Block Paving: Why Re-Sanding After a Clean Is Not Optional

Block paving does not hold together because of the blocks. It holds together because of the kiln-dried sand packed into the joints between them, which locks each block against its neighbours and lets the whole surface behave like one slab that can still flex.

A pressure washer removes that sand. It has to: the sand is exactly where the moss and weeds are rooted. The question is what happens next.

Blocks with empty joints have nothing to lean on. They start to rock under car wheels, then to sink at the edges, and once one block drops the ones around it follow. On a slope, the whole field can creep downhill a few millimetres a year.

Empty joints also hold water instead of shedding it, and in a Scottish winter that water freezes and expands. Then, come spring, the empty joints are perfect seedbeds, so the weeds come back faster and thicker than they were before the clean.

The driveway looks superb for about six weeks and then starts going backwards. That is not the wash failing, that is the missing step.

The surface has to be properly dry first, which in practice often means coming back rather than finishing the same afternoon. Damp sand bridges in the joint instead of dropping to the bottom, and you end up with a half-filled joint that looks full.

Dry kiln-dried sand is brushed across the whole area and worked into every joint, then the surface is vibrated or firmly swept so the sand settles and drops, then topped up again. It usually takes two or three passes before the joints stop taking more.

It is unglamorous work and it is the difference between a driveway that stays put and one that does not. Our driveway cleaning treats it as part of the job on block paving, not an upsell.

Sometimes. A sealer soaks into the blocks and the sand, binding the joints, slowing weed germination and making oil and tyre marks far easier to lift later. On a driveway you have just spent money restoring, it protects that spend.

It is not free of trade-offs. Sealer changes the look, usually darkening the blocks and adding anything from a slight damp appearance to a noticeable sheen depending on the product. It needs redoing every few years. And it must go onto a properly clean, properly dry, properly re-sanded surface, or you are sealing dirt and damp under a layer you now have to strip.

Imprinted concrete is a different case: there, sealing is not optional, because the colour and pattern live in the seal coat rather than in the concrete itself. Once that has worn through, the surface looks tired no matter how clean it is.

Push a screwdriver or a key into a joint. If it goes down easily past a few millimetres, the joint is empty. Look along the driveway at eye level, too: rocking or dipped blocks near where car wheels track are the early warning.

Weeds coming back within weeks of a clean is the other giveaway, and it is normally a sign the driveway was washed and walked away from.

Slabs and natural stone on a patio have their own version of this, though pointed patios need repointing rather than sanding, and it is a slower, more careful job.

And nothing on this page applies to the walls of the house. Render, roughcast and cladding must never see this kind of pressure at all, for reasons covered in soft washing vs pressure washing and handled by render and cladding cleaning.