-and-after photos. It's also worth asking for a follow-up photo a couple of months later.

Is roof cleaning necessary at all?

Not always. Light, even discolouration on a steep, sunny roof is mostly cosmetic. Cleaning becomes worthwhile when:

• moss and lichen are building up. They hold moisture against the roof surface.
• growth and debris are washing into gutters and valleys, which adds to blockages.
• you're preparing to paint, restore or sell. A clean surface is the starting point for any coating, and a clean roof photographs better.
• growth is spreading onto solar panels, where rain won't shift it.

Be wary of anyone who knocks on your door and tells you your roof is in urgent trouble. In one review we read, a Sydney homeowner described a door-knocker offering gutter cleaning who "tryed to scare me into having a defected roof". If someone makes big claims about your roof, get a second opinion and ask for photos.

How to make a roof clean last longer

• Keep gutters and valleys clear. Dead lichen and leaves wash into them after a clean, so book a gutter clean for a few weeks after a soft wash.
• Trim overhanging branches to let in sun and cut leaf drop. Check whether you need council approval first.
• Look at the roof from the ground once a year, especially the shaded slopes.
• Deal with regrowth early. Small patches are easier and cheaper to treat than a roof-wide layer.

Questions to ask before you book

• What product will you use, and at what strength? Can I see the safety data sheet?
• Is a treatment to slow regrowth included?
• How long do you expect the result to last on my roof, and what is that based on?
• What do I need to do about my rainwater tank? One Sydney operator advises that tank water "should not be used for a few weeks" after a soft wash if it hasn't been diverted.
• Will you clean the gutters afterwards, or should I book that separately?
• Is any re-treatment offer written into the quote, with its conditions spelled out?

A clear, written answer to each of these tells you more than any "lasts for years" headline. For our method and typical prices, see our roof cleaning service.