# Roof cleaning chemicals: what's safe for tiles, Colorbond and rainwater tanks (2026)

> Which roof cleaning chemicals are safe on tiles, terracotta and Colorbond, what BlueScope says about bleach, and how to keep it out of your rainwater tank.

Canonical: https://rnrroofrestorations.com.au/blog/roof-cleaning-chemicals
Last updated: 2026-11-02

The safe roof cleaning chemical depends on what your roof is made of. For a Colorbond roof, BlueScope names one in its own bulletin: a 2% sodium hypochlorite solution (diluted bleach), brushed on, left for 3 to 5 minutes, then rinsed off with plenty of fresh water. For concrete and terracotta tiles, a moss and lichen treatment labelled for that tile does the work, and water or weather takes the dead growth away. Acids, paint strippers and full-strength pool chlorine don't belong on a house roof. No treatment should ever reach your rainwater tank. At R&R Restorations, our [roof cleaning service](/services/roof-cleaning-gold-coast) covers tile and metal roofs across the Gold Coast, Brisbane and Logan, and the Tweed and Northern Rivers, with the method matched to the roof's material and its condition.

Chances are you're standing in a hardware aisle, or about to be. The bottle in your hand says it suits "most types of roofs". It can't see your roof. It doesn't know that your downpipes feed the tank your kettle drinks from.

Here's what the bottle won't tell you.

## In summary

- **Colorbond and painted steel:** BlueScope's TB-27 bulletin allows a 2% sodium hypochlorite wash for 3 to 5 minutes, then a thorough rinse. It warns that stronger mixes, longer contact and commercial fungicides can harm the paint's long-term performance.
- **Concrete and terracotta tiles:** use a treatment labelled for the tile, never an acid. Terracotta's colour is in the clay, so a good clean can bring it back without paint.
- **Never use:** acid brick cleaners, paint strippers, neat pool chlorine, or two products mixed together. Never use any treatment on an asbestos cement roof without specialist advice.
- **Rainwater tanks:** disconnect the downpipes before anything is sprayed. If the roof is being recoated, leave them off for at least two good rains, or until the roof is washed after three weeks of curing (Dulux's roof coating guide).
- **Our price:** high-pressure roof cleaning generally costs $500 to $1,200 (our published cost guide), on its own or as the first stage of a roof restoration.

## What chemicals do professional roof cleaners use?

Professional roof cleaners mostly use sodium hypochlorite, the active ingredient in bleach, or a "spray and leave" biocide that kills moss, mould and lichen slowly. A few use oxygen bleach, and some coating systems use a copper-based moss killer before the roof is pressure cleaned. Each one suits some roofs and not others.

A biocide is simply a chemical that kills living growth. That covers almost everything sold as a roof treatment, so the label word to look for is the active ingredient, not "biocide" or "eco".

| Chemical | What it does | How fast you see it | Where it fits | Watch out for |
|---|---|---|---|---|
| Sodium hypochlorite (bleach, liquid chlorine) | Kills and whitens mould, moss and lichen on contact | The same day | Colorbond at BlueScope's 2% recipe; many tile soft washes | Strength and contact time; burns plants; must stay out of tanks |
| Spray-and-leave biocides (some contain quaternary ammonium compounds, called "quats") | Kills growth, which rain then wears away | Weeks to months | Tiles where you can wait; some labels list metal | BlueScope warns commercial fungicides can harm Colorbond's finish |
| Oxygen bleach (sodium percarbonate or hydrogen peroxide) | Kills moss and mould with a peroxide base | Varies by product | Jobs where plants sit close; roof cleaners call it gentler on them | Still a chemical: keep it out of tanks and follow the label |
| Copper sulphate with ammonium carbonate | Kills moss and lichen before a high-pressure clean | 3 to 6 days, then washed off | Concrete tiles being prepared for a coating | Written for concrete tiles, not metal roofs or metal gutters |
| Non-ionic detergent and water | Loosens dirt and salt | The same day | Unpainted Zincalume and routine metal washes | Not a moss or mould killer |
| Acids and caustic strippers | Dissolve cement, metal coatings or paint | Immediately, often permanently | No house roof | Etched tiles, stripped coatings, damaged pointing |

Two of those rows come straight from manufacturers' instructions. [BlueScope's TB-27 bulletin](https://cdn.dcs.bluescope.com.au/download/tb-27) (Revision 6, May 2022) sets out the Colorbond recipe. Dulux's datasheet for [Acratex Roof Membrane](https://files.duspecplus.com.au/public/pdf/datasheet/29659596-3140-4A9D-863C-3D3C28DC1E8E) gives the copper mix for concrete tiles: 20 grams of copper sulphate and 70 grams of ammonium carbonate in a litre of water. It's left for 3 to 6 days, then the roof gets a high-pressure wash at a minimum of 4,000 PSI before coating.

Some products sold as one bottle are really two of these rows combined. One professional Australian roof wash concentrate contains a quat biocide and is mixed on site with 12.5% sodium hypochlorite. Its label mix is 200 mL of concentrate, 2 litres of 12.5% hypochlorite and 17.8 litres of water. That works out at about 1.25% hypochlorite, left to dwell for 10 to 30 minutes ([Evolving Elements, Roof Restore Pro](https://evolvingelements.com.au/product/roof-restore-pro/), November 2026).

## Can you clean a roof with bleach?

Yes, but only diluted, only for a few minutes and only on a roof it suits. BlueScope's TB-27 recipe for Colorbond is a 2% sodium hypochlorite solution, applied with a soft-bristle broom, left for 3 to 5 minutes and washed off with "copious amounts of fresh potable water". Neat bleach, long soaks and home-made "stronger" mixes are a different job, and a riskier one.

Bleach strength is the part most people get wrong. Unscented household bleach in Australia is about 4 to 5% available chlorine ([NSW Health](https://www.health.nsw.gov.au/environment/water/Pages/rainwater.aspx)), and pool liquid chlorine is about 12.5%. So "2%" is the final mix, not 2% bleach in a bucket of water.

| Bleach strength on the label | Bleach per litre of 2% mix | Water per litre |
|---|---|---|
| 12.5% (pool liquid chlorine) | 160 mL | 840 mL |
| 10% | 200 mL | 800 mL |
| 5% (typical household bleach) | 400 mL | 600 mL |
| 4% | 500 mL | 500 mL |
| 2% | No dilution | None |

The 10%, 5%, 4% and 2% rows are BlueScope's own Table 1 in TB-27. The 12.5% row is the same sum. BlueScope also says a little non-ionic detergent, such as dishwashing liquid, can be added to help the mix wet the surface.

A popular American roof cleaning video ([Exterior Cleaning 4 Real, September 2024](https://www.youtube.com/watch?v=SmKK6u3muWs)) puts the general roof washing mix at 5 to 6% hypochlorite. That's two and a half to three times BlueScope's figure for Colorbond. Advice written for one roof doesn't transfer to another just because the chemical has the same name.

Two more rules apply to every bleach job. Never mix bleach with another cleaner, because some combinations release toxic chlorine gas. And bleach kills plants, so garden beds under the eaves need wetting down and covering before the job starts and rinsing afterwards.

## Which chemicals are safe on a Colorbond or metal roof?

On Colorbond, BlueScope's own TB-27 recipe is the safe choice for black mould and lichen: 2% sodium hypochlorite for 3 to 5 minutes, then a thorough fresh-water rinse. BlueScope warns that higher concentrations, longer contact times or "the use of commercially available fungicides may be detrimental to the long term performance" of Colorbond steel. For plain dirt and salt, fresh water is enough.

That warning is where the bottle and the roof maker part ways. A spray-and-leave product works by sitting on the roof for weeks, and some list "colour steel" among their surfaces. BlueScope's bulletin is about the paint that is meant to keep your roof from rusting. When the two disagree, we'd follow the people who made the paint.

The common Colorbond problems, and what suits each:

- **Black spots near the ridge capping.** TB-27 calls this the "ridge cap effect": moisture and nutrients escaping from the roof space feed fungus along the ridge. It's growth, not rust, and the 2% wash removes it.
- **Dark staining across the whole roof.** Also usually fungus. BlueScope says neighbouring fences and buildings typically show the same growth. Its spot test is one drop of bleach on the mark: fungus clears within a few minutes, and ordinary dirt doesn't. It isn't foolproof, but it separates growth from dirt. Try it on the fence, not from the roof.
- **Salt build-up on coastal homes.** Fresh water, more often on the areas rain doesn't reach, such as under the eaves. BlueScope's [Colorbond maintenance page](https://colorbond.com/why-colorbond-steel/maintenance) says to hose those down at least every six months, and more often near the sea.
- **Unpainted Zincalume.** BlueScope's [overpainting bulletin TB-2](https://cdn.dcs.bluescope.com.au/download/tb-02) has it washed with household detergent and water before any paint goes on.

Rust, chalky paint and failed screws are not cleaning problems. No chemical fixes them. Our step-by-step [guide to cleaning a Colorbond roof](/blog/how-to-clean-a-colorbond-roof) covers the washing method itself. When the clean shows rust or failed screws, those go into a separate repair or roof restoration quote.

## What's safe for concrete and terracotta roof tiles?

For both concrete and terracotta tiles, use a moss and lichen treatment labelled for that tile, then let water remove the dead growth: rain over the following months, or a pressure clean on the day. Never put acid on either. Concrete tiles and the mortar under your ridge caps are cement, and acid eats cement.

**Concrete tiles** take most treatments well, but their colour coat is thin and wears away over the years. A clean can make a worn concrete roof look patchier, because it washes off the dirt that was hiding the bare spots. That's normal, and it's why a clean before a recoat has to be thorough.

**Terracotta tiles** are fired clay, and the colour runs right through them. That makes chemical cleaning terracotta roof tiles the most rewarding job on this page. Kill the growth and wash it off, and the original red or orange comes back with no paint at all. Unglazed terracotta is porous, so test any product on one tile first and rinse well. Glazed terracotta has a glassy surface that needs only a gentle treatment. Never scour it.

| Your roof | Suitable treatment | Avoid | What usually comes next |
|---|---|---|---|
| Concrete tiles, original finish | A roof treatment labelled for concrete tiles | Acid, caustic strippers | Sealing and recoating if the colour coat has worn through |
| Concrete tiles, previously painted | A gentle, labelled treatment and a careful rinse | Anything that softens or lifts paint | Recoating over a sound, prepared surface |
| Unglazed terracotta | A labelled treatment, tested on one tile first | Acid, scouring, guessed bleach mixes | Broken tiles and ridge joints; a coating is optional |
| Glazed terracotta | A gentle treatment and rinse | Scouring, paint | Repairs only |
| Colorbond | BlueScope's 2% hypochlorite recipe, or fresh water | Stronger mixes, long soaks, commercial fungicides | Rust and screw repairs if the clean finds them |

A pale white haze on concrete tiles is often efflorescence, natural salts coming out of the concrete, not a stain to scrub. Our [tile roof cleaning guide](/blog/tile-roof-cleaning) explains how to tell it apart from growth.

## What should you never put on a roof?

Never put acid, paint stripper, undiluted pool chlorine or a mix of two cleaners on a house roof. And never treat an asbestos cement roof yourself. These are the mistakes that turn a $60 bottle into a damaged roof, a dead garden bed or an injury.

1. **Acid and "brick cleaner".** Hydrochloric (muriatic) acid etches concrete tiles, eats cement-based ridge bedding and attacks the zinc and aluminium coating that protects metal roofing.
2. **Paint strippers and heavy degreasers.** They're made to remove coatings. On a painted roof that's exactly the problem.
3. **Neat pool chlorine.** At 12.5% it's more than six times BlueScope's Colorbond strength, and it scorches plants on the way down.
4. **Two products at once.** Bleach mixed with acids or ammonia-based cleaners can release toxic gas. Use one product, rinse it off, and then use the next.
5. **More than the label says.** If a patch survives, it needs more time, not a stronger dose. Lichen can stay visible for months after it's dead.
6. **Anything on an asbestos cement roof without advice.** One spray-and-leave label at Bunnings lists "super 6" among its surfaces ([30 Seconds Roof Treatment](https://www.bunnings.com.au/30-seconds-5l-roof-treatment-concentrate_p4460168), November 2026). Super Six is a James Hardie corrugated fibro sheet, and older sheets can contain asbestos. [WorkSafe Queensland warns](https://www.worksafe.qld.gov.au/news-and-events/alerts/incident-alerts/2026/high-pressure-water-cleaning-on-an-asbestos-roof) that high-pressure cleaning of asbestos is illegal. We don't work on asbestos roofs. Our [asbestos roof guide](/blog/asbestos-roof) explains your options.

## Are roof cleaning chemicals safe for rainwater tanks?

No. No roof cleaning chemical should go into a rainwater tank. The safe method is to disconnect the downpipes that feed the tank before anything is sprayed, keep them off until the roof has been rinsed clean, and only then reconnect. [Bunnings' DIY guide](https://www.bunnings.com.au/diy-advice/building-construction/roofing/how-to-clean-roof-tiles) says to "disconnect any water tanks from the gutters so no chemicals get in to your water storage", and BlueScope's TB-27 says care "MUST be taken" to keep hypochlorite out of tanks.

The numbers show why. NSW Health disinfects a contaminated tank with about 5 milligrams of chlorine per litre of water ([NSW Health, rainwater tanks](https://www.health.nsw.gov.au/environment/water/Pages/rainwater.aspx)). A 2% roof wash is 20 grams per litre. Just 10 litres of that wash running into a 5,000 litre tank would add about 40 milligrams per litre: eight times the dose NSW Health uses to disinfect a contaminated tank. Spray-and-leave biocides aren't made for drinking water at all, and they're designed to stay on the roof through the next rains.

### Tank checklist for a roof clean

**Before the clean**

- Find every downpipe that feeds a tank, including any wet system pipes running underground.
- Disconnect them, or divert them to a garden-safe or lawn outlet. Dulux's [metal roof coating guide](https://www.duluxtrade.com.au/technical-advice/application/roof-application-guides/roof-coating-system-for-metal-and-commercial-roofs/) says to disconnect tank downpipes before roof restoration work starts.
- If the tank is your only drinking supply, fill a few containers of drinking water first.

**During the clean**

- Keep treatment off solar panels and skylights unless the product label says they're fine.
- Keep wash water out of the stormwater drain. Under Queensland's Environmental Protection Act 1994, letting polluted roof-cleaning water into drains or waterways is an offence ([Fraser Coast Regional Council fact sheet](https://www.frasercoast.qld.gov.au/files/assets/public/v/1/council/fact-sheets/documents/water_pollution__roof_cleaning_and_restoration.pdf)).

**After the clean**

- After a clean alone, rinse the roof and gutters thoroughly and let at least one decent rain run to waste before reconnecting. NSW Health's advice after a chemical overspray is to clean the roof or wait until after the next rainfall before reconnecting a drinking tank.
- After a recoat, wait longer. Dulux says to leave the downpipes disconnected "for at least two significant rains", or wash the roof after three weeks of curing.
- Empty and clean your first flush diverter, the fitting on the downpipe that catches the dirtiest first water, before reconnecting.

If you think treatment has reached your tank, don't drink from it. Call your local public health unit for advice before you use the water again.

If your roof feeds a drinking tank and you're planning a roof restoration, ask whether the coating is made for drinking water collection. Some are. Dulux's Acratex Roof Membrane datasheet, for one, says it is "formulated for collection of potable drinking water".

## Spray-and-leave, soft wash or pressure clean: which do you need?

Use a spray-and-leave treatment if you can wait months for a roof to clear and only want the growth gone. Use a soft wash (a low-pressure chemical clean, rinsed the same day) on a roof that needs to look clean now but shouldn't take pressure. Use a high-pressure clean when the roof is being repaired or recoated, because the coating needs a fully clean surface to stick to.

| | Spray-and-leave | Soft wash | High-pressure clean |
|---|---|---|---|
| Visible result | Weeks to months | The same day | The same day |
| Chemical left on the roof | Weeks | Minutes, then rinsed | Little or none, depending on pre-treatment |
| Best for | Light growth, sheds, patient owners | Growth on roofs that can't take pressure | Preparation for repairs and recoating |
| Coating preparation | Not on its own | Not on its own | What Dulux specifies before its roof coatings (4,000 PSI minimum) |
| Tank risk | Until the product has washed off | During the job | During the job, if a treatment is used first |

Our high-pressure roof cleaning is matched to the roof's material and its condition, so a concrete tile, a terracotta tile and a Colorbond sheet aren't treated the same way.

## Should you clean your roof yourself or call us?

A DIY treatment is cheap, and the bottle isn't the expensive part. Bunnings sells a 5 litre roof treatment concentrate for $59.99 that makes 25 litres (November 2026). The real costs are the fall risk, the cracked tiles from walking, and a tank or garden full of chemical. Monier, the tile maker, recommends homeowners don't clean their own roof tiles at all, because of falls and tile damage.

A professional clean with us generally costs $500 to $1,200, depending on roof size, height, pitch and how much growth has built up. Our [roof cleaning cost guide](/blog/roof-cleaning-cost) breaks that down by roof type. The bigger difference is what happens next. A clean shows every cracked tile, loose ridge cap and rusted screw that the moss was hiding. We tell you what we found and quote any repairs separately, so you know exactly what you're paying for.

Julie put it this way in her Google review of 20 October 2026:

> "They did a phenomenal job bringing our tired roof back to life, including fully cleaning it and completely repointing the ridge caps. The ridge line looks incredibly clean and secure now, and the whole roof is completely watertight."

## Is R&R Restorations right for you?

We're the right call if you want a clean tile or Colorbond roof without gambling on the bottle, and you'd rather the tank, the garden and the paint all come through untouched. We're owner-operated by Alex Rowe, we hold QBCC Licence No. 15404087, and we're rated 5.0 from 10 Google reviews.

**We're a good fit if:**

- Your roof is concrete tile, terracotta or Colorbond metal, on the Gold Coast, in Brisbane or Logan, or in the Tweed and Northern Rivers.
- Your roof feeds a rainwater tank and you want the clean planned around it. Tell us about every tank connection when you enquire.
- You want to know whether a clean is enough, or whether the roof needs repairs or a full roof restoration. Each item is listed separately in our itemised quote.
- You're preparing a house for sale and want the colour back on the day, not months later.

**We're not the right fit if:**

- Your roof is asbestos cement, or might be. We don't clean, seal or repair asbestos roofs.
- You'd rather treat a small, low shed roof yourself and let the weather finish the job. A labelled product, a ladder and some patience may be all it needs.
- You're outside our service areas.

Rodney Petersen's Google review of 22 April 2024 says Alex "turned my old rusty roof to look brand new."

Pick R&R Restorations if you want your roof clean and its true colour back without guessing at chemicals, so you spend only on what the roof actually needs.

## Get your roof cleaned without risking the tank

Send us your suburb, your roof type if you know it, and whether the roof feeds a tank, through our [roof cleaning enquiry form](/services/roof-cleaning-gold-coast#quote), or call us on 0422 630 075. Photos taken safely from the ground help, but you don't need to climb up. We'll assess the roof's material, condition, pitch and access, and give you an itemised quote before any work is booked.

## Frequently asked questions

### What is the best chemical to clean a roof?

The one your roof maker or the product label names for your roof material. On Colorbond, that's BlueScope's 2% sodium hypochlorite wash. On tiles, it's a moss and lichen treatment labelled for concrete or terracotta. For plain dirt and salt on metal, fresh water does the job.

### Is it OK to use bleach on roof tiles?

Diluted bleach is used on many tile roofs, but it must suit the tile and any coating, be rinsed off, and stay out of the tank and the garden. Never use it neat, and never mix it with another cleaner. Test it on one tile first, especially unglazed terracotta.

### Does bleach damage Colorbond?

It can, if the mix is too strong or left on too long. BlueScope's limit is a 2% solution for 3 to 5 minutes, rinsed off with plenty of fresh water.

### How long after roof cleaning can I reconnect my water tank?

After a clean alone, reconnect once the roof and gutters have been rinsed and at least one good rain has run to waste. After a recoat, Dulux says to wait for at least two significant rains, or wash the roof after three weeks of curing. Always follow the product label if it says longer.

### Do spray-and-leave roof cleaners work?

Yes, slowly. They kill moss, mould and lichen, and rain wears the dead growth off over weeks or months. They suit owners who can wait, but they don't prepare a roof for a coating.

### Is vinegar a good roof cleaner?

Vinegar is a weak acid. It isn't a roof treatment, and acids don't belong on concrete tiles, cement-based ridge bedding or metal roofing. Use a product made and labelled for your roof material instead.


## Related

- [Roof restoration advice](https://rnrroofrestorations.com.au/blog)
- [Get a roof restoration quote](https://rnrroofrestorations.com.au/contact)