Quick answer: House washing and water blasting are different jobs. A soft house wash uses low pressure and cleaning chemistry to lift dirt, mould and salt off painted cladding without forcing water into the building. Water blasting uses high pressure to strip hard surfaces like concrete and unsealed paths. Most Auckland homes need the wash, not the blast.
Hire a water blaster on a Saturday morning and you can strip fifteen years of grime off a driveway before lunch. Point the same machine at a painted weatherboard wall and you can drive water behind the cladding, lift the paint edges, and turn a cosmetic job into a repair bill. The machine does not know the difference. That decision sits with whoever is holding the wand.
This is the question we get asked more than almost any other before an exterior repaint: does the house need a wash or a blast? People use the two terms interchangeably, and plenty of Auckland quotes list “water blasting” when what is actually being sold is a soft chemical wash at garden-hose pressure. The words matter, because the wrong method on the wrong surface causes damage that shows up months later as peeling paint, blown plaster, or a damp patch on an internal wall.
What follows is the working version we use on site. Which method suits which Auckland cladding, what actually kills moss and mould (and how long it takes), what the New Zealand codes and Auckland Council rules say about pressure and wash water, and how often the outside of your place should be cleaned. Real product data, real intervals, no guesswork.
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House Washing and Water Blasting Are Two Different Jobs
The simplest way to separate them: one relies on chemistry and time, the other relies on force. Once you see it that way, most of the confusion disappears and the right method for your place becomes obvious.
What a soft house wash actually does
A soft wash applies a diluted cleaning solution to the cladding, gives it time to work, then rinses it off at low pressure. The pressure is there to carry the solution and flush the residue away, not to cut through the dirt. On a painted surface that is exactly what you want, because the paint film is the thing you are trying to protect.
The chemistry does the heavy lifting. Resene’s Paint Prep and Housewash is a blend of anionic and nonionic surfactants supplied as a concentrate, diluted one part product to three parts water, and covering roughly 10 to 20 square metres per litre of diluted mix. It is designed to clean surfaces ready for painting, or to be used annually to keep a house looking fresh. That annual use is the part most homeowners skip.
Two limitations on that data sheet are worth knowing before anyone starts spraying around your windows. Mild steel and aluminium are attacked by the product, and concentrate left on glass can etch the surface. On a home with aluminium joinery, which is most Auckland homes built since the 1970s, that means controlled application and a proper rinse rather than a blanket soaking.
What water blasting actually does
Water blasting removes material by force. It is the right tool for concrete driveways, paths, retaining walls, unpainted block, tilt slab and pavers, where the surface is hard enough to take the impact and there is no coating to lift. On those surfaces it is fast, effective and hard to beat.
The trouble starts when high pressure meets a building envelope. Cladding systems are designed to shed water travelling downward under gravity and wind. They are not designed to resist a jet driving water upward under laps, into vent gaps, around window flashings and behind soffits. Water that gets in that way has no obvious path out.
Pressure, with an actual number attached
Vague advice to “use low pressure” is not much use when you are standing in a hire shop. The New Zealand Metal Roof and Wall Cladding Code of Practice puts a figure on it for metal roofing and cladding: low-pressure water blasting up to 20 MPa, with the jet directed away from openings and sheet laps. That is around 2,900 psi, and it is a ceiling rather than a target. The same guidance recommends washing with a hose and a soft bristle brush using fresh water or a mild household detergent solution, cleaning from the top down and rinsing thoroughly.
Plenty of consumer machines advertise pressures at or above that ceiling. Being legal to buy is not the same as being safe to point at your house.
What a house washing quote should actually tell you
Because the two terms get used loosely, a one-line quote saying “wash exterior” leaves you guessing. Three things separate a real scope from a hose-off. First, whether a moss and mould treatment is included as a separate step with its own dwell time, or whether the whole thing happens in one visit. Second, what happens to the runoff, which tells you whether the crew has thought about your garden and the nearest street drain. Third, whether the wash is a pre-paint preparation stage or a standalone maintenance clean, because those are priced and sequenced differently.
A pre-paint wash has to leave a surface that a coating can bond to, which means dead growth scrubbed off, chalk removed, and the surface dry before anything else happens. A maintenance wash between repaints is a lighter job with a different goal: keep the existing coating clean so it reaches the far end of its service life. Asking which one you are being quoted for usually clears up a surprising amount.
| Method | How it works | Right for | Main risk |
|---|---|---|---|
| Soft house wash | Diluted detergent, dwell time, low-pressure rinse | Painted weatherboard, plaster, fibre cement, joinery, soffits | Overspray onto plants and adjoining surfaces |
| Moss and mould treatment | Hypochlorite wash left to kill growth, then rinsed | Any surface with visible mould, lichen or moss | Bleaching of fabrics and foliage; needs 48 hours |
| Low-pressure blasting | Controlled pressure under the 20 MPa ceiling | Metal roofs, hard-wearing unpainted cladding, sound concrete | Water forced under laps and flashings |
| High-pressure blasting | Mechanical stripping by force | Driveways, paths, retaining walls, unpainted block | Gouging soft substrates; stripping coatings |
🎨 Painting tip: If a quote says “water blast and paint” with no mention of a moss and mould treatment, ask what is being used and how long it will be left on. A same-day wash-and-paint on a mouldy wall is a coating failure waiting to happen.
Most Auckland homes need a soft wash on the house and a genuine blast only on the hard landscaping. That is why how we wash down an Auckland home before the first coat goes on is treated as its own stage of the job rather than a five-minute hose-off on the morning we start.
Match the Method to Your Cladding, Not to the Machine You Hired
Auckland has an unusually mixed housing stock, and each cladding type fails in its own way when it is cleaned wrong. Walking a street in Mt Eden you will pass a 1910 villa, a 1950s bungalow, a 1998 plaster townhouse and a 2019 fibre cement infill. Four different answers to the same question.
Painted weatherboard
Timber weatherboard is the classic Auckland exterior, and it is more fragile than it looks once the paint film has aged. High pressure lifts the bottom edge of each board, forces water up behind the lap, and pushes it into end grain at the scarf joints and corners. The timber swells, the paint loses adhesion from behind, and within a season you get flaking along board edges that had been sound for years.
Soft washing is the answer here. Solution on, dwell, rinse down at low pressure, working top to bottom so dirty water never runs over a surface you have already cleaned. On older villas in Grey Lynn, Ponsonby and Kingsland, the paint film may be decades old in places, so the wash also doubles as a diagnostic. Anything that comes off under a gentle rinse was never going to hold a new coat.
Monolithic plaster and the leaky-building era
This is where the stakes are highest. Monolithic plaster cladding from the 1990s and early 2000s, especially the direct-fixed systems built without a drained cavity, has very little tolerance for water pushed in from outside. A jet aimed at a hairline crack, a failed sealant joint, or the junction between cladding and joinery can put litres of water into a wall that has no way to drain or dry.
We treat these homes as soft-wash-only, and we look at the sealant lines and window junctions before anything is turned on. If a home has known weathertightness history, cleaning is a conversation to have with the owner first, not a task to hand to whoever has the biggest machine.
Brick, block and fibre cement
Unpainted clay brick is the most forgiving surface on a typical Auckland house and tolerates a moderate wash comfortably, though old mortar joints can be eroded by a concentrated jet held too close. Painted block and painted fibre cement behave like any other coated surface, so they get the soft wash treatment. Unpainted fibre cement can be chalky as it weathers, and aggressive cleaning removes surface material rather than dirt.
Roofs: Coloursteel, decramastic and concrete tile
Roofs are a category of their own, and the material decides the method. Pre-painted steel roofing follows the metal roofing code guidance above: keep under the pressure ceiling, keep the jet off the laps, wash from the ridge down. COLORSTEEL’s own care guidance makes the point that rain does most of the washing for you, and the areas that actually need manual attention are the sheltered ones that rain never reaches, more often near the coast.
Decramastic and pressed metal tiles are the ones that punish high pressure. The stone chip is bonded into a bituminous coating, and once it has aged, a strong jet strips chip that was still doing a job. Concrete tiles grow moss in their surface porosity rather than just on top of it, so they need a kill treatment and time, not force. Painting is a separate decision again, and we have covered where that line sits in our guide to roof painting versus replacement.
Decks, fences, driveways and the hard landscaping
Outside the building envelope the rules relax, and this is where a water blaster earns its keep. Concrete driveways, exposed aggregate, paths, retaining walls and unpainted block all take real pressure without complaint, and there is no coating to lift. Auckland driveways in particular collect a slick of algae on the shaded side of the house that no amount of detergent will shift on its own.
Timber decking is the exception that catches people out. A high-pressure jet held close to soft pine raises the grain, cuts furrows along the softer early wood, and leaves a fuzzy surface that then holds dirt worse than it did before. Decks want moderate pressure, a wide fan, constant movement and a wash with the grain. If the deck is due for a stain or oil, the same rule applies as on cladding: kill any mould first, let it work, then clean and let the timber dry right through before coating.
Painted fences and painted retaining walls go back into the soft-wash category, because they are coated surfaces regardless of what they are attached to. The material underneath does not decide the method. The coating on top does.
— Superior Painters Team
Important note: If your home was built or reclad between roughly 1994 and 2005 with monolithic plaster, treat any exterior cleaning as a job for someone who understands weathertightness. Water forced into an undrained wall cavity does not evaporate on a timeline that suits you.
Moss, Mould and Lichen: Why Washing Before Painting Is Not Optional
Auckland’s humidity, mild winters and shaded south-facing walls make this city very good at growing things on paint. Understanding what that growth does to a coating is the difference between a repaint that lasts a decade and one that disappoints you in three years.
How mould gets under your paint
Resene’s technical guidance is direct about the mechanism. On painted surfaces mould looks like dirt and often cannot be told apart from it by eye. Wet the surface and rub it, and mould shows up as slime. Left alone, it does more than sit on the surface: unless it is removed within a few months of appearing, mould penetrates the body of the existing paint film, and once established it grows through subsequent coatings, impairing adhesion and ruining the appearance of the paintwork.
Read that again, because it is the single most important sentence in this article. Painting over live mould does not seal it in. It grows through the new coat. Every dollar of the repaint is riding on whether the growth was killed first.
What actually kills it
A detergent wash removes dirt. It does not kill mould. Killing it takes a hypochlorite-based treatment, and the numbers on Resene Moss & Mould Killer’s data sheet tell you how the job should be sequenced:
| Specification | Resene Moss & Mould Killer |
|---|---|
| Type | Sodium hypochlorite concentrate |
| Dilution | 1 part product to 5 parts water |
| Coverage (diluted) | Light infestation 20 m²/L, medium 10 m²/L, heavy 5 m²/L |
| Time to full kill | 48 hours after application |
| After the kill | Scrub vigorously with water to remove dead residues before painting |
| Practical takeaway | Mould treatment is a separate visit, not a step in the painting day |
Gross growth is physically removed first, then the diluted solution goes on by brush, knapsack or spray. The 48-hour window is the detail that reshapes a schedule. A crew that arrives, treats, waits, returns and rinses is doing it properly. A crew that treats and paints on the same visit has skipped the part that matters.
Lichen is the slow one
Mould, moss and lichen get lumped together, but lichen behaves differently and needs more patience. It grows as a crusty grey, green or orange patch, most often on roofs, north-facing fence caps and older concrete tiles, and it anchors itself into the surface rather than sitting on it. Scraping it off takes the surface with it. Blasting it off a tile roof takes the tile finish with it.
The approach is the same treatment, applied properly, and then time. Heavy lichen typically needs a stronger dilution, full coverage, and often a second application weeks later once the first has killed and weathered off. On a badly infested roof, expect the cleaning to run across more than one visit. Anyone promising a lichen-covered roof back to bare surface in an afternoon is planning to use pressure that the roof cannot afford.
Housewash cleans, mould killer kills
These are two different products doing two different jobs, and using one where the other is needed is the most common mistake we see in DIY prep. Paint Prep and Housewash strips dirt, salt and airborne grime so a new coat can bond. Moss & Mould Killer deals with the biology. A wall with visible black staining or green film needs both, in that order, with the wait in between.
Where the runoff goes
Moss & Mould Killer is a strong bleaching agent. Even diluted, it can damage or bleach fabrics, textiles, carpets and foliage, and the data sheet asks you to avoid run-off into gardens and lawns and to flush any affected area immediately with clean water. In practice that means covering or wetting down plants, moving outdoor furniture, and thinking about where the water at the bottom of the wall ends up before you start.
🎨 Painting tip: South and south-east walls, anything under a large tree, and the shaded side of a fence line will regrow first. If only part of the house is treated, that is where the next outbreak starts, and it will spread back across the clean walls.
How Often Should an Auckland Home Be Washed, and What the Rules Say
Washing is not a cosmetic extra. It is maintenance that the New Zealand Building Code framework assumes you are doing. Most homeowners have never been told that, which is why so many exteriors arrive at repaint time in worse condition than their age suggests.
The six-monthly benchmark
BRANZ’s LEVEL guidance on maintenance is explicit that durability is conditional on upkeep. It quotes E2/AS1: “Maintenance shall be carried out as necessary to achieve the required durability of materials, components and junctions”, and the Acceptable Solution names washing exterior surfaces among the specific requirements. The worked example in that guidance is washing down painted weatherboard cladding at least six-monthly, with repainting every 8 to 10 years.
That six-monthly figure surprises people. It lines up closely with what we see on Auckland exteriors: the homes that get washed once or twice a year hold their coatings toward the long end of the 8 to 12 year range we expect from a properly prepared exterior repaint. The ones that never get touched between repaints lose years off the finish.
Coastal Auckland runs a different schedule
Salt is the accelerant. On the North Shore beaches, the Hibiscus Coast, the eastern bays and anywhere within a few hundred metres of open water, salt deposits sit on the cladding and roof between rain events, hold moisture against the surface, and drive corrosion and coating breakdown faster than the same house would experience in Mt Roskill.
Coloursteel guidance makes the same point about sheltered areas that rain never reaches: under eaves, behind downpipes, on the underside of soffits, and anywhere covered from natural washing. These are the spots where salt accumulates unchecked, and they are exactly the spots a quick hose-off misses. Add Auckland’s UV load on top of that. NIWA records peak summer UV index values around 12 to 13 here, well into what is classed as extreme, and UV plus salt is a harder combination on a coating than either alone.
Where the wash water is allowed to go
This is the part almost no painting blog covers, and it applies to homeowners as well as contractors. Auckland’s stormwater network runs untreated to streams, harbours and the sea, so anything that goes down a street drain arrives there unfiltered. Auckland Council publishes specific best-practice guidance on pressure washing and water blasting, and the expectation is that controls are in place before work starts so wash water carrying detergent, paint residue or biocide does not reach the stormwater system.
On a practical job that means directing runoff to unsealed ground or garden where it can soak away, blocking or bunding nearby drains, and collecting wash water where the surface makes that impossible. Roof washing adds one more step: if the roof feeds a drinking water tank, the downpipes come off the tank before anything is applied. The metal roofing code makes the same point.
Height is the line most people should not cross
The other reason to think twice before hiring a machine is where you would have to stand to use it. A wet roof, a wand that kicks back, and a ladder on soft ground is a genuinely dangerous combination, and falls from height remain one of the most serious hazards WorkSafe New Zealand publishes guidance on. A single-storey wash from ground level with a soft brush and a hose is reasonable for most people. Anything involving a roof, a second storey, or a ladder set up on a slope is not a Saturday project.
🎨 Painting tip: Put a wash in the diary for late autumn and again in early spring. The autumn one clears the summer’s dust and salt before the wet months, and the spring one takes off winter’s mould growth before the dry painting window opens.
Timing matters for the repaint too, and it is worth reading alongside our guide to the best time to paint a house exterior in Auckland, because the wash has to fit in front of the paint with enough dry time behind it.
How We Wash Before a Repaint, and What We Look For While We Are Up There
On a Superior Painters exterior job the wash is a scheduled stage with its own time allowance, not a warm-up. It is also the most informative half-day of the project, because a clean wall tells you the truth about what you are actually painting.
Wash, assess, then finalise the system
Dirt and mould hide a lot. Once cladding is clean and dry, the failures become obvious: hairline cracks in plaster, split weatherboard ends, rusting fixings, failed sealant at joinery, chalking on a sun-hammered west wall. The project manager assigned to your job walks the exterior after the wash and confirms the prep and coating system against what has actually been uncovered, rather than what could be seen through a layer of grime at quoting stage.
Sequence and dry time
Where there is mould, the kill treatment goes on first and gets its 48 hours. The wash and rinse follow. Then the cladding has to dry properly before any coating goes near it, and in Auckland that is weather-dependent rather than clock-dependent. Paint applied over cladding that is still carrying moisture is one of the reliable causes of early adhesion failure, and it is invisible on the day.
Only then does prep proper begin: scraping, sanding, filling, priming bare areas, sealing joinery gaps. The washing stage is what makes all of it possible. Prep is most of an exterior repaint, and washing is the first move in the prep, which is also covered on the page explaining how we work through an exterior repaint on an Auckland home.
The checks at the other end
Our 97-point inspection checklist runs at completion and includes the surfaces most likely to have been rushed at the wash stage: soffit undersides, the tops of window heads, behind downpipes, gutter faces, and the sheltered corners that never get rain-washed. The Picture Perfect guarantee sits behind that. A dedicated project manager is on site daily through the job, so the questions you would otherwise be saving up for a phone call get answered on the spot.
If you only ever do one piece of exterior maintenance between repaints, make it an annual wash. It is the cheapest thing on this list and it protects the most expensive one. Our advice on how often you actually have to repaint assumes the house is being washed in between.
Making the Call on Your Place
Take the machine out of the decision and start with the surface. Painted cladding, joinery and soffits get a soft wash with the right chemistry and a low-pressure rinse. Concrete, paths and unpainted hard surfaces can take real pressure. Metal roofing sits under a defined pressure ceiling with the jet kept off the laps. Anything with visible moss, mould or lichen needs a kill treatment and 48 hours before it is touched again.
Get that sequence right and an exterior repaint sits at the long end of its expected life. Get it wrong and you can undo a good paint system in an afternoon with equipment that cost less to hire than a tank of petrol.
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House Washing vs Water Blasting: Frequently Asked Questions
What is the difference between house washing and water blasting?
House washing is a chemical process at low pressure: a diluted cleaning solution is applied, given time to work, then rinsed off gently so the paint film is not damaged. Water blasting is a mechanical process that removes material by force and suits hard, uncoated surfaces such as concrete driveways and paths. Most Auckland houses need the wash on the cladding and the blast only on the hard landscaping.
Can water blasting damage my house?
Yes. High pressure aimed at painted weatherboard can lift board edges and force water behind the cladding and into end grain. On monolithic plaster cladding, especially direct-fixed systems from the 1990s and early 2000s without a drained cavity, water pushed through a crack or a failed sealant joint has no easy way out. It can also strip stone chip from aged decramastic roof tiles and erode old mortar joints in brickwork.
How often should I wash my house in Auckland?
BRANZ's LEVEL maintenance guidance uses washing down painted weatherboard cladding at least six-monthly as its worked example, alongside repainting every 8 to 10 years. Coastal Auckland homes on the North Shore, Hibiscus Coast and eastern bays benefit from more frequent attention because salt deposits accelerate coating breakdown, particularly in sheltered spots under eaves and behind downpipes that rain never washes.
Do I need to wash my house before painting it?
Yes, and it is not optional. New paint cannot bond properly through dirt, salt, chalk or mould. Resene notes that mould left in place penetrates the existing paint film and grows through subsequent coatings, impairing adhesion and ruining the finish. Washing also uncovers the cracks, split boards, rusting fixings and failed sealant that need repair before any coating goes on.
What kills moss and mould on a house exterior?
A hypochlorite-based treatment such as Resene Moss and Mould Killer, which is diluted one part product to five parts water. Coverage of the diluted mix runs about 20 square metres per litre on light infestation, 10 on medium and 5 on heavy. It needs 48 hours to achieve a full kill, and the dead residue is then scrubbed off with water before painting. Ordinary housewash detergent cleans but does not kill.
How much pressure is safe on a metal roof?
The New Zealand Metal Roof and Wall Cladding Code of Practice allows low-pressure water blasting up to 20 MPa, roughly 2,900 psi, with the jet directed away from openings and sheet laps. It also recommends washing with a hose and a soft bristle brush using fresh water or a mild household detergent solution, working from the top down and rinsing thoroughly with clean water afterwards.
Can I do house washing myself?
A single-storey wash from ground level using a hose, a soft brush and a mild detergent is reasonable for most people. The point to stop is height. A wet roof, a pressure wand that kicks back and a ladder on uneven ground is a serious fall risk, and WorkSafe New Zealand treats working at height as a major hazard. Roofs, second storeys and anything needing a ladder on a slope are better left to a crew with the right access gear.
Where is the wash water supposed to go?
Not into the stormwater system. Auckland's stormwater runs untreated to streams, the harbour and the sea, so detergent, biocide and paint residue reaching a street drain ends up in the environment. Auckland Council publishes best-practice guidance for pressure washing and water blasting that expects controls to be in place before work begins: divert runoff to unsealed ground, block or bund nearby drains, and collect wash water where that is not possible.
Will house washing damage my plants?
It can if the runoff is not managed. Resene's data sheet for Moss and Mould Killer warns that even diluted it can damage or bleach fabrics, textiles, carpets and foliage, and asks that run-off into gardens and lawns be avoided. In practice that means wetting down and covering plants before treatment, moving outdoor furniture clear, and flushing anything that is splashed with plenty of clean water straight away.
Can you wash and paint on the same day?
Not where there is mould. The treatment needs 48 hours to fully kill the growth, and the dead residue has to be scrubbed off afterwards. Even on a clean surface the cladding needs to dry properly before coating, and in Auckland that depends on weather rather than the clock. Paint applied over damp cladding is one of the more common causes of early adhesion failure, and the problem is invisible on the day it happens.
References
- Resene — Moss & Mould Killer data sheet (D80)
- Resene — Paint Prep and Housewash data sheet (D812)
- Resene — Moss and mould problems (paint and decorating problem solver)
- BRANZ LEVEL — Maintenance of buildings and building materials
- NZ Metal Roofing Manufacturers — Code of Practice, Maintenance: Washing
- COLORSTEEL — Care and maintenance guidance
- Auckland Council — Pressure washing or water blasting: preventing pollution in our waterways
- WorkSafe New Zealand — Working at height
- NIWA — UV and ozone