To soft wash a roof, you apply a diluted, bleach-based cleaning solution at low pressure, let it dwell for 10 to 30 minutes so it kills the moss, algae and lichen at the root, then rinse gently or let the rain wash the dead growth away. No high pressure, no scrubbing. The chemistry does the work, and the water just carries it. That is why soft washing is the standard way to clean a roof without stripping it.
If your roof has gone green, black or furry with mould and lichen, you are not doing anything wrong. Brisbane’s humidity does that to almost every roof. The real question is whether to climb up and do the roof soft wash yourself or hand it to someone who does it every week. Here is how the job works, and an honest look at the risks.
What Is Soft Washing, and Why Do Roofs Need It
Soft washing is a roof cleaning method that uses low pressure, well under 100 PSI at the nozzle, with a diluted cleaning solution. The solution kills moss, algae and lichen at the root while the low pressure keeps the surface intact. High pressure does the opposite: it strips the roof and leaves the growth alive.
Instead of a 3,000 PSI blast, you lay a cleaning solution over the roof with a gentle stream, usually under 500 PSI. That solution is diluted sodium hypochlorite, the active ingredient in household bleach, with a surfactant so it clings long enough to work. High pressure only knocks the top growth off and leaves the roots and spores, so the moss and lichen return within months. A soft wash kills the organism at the root, and the dead growth then rinses off or weathers away with the rain.
Brisbane’s climate is close to ideal for this growth. Humid, warm air, summer storms and shade under leafy trees keep tiles damp, and damp is all moss, algae and lichen need. On the northside, from Chermside to Albany Creek, heavier tree cover means established lichen and roof mould are common, so regular roof cleaning here is ongoing maintenance rather than a one-off. More on our roof cleaning service page.
Soft Washing Versus High Pressure on a Roof
High pressure is the wrong tool for a roof. It can crack tiles, strip the protective granules or coating, force water under the overlaps, and it often voids the manufacturer’s warranty. Worse, it does not kill the root, so the growth returns. A soft wash avoids all of that.
On concrete tiles, a high-pressure wand blasts off the fine granules that protect the surface and give the tile its colour, leaving it porous and quick to regrow. On terracotta, it can crack ageing tiles. On the asphalt shingles common overseas, high pressure tears the granules off, and once shingles lose their granules, they fail early. Colorbond and metal roofs fare better, but pressure washing can still drive water under the sheets and strip the coating that holds back rust.
It also forces water past the overlaps and under the tiles, which most manufacturers say voids the warranty, and where it causes leaks and, in time, structural damage. On top of that, a pressure washing clean that only strips the surface is short-lived: the roots survive, regrowth is quick, and you are back to square one within a year. A soft wash that kills the growth at the root lasts three to five years, which makes it cheaper over time, even though it looks gentler.
How to Soft Wash a Roof, Step by Step
If you have weighed the risks below and still want to tackle it, here is how to soft wash a roof properly.
1. Inspect and clear debris
Pick a still, overcast day, never full sun, which dries the solution before it works. Set up a stable ladder and, if you can do it safely, check for cracked or loose tiles, lifted flashing and blocked gutters. Clear leaves and debris, and flush the gutters while you are up there, since you will rinse down into them.
2. Protect plants and mix the solution
The cleaning solution is bleach-based, so soak the surrounding plants with plain water first and cover the ones near downpipes. Mix diluted sodium hypochlorite with a surfactant to the right strength, and wear proper PPE for safety: eye protection, gloves and old clothes. A biodegradable, low-concentration mix is kinder to the garden and still does the job.
3. Apply, let it dwell, then rinse
Apply the cleaning solution at low pressure, working from the bottom up so the roof stays wet and streaks do not form, then let it dwell for 10 to 30 minutes. The moss and algae will darken and die. Rinse gently at low pressure, or on a lightly soiled roof, let the rain carry the dead growth away. Stubborn lichen usually needs a second pass. Never use high pressure to speed it up.
The Safety and Damage Risks to Know
Here is the honest part. Roof work is one of the more dangerous jobs around the home, and safety is the main reason a roof soft wash is often best left to a professional.
Falls from heights are a leading cause of serious injury at home, and a roof made slick with water and cleaning solution is far worse than a dry one. Add a ladder on soft ground and a steep pitch, and it stacks up fast. The chemicals need respect, too, as sodium hypochlorite can burn skin and eyes and harm plants if it runs off, and mixing it wrong is where people get hurt, so PPE and a good rinse-down are not optional.
Then there is the roof itself. Old terracotta and concrete tiles can be brittle, and simply walking on them can crack tiles that you then have to replace, while rusted metal roofs are worse again. Part of the skill in roof cleaning is knowing where you can safely step and reading the condition of the surface before you trust it with your weight.
When to Call a Professional
For most homeowners, a roof soft wash is worth handing to a professional. The mix of working at heights, handling bleach-based chemicals and treading on fragile tiles carries real risk, and a professional brings the gear, the safety training and the insurance to manage it.
Do an honest sum. DIY saves a call-out fee, but only if nothing goes wrong. Weigh that against a fall, a cracked run of tiles or a voided warranty, and the professional job usually wins. If your roof is steep or two storeys, the decision more or less makes itself.
A professional roof soft wash means the right low-pressure equipment, biodegradable chemistry matched to your roof, full plant protection, and a gutter flush while we are up there. With North Brisbane Pressure Washing, you also get an honest report on anything we spot, cracked tiles, lifted flashing or rust, and you deal directly with Tim, the person who quotes it. See the results in our before-and-after gallery.
Because a soft wash kills the growth at the root, results on a Brisbane roof last three to five years, whether you have concrete tiles, terracotta or Colorbond, rather than the twelve months a high-pressure blast tends to buy. A light wash every few years, as maintenance slows regrowth, protects the surface and keeps the curb appeal of the whole house up. It is the same low-pressure method we use for exterior house washing.
Frequently Asked Questions
What is soft washing a roof?
Soft washing a roof cleans it with low pressure and a diluted, bleach-based cleaning solution instead of high pressure. The solution kills moss, algae and lichen at the root, then rinses off or weathers away, leaving the surface clean and undamaged. It is the method most roofing manufacturers recommend.
Why should you not pressure wash a roof?
High pressure strips the protective granules off tiles and shingles, can crack ageing tiles, and forces water under the roof, where it causes leaks. It also voids many warranties and does not kill the growth at the root, so moss and algae return within months. Low-pressure soft washing avoids all of it.
What chemicals are used to soft wash a roof?
The main ingredient is sodium hypochlorite, the active part of household bleach, diluted to a roof-safe strength. A surfactant is added so the cleaning solution clings and dwells long enough to kill the growth. Good operators use biodegradable, low-concentration mixes and rinse surrounding plants to limit runoff.
How long does a roof soft wash last?
Because soft washing kills moss, algae and lichen at the root, results usually last three to five years. A high-pressure clean can look good on the day, but often regrows within twelve months. In Brisbane’s humid climate, a light soft wash every few years keeps regrowth in check.
Ready to have your roof soft-washed without climbing up yourself? Get a free, no-obligation quote from North Brisbane Pressure Washing. Deal directly with Tim, get a fixed price within 24 hours, and have it done safely by a local who cleans roofs every week.
About the Author
The North Brisbane Pressure Washing Team, Soft wash specialists, Brisbane Northside
North Brisbane Pressure Washing is an owner-operated exterior cleaning business serving Brisbane’s northside, from Chermside to North Lakes, Albany Creek and the bayside suburbs. The team specialises in soft washing roofs, houses and driveways, matching the method to the surface so growth is killed at the root without damage. Fully insured, 5-star rated, and local. You deal directly with Tim, the person who quotes the job.
