Two quotes for the same Hamilton house, a few hundred dollars apart: one to soft wash it, one to water blast it. The tempting move is to take the cheaper one.

It depends which surface each quote is for. For concrete, blasting is the right tool. For the house walls, New Zealand’s own building guidance says not to use a water blaster at all. This covers where each method belongs, and why the two quotes may not be competing.

For your house walls, this isn’t a close call

New Zealand’s building maintenance guidance is direct on this. Wash cladding with a soft brush and a low-pressure hose, and don’t use a water blaster — it can damage claddings and force water through gaps and joints.

That guidance comes from MBIE, not from a cleaning company with a preference. It’s worth reading before you decide, because it settles the wall question without anyone’s opinion being involved.

Cladding manufacturers say much the same thing, and their version carries more weight. Many cladding systems need washing at set intervals to keep the warranty valid. If yours specifies low pressure, the decision is already made, and it was made before you got a quote.

Read your warranty before booking either method. It usually names the method.

The reason blasting is wrong on cladding isn’t that it’s too strong for the paint. It’s that water under pressure doesn’t just clean the surface. It goes where it shouldn’t: behind joints, into the cavity, into the end grain of timber. Cladding is designed to shed water, not to resist it being driven in.

The pressure that’s right for a driveway is the pressure that’s wrong for a house.

Why the water blasting quote comes in cheaper

It’s usually cheaper for real reasons, not because someone is cutting corners.

Water blasting is one pass with one machine. Water goes on at pressure, dirt comes off, and the operator moves on. There’s no dwell time, no treatment to mix, and nothing to wait for. On a hard surface that’s genuinely efficient.

Soft washing adds steps. A cleaning solution goes on first, then it’s left on the surface long enough to break the growth down, then it’s rinsed off at low pressure. More time, more material, more of the day used up.

The extra step is also the part that keeps working after the crew has gone. That’s the next section.

There’s a practical reason the gap exists on houses specifically. Blasting cladding is quick for the same reason it’s a bad idea: the water hits the surface hard and moves on, including into places it shouldn’t go.

So before comparing the two totals, check what each quote actually covers. If one is for the house and the other is for the driveway, they were never in competition, and the cheaper number isn’t cheaper.

A cheap quote for a different surface isn’t a cheap quote for yours.

The difference that shows up months later

Pressure moves growth. It doesn’t kill it.

Blasting takes the visible layer off the surface: the green film, the black staining, the lichen that was sitting on the board. What it leaves is everything holding on underneath — spores in the timber, in the coating, and in the joints between boards.

Those are still there when the surface dries. So the walls that were blasted tend to go green again, and usually on the same shaded side they went green on last time. Nothing about the conditions changed. Only the top layer was removed.

Treatment works differently. A solution left on the surface long enough breaks the growth down rather than shifting it, and the rinse then carries it away. That’s why the dwell time isn’t padding on the quote. It’s the part doing the work.

The cost question follows from that. Results that come back sooner mean paying again sooner, and a Hamilton house that goes green every season will show the difference more than once.

Cheaper per visit isn’t cheaper over a few years.

Where water blasting is genuinely the right tool

Concrete. Driveways, paths, paving, hardstands, and any dense unpainted surface that can take pressure.

This is what the machine is for, and it does a job soft washing can’t match. On a concrete driveway that’s greyed and stained, blasting is often what brings the surface back rather than merely tidying it. It’s also faster across large flat areas, which matters on a long driveway or a big parking area.

So the honest position is that a Hamilton house with a concrete driveway normally needs both methods, and they aren’t competing with each other.

Cladding gets cleaned at low pressure with a treatment. Concrete gets blasted. Same property, two methods, two quotes that were never rivals.

Booking them together usually makes more sense than splitting them. One visit, one setup, one crew already on site with the equipment out.

What we don’t do is decide by surface alone. A surface that looks like it can take pressure sometimes can’t, and it’s worth having someone look before the machine comes out.

The right method depends on what’s under the dirt, not on which one sounds better.

What the age of your cladding changes

Plenty of Waikato housing dates from the 1960s through to the 1990s, and cladding from those decades doesn’t behave like modern systems. Older paint, older seals, and boards that have been through a few decades of weather.

Fibre-cement cladding from that era needs particular care. Manufacturer guidance for these systems is low-pressure water and a brush, and a water blaster is specifically not recommended.

There’s a more serious reason too. WorkSafe advises that high-pressure water blasting can disturb asbestos fibres in roofing or exterior cladding, and that material containing, or possibly containing, asbestos should not be blasted. Homes built or renovated before the 2000s can fall into that category, and the material isn’t always obvious.

That’s not a reason to leave an older house dirty. It’s the reason some jobs get declined, and why the method matters more on an older home, not less.

If you’re unsure what’s on your house, that’s worth resolving before anyone brings equipment onto site.

On older cladding, the assessment matters more than the machine.

If you’re thinking of hiring a water blaster yourself

The machine is cheap to hire for a day, and that’s what makes this route tempting.

It’s worth knowing what the hire companies say, given they profit from renting you one. Hirepool advises avoiding high pressure on exterior walls altogether, and warns about stripped paintwork, marked cladding and water forced into cracks and crevices. When the company renting you the equipment tells you not to use it on your walls, that’s worth listening to.

What a hired machine doesn’t come with is judgement. It arrives with a fixed pressure, a nozzle, and no view of what’s under the paint, how the joints are sealed, or how old the boards are. The method isn’t the equipment. It’s the decision about where to point it.

What tends to go wrong is predictable: paint lifted along board edges, water driven behind cladding, damaged window seals, and protective coatings stripped back. Often it doesn’t show up until the next wet season.

On a concrete driveway or path, hiring a blaster is a reasonable way to spend a Saturday.

You can hire the machine. The judgement doesn’t come with it.

Soft washing vs water blasting in Hamilton: quick answers