What sand do you use for block paving joints?

The sand in the joints is what makes block paving interlock and carry load. Using the right type, dry and fully filled, is more important than it looks.

Why kiln-dried sand

The standard material for filling block paving joints is kiln-dried sand: a clean, fine silica sand that has been dried in a kiln so it contains virtually no moisture. The dryness is the whole point. Because it is bone-dry and free-flowing, it pours readily into the narrow joints between blocks and, when the surface is vibrated, works its way down to fill the joint completely. That full, tightly packed column of sand is what creates the interlock — it stops adjacent blocks rocking or tipping and lets the paving spread vehicle loads as a unit. Damp sand cannot do this job: it clumps, bridges across the top of the joint, and leaves voids below, so the blocks never lock together properly. This is why jointing is done in dry conditions with dry sand.

Jointing sand versus laying-course sand

A block paving build uses sand in two different places for two different jobs, and they are not interchangeable. Confusing them is a common cause of trouble:

• Laying course (under the blocks): uses sharp (grit) sand — a coarser, angular, free-draining sand that the blocks bed into and that holds its shape under compaction. Soft building sand here holds too much water and moves under load, so the blocks settle unevenly.
• Joints (between the blocks): uses kiln-dried sand — finer and very dry, so it flows into the tight joints and packs down to lock the blocks.
• Building (soft) sand: the fine, cohesive sand used in mortar is not suitable for either the laying course or the joints; it retains moisture, does not drain, and does not provide the stability the paving needs.

So the short answer is: sharp sand below, kiln-dried sand in the joints, and building sand nowhere near a block paving build.

Which sand goes where in a block paving build. Source: Pavingexpert; Marshalls installation guidance.

Alternatives to plain kiln-dried sand

Plain kiln-dried sand works well and is the traditional, low-cost choice, but it has one weakness: it can be washed out by heavy rain or blasted out by careless pressure washing, and open joints then let in weeds and ants. Because of this, firmer jointing products have become popular, and they are worth understanding:

• Polymeric jointing sand: kiln-dried sand blended with polymers that, once brushed in and lightly wetted (per the product instructions), bind into a firmer, semi-flexible joint. It resists washout and weeds better than plain sand, but is fussier to apply — it must be cleaned fully off the block faces before activating, or it can leave a haze, and it does not suit very narrow or very wet joints.
• Resin or slurry jointing compounds: two-part resin systems poured or slurried into the joints that cure to a hard, durable fill, strongly resisting weeds and washout. They are more expensive, demand careful application and the right joint width and weather, and are less easily topped up later — but give the most robust, low-maintenance joints.

Which to choose depends on priorities. Plain kiln-dried sand is cheap, simple to top up, and fine for most domestic drives if the joints are kept full and the paving is not aggressively power-washed. The firmer compounds cost more and are harder to apply but cut down on re-sanding, weeds and washout, which appeals where maintenance is a nuisance or the paving sees a lot of water. Whichever is used, the underlying principle is the same: the joints must be completely filled, because a half-empty joint — of any material — undermines the interlock the paving depends on. For plain sand that means brushing in, vibrating, and topping up in the first weeks as it settles; for the bound products it means following the application steps exactly so the joint cures full and solid.

How jointing sand is applied and topped up

Using the right sand only works if it is applied properly, and the method is as important as the material. The jointing stage comes after the blocks have been laid, cut in and given a first pass with the plate compactor. Dry kiln-dried sand is spread across the surface and brushed into the joints in several directions so they fill completely. The plate compactor is then run over the blocks again, which vibrates the sand down into the joints and settles it; more sand is swept in and the process repeated until the joints are full to the top. The key conditions are that everything is dry — the surface, the joints and the sand — because damp sand bridges across the joint instead of flowing down it, and that the joints end up genuinely full, since a half-filled joint provides little of the interlock the paving relies on.

Jointing is not entirely a one-off. In the first few weeks of use the sand continues to consolidate and settle, so the joints often drop slightly and need topping up — a good installer either leaves a dressing of kiln-dried sand on the surface to work in with traffic and weather, or returns to re-sand. Over the longer term, rain and routine cleaning gradually deplete the joints, and aggressive pressure washing can strip them outright, so periodic re-sanding is part of normal maintenance for plain kiln-dried joints. This is exactly the weakness the firmer products are designed to reduce: polymeric sand binds into a more washout-resistant joint once activated, and resin or slurry compounds cure hard and largely eliminate routine re-sanding, at the cost of more demanding application and less easy topping-up later. Whichever route is taken, the discipline that matters is keeping the joints full — brushing in fresh kiln-dried sand whenever they look low for plain joints, or following the application steps precisely for the bound products — because the interlock that carries the load lives in those filled joints, not in the blocks alone.

Frequently asked questions

Sources & further reading

• Pavingexpert — jointing sands and compounds
• Marshalls — jointing block paving advice
• Brett Landscaping — block paving jointing guidance

Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.