Why Dublin Block Paving Goes Green Again Within a Year

Cobblelock and block paving went down across Dublin’s suburbs in enormous quantities from the 1990s onward, and most of it is now twenty to thirty years old. It is the surface we are asked about most, and the question is almost always the same one: why is it green again already.

The honest answer is that cleaning removes the growth but does not remove the reason for it. What decides how long you get is what happens between the blocks and underneath them.

Joint sand is the whole argument

Block paving is not a surface. It is several thousand separate blocks held in position by kiln-dried sand packed into the joints, and that sand is doing structural work — it transfers load sideways from one block to the next, which is what stops a drive rutting under a car.

Washing moves some of that sand. That is unavoidable; it is the nature of putting water into a joint. What matters is whether it goes back.

An unfilled joint is a narrow, sheltered, permanently damp channel with a bit of grit in the bottom. That is close to an ideal seedbed. Moss establishes in it within a season, weeds follow, and because the roots are now below the surface of the blocks, the next clean has to work much harder for a worse result.

This is why re-sanding is part of the job rather than an upsell, and it is the most common thing missing when a drive greens up unusually fast after a cheap clean.

Shade decides the pace

Two identical drives on the same Dublin road will green at very different rates depending on which way they face and what is over them.

A south or west-facing drive with open sky dries within an hour of rain stopping. A north-facing one behind a hedge, or a side passage between two houses, can stay damp all day in winter and effectively never dries between November and February. Algae and moss need moisture and time, and shade supplies both.

Dublin’s denser suburbs make this worse than it would be in open country, simply because houses are closer together and there is more of the day in shadow.

Trees do two separate things

Under the mature limes, planes and sycamores of Rathgar, Terenure, Clontarf and Drumcondra, paving takes on a staining that is not really dirt at all.

The first thing is leaf litter: it sits, holds water against the blocks, and rots down into exactly the organic film that growth feeds on. The second is tannin, which leaches out of the leaves and berries and stains the block itself a brownish grey. Tannin is a stain, not a soiling, and it behaves differently — it does not simply lift the way traffic film does, and a drive under heavy tree cover will usually come back clean rather than back to the original colour of the block.

Worth knowing before rather than after. Sweeping leaves off promptly through October and November does more for the following spring than most things you could pay for.

Where the water sits

If one strip of a drive greens up ahead of the rest, look up rather than down.

A blocked or overflowing gutter puts a concentrated line of water onto the same few square metres every time it rains, and that patch never gets the chance to dry. The same goes for a downpipe discharging onto the paving instead of into a gully, and for a gully that has silted up and no longer takes what reaches it.

Cleaning that strip treats the symptom. It will be back, in the same shape, in months. The gutter side of this is covered in why gutters need clearing in an Irish climate.

Settlement, and the low corner

Older Dublin cobblelock frequently has a dip somewhere — usually where a vehicle turns, or over a service trench that was reinstated and settled afterwards.

A dip holds standing water after every shower. It will be the greenest part of the drive no matter how often it is cleaned, and no amount of washing changes that, because the cause is the level rather than the surface. Lifting and relaying that section is a paving job rather than a cleaning one, and it is more honest to say so than to keep cleaning it twice a year.

So how long should a clean last?

On an open, well-drained Dublin drive with joints properly re-sanded, a couple of years before it is noticeably back. In shade, under trees, or on the coastal side of the county where surfaces stay damp longer, a year to eighteen months is more realistic.

Under a year means something is wrong: joints that were never refilled, water being delivered onto the surface from above, or a level that holds it. Those are all findable, and worth finding before booking another clean. What a visit covers is set out on the pressure washing in Dublin page.

Common questions

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