Green algae, dark mould and black staining on a Hamilton house are usually three different problems, often on the same property at the same time. They are driven by the same thing: surfaces that stay wet long enough for organic growth to establish. Hamilton gives growth plenty of that.

This guide explains what each one actually is, why Waikato conditions cause it, and why the same growth can look green one year and black the next.

Three problems that look alike on a Hamilton house

Most homeowners describe all of it as mould. That single word covers at least three different things, and they do not respond to the same treatment.

Green algae spreads as a flat film across a surface. It looks like a green wash or tint rather than something sitting on top, and it usually has no clear edge.

Mould grows in clusters. You get dark spots, patches or speckling, often around window edges, corners and under eaves. It reads as marks rather than a coating.

Lichen sits on the surface as a patch you can see the edge of. Often grey-green or yellow-green, dry and crusty to touch, and circular or irregular in shape.

Mixed growth is normal. A shaded Hamilton wall can carry green algae across the surface, mould spotting around the window frame, and lichen on the fence a few metres away. One method rarely finishes all three.

For removing green algae specifically, there is a full guide to cleaning it from Hamilton house walls.

Why green growth turns black

A patch that was green last summer and black now has usually not changed species. It has matured.

Young algae is bright green and thin. As a colony thickens it develops a darker outer sheath that screens the growth underneath from sunlight, and the colony becomes far more resilient to drying out. From a distance, it reads as dark green, grey or black.

That matters for two reasons. A house that “suddenly went black” often had the problem for years; it was simply visible as green before. And black staining generally signals older growth, not a worse type. Age, not severity.

Not all dark staining is matured algae, though. Under eaves and around window frames it is more often mould. On fences and lower walls it can be lichen, which was never green and never will be.

Same colour, different organisms, different treatment. That is why one product applied across a whole house rarely clears everything.

Why Hamilton homes grow it faster than most of New Zealand

Organic growth needs one thing: a surface that stays wet long enough to establish. Hamilton meets that unusually well.

The city averages around 1,265 mm of rain a year across roughly 140 to 160 rain days. That count matters more than the total. A heavy downpour runs off and the sun returns. Frequent lighter rain wets a surface, lets it dry, then wets it again, which is the cycle growth exploits.

Humidity does the rest. Hamilton’s sits between roughly 72% and 85% across the year, peaking in winter, and rarely drops low enough for a damp wall to dry quickly.

Then there is winter sunshine. In June and July, Hamilton averages only around four hours of sun a day. A wall that gets wet in the morning may not dry before it rains again.

This is what sets Hamilton apart from the coastal cities. Auckland and Tauranga deal with salt but also get more wind. Hamilton is inland and still. There is no salt here. The problem is drying time, which is why growth in the Waikato behaves like a maintenance cycle rather than a one-off job.

Why it appears on some walls and not others

Every exterior surface on your house gets wet. Only some stay wet. That difference, more than rainfall, decides where growth appears.

South-facing walls get the least direct sun, so they hold moisture longest. This is usually where a Hamilton homeowner first notices green.

Under eaves, gutters and downpipes. The roof overhang shades the wall directly below it, and a gutter that overflows keeps a narrow strip of cladding wet every time it rains. Staining there is worth tracing back to its source rather than just cleaning off.

Lower walls near the ground. Splashback from paths and decks, plus soil and planting against the cladding. Airflow is poorest at the base of a wall, which slows drying further.

Beside trees and hedges. Shade, leaf litter and still air combine. This is often the worst-affected part of a property.

Two houses on the same street can differ completely on orientation and tree cover alone. That is why a neighbour’s clean wall proves nothing about yours.

What lichen does that algae and mould don’t

Lichen is not one organism. It is a partnership between a fungus and an alga, which is why it behaves like neither.

The important difference is how it attaches. Algae and mould sit on the surface. Lichen works root-like structures into it, so it becomes part of the cladding rather than something resting on it. There is nothing raised to blast off.

It also grows slowly, which means a lichen patch has usually been there for several years before anyone notices it.

The practical consequence matters. Lichen has to be killed before it will release, and it lets go gradually rather than at once. Scrubbing or blasting directly does not remove it; it damages the surface underneath. On painted cladding, that means a repaint.

This is the growth most often mistaken for stubborn mould, and the one most likely to leave a lasting mark when treated as mould for years.

For how soft washing handles growth like this, see soft wash house cleaning in Hamilton.

Why it comes back after cleaning

Growth returning in the same spot months later is the most common complaint about exterior cleaning. It usually comes down to one of three things.

The wrong growth was treated. A mould treatment applied to lichen, or a general wash on a mould problem, leaves the organism alive. It regrows, and it looks like the cleaning failed when the diagnosis did.

The surface was cleared, the cause was not. A gutter that keeps overflowing onto the same wall keeps it wet. Cleaning the wall resets it; it does not fix it. The same applies to a downpipe discharging against the cladding.

Only what was visible was removed. Growth washed off without being killed returns from what remains. This is why a blast can look perfect for a few weeks and then fail.

The pattern tells you which you are dealing with. Growth returning in the same place points to a cause still there. Growth returning somewhere new is a different conversation.

For removing mould specifically, see mould removal in Hamilton.

Which growth actually damages the house

Most of what grows on a Hamilton exterior is a maintenance problem before a structural one.

Algae and mould on painted cladding attack the coating, not the wall behind it. Growth holds moisture against the paint, and repeated wetting breaks a coating down faster than weather alone. The damage is real, but slow. That makes regular cleaning cheaper than a repaint. BRANZ publishes maintenance guidance for exterior cladding if you want to check your own.

Lichen is the exception. It works into the surface rather than sitting on it, so the longer it stays, the more likely removal leaves a mark.

The more useful question is what the growth is telling you. Staining running down from a gutter usually means the gutter is overflowing, not that the wall needs scrubbing. Staining at the base of a wall often means water is sitting where it should not.

Those are worth fixing regardless of how the wall looks. Cleaning that ignores them is a repeat booking.

For what a full exterior clean costs, see exterior cleaning costs in Hamilton.

Green algae, mould and black stains in Hamilton: quick answers

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