The usual explanation is that Tauranga is damp. It’s true, and it isn’t the useful part. Your neighbour’s patio is just as damp, and half of them stay grey.

The reason a driveway greens over isn’t only the weather falling on it. It’s the material it’s made from, what drains onto it, and where the water goes afterwards.

Concrete doesn’t behave like a wall or a roof. Most advice about green outside surfaces is written for those, which is why it hasn’t helped your drive.

The surface is the reason, not the weather

Tauranga’s climate matters, but it isn’t what separates a driveway from a patio or a forecourt. The slab itself is.

Concrete is porous, so water doesn’t sit on it and run off. It goes into the surface, and the slab gives it back slowly. A wall behaves the other way. Rain hits a painted surface, most of it drains within seconds, and the rest evaporates from the outside in.

So a driveway spends its life a little damp. It isn’t that Tauranga rains more than elsewhere; it’s that a slab here rarely gets fully dry. Which month you wash makes more difference than which product, and that’s covered separately. In summer it can look bone dry at 4pm and still be holding water inside it.

Growth only needs that. Algae, moss and lichen don’t need soil and they don’t need a wet day — they need a surface that stays moist long enough to establish, and a slab supplies both in January as readily as July. What each looks like, and why overnight damp matters more than rainfall, is in the guide to why Tauranga homes go green.

Which is why hosing a green driveway achieves so little. You’ve removed what was growing. The conditions that let it grow are still the slab.

Why one half of it stays clean

Ask someone with a patchy drive which half is clean and they’ll tell you the sun side dries faster. Sometimes that’s true. It rarely explains a drive that’s green to a straight line.

Look at where the green stops. If it stops at a hard edge rather than fading, the boundary is usually something above the slab. Rain running off galvanised steel and copper takes trace amounts of metal with it, and those metals hold back the organisms that make a surface green. Wherever that water lands, the concrete stays bare. That’s why a strip under a galvanised flashing or downpipe can stay clean for years while the ground either side furs over.

The pattern is a clue, not a verdict. It tells you what’s above the slab, not that your roof is to blame. Paint stops the leaching, so a coated roof doesn’t feed a drive the way bare galvanised does.

Two other asymmetries worth knowing. A slab under a wide eave never dries, so it greens on schedule whatever the roof is doing. And near the coast, salt draws moisture into the surface, which is why concrete at Papamoa ages faster than concrete three kilometres inland.

The slab is a drainage device

A driveway is meant to move water sideways. It’s poured at a fall, away from the house, toward a channel, a swale or the street. When that works, most rain is gone in minutes and the slab keeps only the damp it can’t avoid.

When it doesn’t, the slab holds water in a place you stop noticing. A low patch the size of a dinner plate is enough — enough for a puddle that stands overnight, every night, in one spot. Puddles like that are usually old, not damage from a recent storm. They form as the base under a slab settles or the fall was finished flat, and once one is there it fills with the same silt and leaf mulch that feeds growth.

That’s the other reason a clean drive goes back. A wash doesn’t lift a low patch or clear a blocked channel.

So look at where the water goes before you look at where the green is. If it’s standing somewhere, that’s the problem and the green is the symptom.

Pavers and exposed aggregate invert the rule. They grow in the joints, where the sand holds water longest.

Why a clean that looks perfect comes back green

A good clean on concrete does what it says. The slab goes grey, the edges are sharp, and for a few weeks it looks finished. Cleaning and killing are separate jobs, though, and a driveway usually gets one of them.

Algae and moss don’t sit on concrete so much as live in it. Spores settle into the pore network and the growth sends holdfasts down. A wash removes what you can see, but the pores are beyond the reach of any nozzle, so what survives has a head start once the slab is damp.

There’s a worse version of this. A clean delivered aggressively strips the cement paste and leaves the surface rougher than it was. Rougher holds more water and gives growth better footing, so the drive comes back sooner, and moss adds its own — kill only what the spray touched and fragments left behind re-root.

Both ways of dealing with this work. One treats, then washes and rinses, and you have a clean surface today. The other lays a no-rinse treatment on and lets rain do the washing, slower and dependent on the weather. Neither is permanent. On a slab that holds water, a clean buys time.

If you coat it, the timing is the job

The obvious answer to a porous slab is to seal it. Coating the surface slows the drying-out and makes the green easier to wash off next time. On a driveway in full sun that can be worth doing.

It is not a cure, and the reason is the same porosity. A coating sits on top of a slab that still holds water; if it goes on before that water has left, it is in there with it.

So the timing is the whole job. The slab has to be genuinely dry, not surface-dry, and the coating wants mild, cloudy weather with no rain expected. A clear summer afternoon is the wrong day.

Follow the tin rather than the forecast. Paint makers’ guidance for concrete coatings states at least four hours between coats and twenty-four before an area is put back into full use — on a driveway, that is no car parked on it overnight.

And sometimes the honest answer is not to coat it. A slab that dries slowly anyway, or one with the low patch from earlier, is a slab where trapping moisture under a film is worse than living with the green.

Why Tauranga driveways turn green: quick answers

If the drive is slippery, that’s the reason to act, not the colour. A quote visit looks at both — what the slab is made of, and where its water goes.