Block paving jointing: which sand to use

Jointing is the last step and the one that decides whether the paving holds together: the sand in the joints locks the blocks to each other, spreads the load and stops them working loose. Kiln-dried sand is the standard here; a polymeric jointing sand costs more and holds up better where the joints keep washing out. Either way the joints need topping up later.

1. The paving has to be dry. Tip the sand out over the blocks.
2. Brush it into the joints with a stiff broom until they are full.
3. Run the plate compactor over a rubber sole plate so the sand drops down.
4. Top the sand up, brush again, and sweep the surplus off the face.
5. With polymeric sand, wet the area down as the maker tells you so it sets.

There is a condition in the standards that most how-to guides leave out: the laying course must be graded so the jointing material cannot work down into it. In plain terms, the jointing sand must not fall through the gaps between the grains of grit underneath.

When it does, here is what you see: the joints empty after every downpour, you are still topping them up in the second and third year, and nobody can work out why. The sand has not gone anywhere — it has simply moved down 30 or 40mm. The blocks lost their side support along the way and start to rock.

That is why the laying course is sharp (grit) sand rather than soft building sand, and why the jointing sand is finer than the bedding but is not dust. If a supplier offers you the same bag for both, something is wrong.

For the vast majority of drives, paths and patios, kiln-dried sand is all you need. Polymeric is worth the money where the joints wash out repeatedly: under a downpipe, on a steep fall, or on an area the customer jet-washes every spring. It costs several times as much and it does not forgive a sloppy application — once it is wetted, anything left on the block face sets there too.

You can work it out from the length of joint. With a 200 × 100mm block there is roughly 15 linear metres of joint in every square metre of paving. At a joint 2 to 5mm wide — 3 to 5mm is what a standard block actually gives you — running the full 60mm depth, that comes to about 3 to 5 kg of sand per m², depending on how far down the joint really fills.

A larger format means fewer joints and less sand; small setts mean a lot more. Allow half as much again for the first top-up after compaction, because the sand always drops further than you expect.

Weeds do not grow up from the ground under the paving. The seed lands from above and germinates in the dirt that collects in the joint. That is why brushing beats chemicals: a clean, full joint gives a seed nothing to root into.

Boiling water and a weed burner both work and neither harms the blocks. Salt and vinegar do not: salt attacks the block face and makes efflorescence worse over the winter, vinegar washes the sand out of the joint. If there are more weeds every year, either the joints are half empty or the area has no fall and is holding water.

Same warning about the jet wash, which is where most British driveways lose their sand. Hold the lance well back and keep it moving across the joints rather than along them; a nozzle held close blows a joint empty in one pass. After any wash, expect to re-sand.

Joints settle and wash out over time — check them in the first year after laying and top them up. Pull weeds out and refill the joint rather than treating it, and keep salt off the paving. Regular brushing and a bag of kiln-dried sand now and then add years to the whole area.

Related: the step-by-step laying guide and the sub-base under block paving.