Why Does Tarmac Go Green?

Why porosity matters so much

Tarmac that's aged, has a more open aggregate mix, or was laid with less compaction tends to be more porous, meaning it absorbs and holds moisture within the surface rather than shedding it quickly. That retained moisture is exactly what algae needs to establish and spread across the surface, which is why older, more weathered tarmac drives often green up faster than a newer, denser surface even under identical weather conditions.

Cracks and surface degradation compound this, since they create additional spots where water pools or gets trapped, giving algae even more places to take hold beyond the general surface porosity.

Why shade accelerates it

A tarmac driveway under trees, next to a tall fence, or on the shaded side of a house stays damp for far longer after rain than a sun-exposed drive, since there's little direct sunlight to dry the surface out. This extended damp period gives algae exactly the conditions it needs, which is why two otherwise identical tarmac drives on the same street can show dramatically different rates of green staining purely based on how much sun each gets.

Overhanging trees add a second problem on top of shade: leaf litter, sap and organic debris that falls onto the tarmac provides additional nutrients that can encourage algae and moss growth even further.

Does green tarmac mean it's damaged?

Not necessarily. Surface algae staining is largely cosmetic in its early stages and doesn't indicate the tarmac itself is structurally failing. However, left untreated for a long period, moss and algae can hold moisture against the surface persistently, which over years can contribute to freeze-thaw damage and accelerate surface degradation, particularly during harsh winters.

Treating it earlier rather than later is sensible less because of imminent damage and more because established, thick moss growth is considerably harder and more expensive to remove than a light surface algae film caught early.

How treatment and prevention help

A pressure wash or soft wash treatment removes existing algae, with soft washing's biocide element generally offering a longer-lasting result on porous, shaded tarmac than pressure washing alone. Improving drainage where possible, and trimming back overhanging branches that contribute to shade and leaf litter, both help slow how quickly the green staining returns after cleaning.

Why some tarmac drives never seem to go green at all

It's worth acknowledging the flip side: some tarmac driveways, generally newer, denser surfaces laid with good compaction on well-draining ground and in a sunny, open position, barely develop any green staining at all for years. This isn't luck so much as the absence of every factor that drives the problem: low porosity, minimal shade, good drainage and no overhanging vegetation. If you're having new tarmac laid and green staining is a concern based on a previous drive's experience, discussing drainage falls and sun exposure with the installer at the planning stage can meaningfully reduce how much of a problem it becomes later.

For an existing drive where none of that is easily changeable, periodic cleaning remains the practical answer, and there's no shame in a shaded, older tarmac surface needing more frequent attention than a neighbour's sunnier, newer one.