The surface decides the method, not the machine. Block paving and resin-bound gravel sit side by side on plenty of Dublin driveways and want almost opposite treatment, and the difference between a drive that looks better for five years and one that is visibly worse within one is usually a decision made in the first ten minutes.
Surface by surface
Concrete and concrete flags. The most tolerant surface there is. Takes direct pressure, responds well to a rotary surface cleaner, and the main risk is striping — the tiger-stripe pattern left by working too fast with a fan jet. Even coverage matters more than pressure.
Block paving. Cleans up dramatically and is the surface most often damaged in the process, because the jointing sand between the blocks washes out at a fraction of the pressure the blocks themselves will take. See below — this is the one to get right.
Natural stone: limestone, sandstone, granite. Common on Irish patios and variable in hardness. Granite is robust. Limestone and sandstone are softer and can be etched or have their surface opened up, which makes them stain faster afterwards. These want lower pressure, wider nozzles, and a chemical treatment doing more of the work.
Porcelain paving. Non-porous, very hard, and effectively immune to pressure damage on the face. The vulnerable part is the grouted joint, not the slab.
Tarmac and asphalt. Treat with real caution. Tarmac is a bound surface held together by bitumen, and high pressure lifts the aggregate straight out of it, leaving a rough patch that cannot be put back. Low pressure and chemistry only.
Resin-bound gravel. Increasingly common on newer Dublin driveways and widely cleaned wrongly. The stone is held in a resin matrix; concentrated pressure breaks that matrix and the stone comes loose in patches. Manufacturers generally specify a low pressure limit, and it is worth following theirs rather than a contractor’s confidence.
Timber decking. Softwood decking is soft. A pressure washer raises the grain, and a raised grain traps dirt and algae, so the deck gets dirty faster after being cleaned than it did before. Low pressure along the grain, or a chemical clean and a stiff brush.
Composite decking. Check the manufacturer’s figure. Most permit modest pressure; some void the warranty outright.
The joint sand problem
This is the part worth reading twice, because it is invisible on the day and expensive a year later.
Block paving works as a structure: the blocks lock against each other through the sand in the joints. Wash that sand out and the blocks are no longer restrained. What follows is movement under vehicle weight, blocks that rock and sink, an uneven surface, and joints that are now an open seed bed — so the weeds come back faster than before it was cleaned.
A driveway cleaned properly is therefore re-sanded afterwards, and that is a distinct step with a cost attached. Kiln-dried sand is swept into the joints and settled; a polymeric or stabilising sand costs more and resists washout and weed growth for longer.
If a quote for block paving does not mention re-sanding, ask about it before the work rather than after. It is the clearest single question for telling apart two quotes that otherwise look the same. Our own approach to this is set out on driveway and patio cleaning.
How a job actually runs
- Clear and protect. Everything moveable comes off, and drains, planting and any pond get covered or diverted. Slurry off a driveway is the runoff that causes most of the collateral damage on this kind of work.
- Treat first where growth is heavy. Killing algae and moss chemically before washing means less pressure is needed to shift it, which is the whole trick to cleaning a soft surface safely.
- Weeds out of the joints by hand or with a knife. A jet cuts the tops off and leaves the roots.
- Wash with even, overlapping passes, normally with a rotary surface cleaner rather than a bare lance. The rotary head is what prevents striping and keeps the working distance consistent.
- Rinse the slurry off properly, including the walls and any adjacent render it has splashed. Dried slurry sets like a thin grout.
- Re-sand the joints on block paving, once the surface is dry enough for the sand to run into them rather than stick at the top.
- Seal, if it is appropriate — see below. This normally means returning on a later day, not finishing the same afternoon.
Sealing, and when not to
Sealing a driveway is genuinely useful on the right surface and actively harmful on the wrong one, and the decision comes down to whether the surface needs to breathe and whether it is properly dry.
- It suits block paving and most concrete: it stabilises the joints, resists oil staining, and slows growth coming back.
- It needs the surface genuinely dry. Sealing over trapped moisture clouds the finish and can lift it. In an Irish autumn that dry window is the constraint on the whole job, and it is the reason sealing is usually a separate visit.
- It is a commitment, not a one-off. A sealed drive needs re-sealing on a cycle. Left to fail it goes patchy, and stripping a failed sealer is harder work than applying one.
- Be careful with natural stone. Some sealers darken it permanently. On anything porous, test an offcut or an out-of-sight corner first.
The expensive mistakes
- Full pressure on tarmac or resin. Not reversible. The aggregate is gone.
- Skipping the re-sand on block paving. The drive is structurally worse than before it was cleaned, and it will not be obvious for months.
- Holding the lance too close. On any soft or porous surface the damage is done by distance and dwell, not by the number on the machine.
- Letting slurry into drains and borders. Sand and silt block gullies, and the biocide in the runoff is not something to put on planting or into a pond.
- Cleaning the drive before the roof. Everything off a roof or wall lands on the ground, so the horizontal surfaces come last. The same logic that puts gutters after the roof puts the driveway after both.
For what the pressure figures on a machine actually mean, and why flow rate does more of the work than pressure does, see what the jet washing pressure numbers mean.