Why does the sand keep washing out of my block paving?

The jointing sand is not decoration. It is what transfers load between blocks, and a block field with empty joints has no interlock — the blocks rock individually, the edges chip and the whole pavement loosens. So joints that keep emptying are a structural problem with a cosmetic appearance, and it is worth finding out which of four causes you have.

Safety

What you'll need

• Kiln-dried jointing sand
• Stiff broom
• Plate compactor with a rubber mat, hired (optional)
• Mask and eye protection
• Block paving sealer (optional)
• Polymeric or resin jointing sand (optional)

Step by step

1. Work out where the sand is going.Washed out downhill by rain and a downpipe discharging onto the paving: you will see the sand fanned out at the low edge. Blasted out by a pressure washer: the joints are uniformly empty and the block edges look scoured. Taken by ants: small conical heaps of fine sand on the surface. Never there in the first place: the paving is recent and the joints have been low since it was laid.
2. Fix the water problem before refilling.A downpipe discharging onto block paving will empty the joints beneath it however many times you fill them. Get the discharge into a gully, a butt or a soakaway. The same applies to a gutter overflowing onto a drive.
3. Wait for genuinely dry paving.Kiln-dried sand only flows into a joint if both the sand and the joint are bone dry. Two or three dry days, and a dry surface with no dew. This is the step that decides whether the job works.
4. Brush sand in and then vibrate it down.Spread sand over the whole area and brush it in diagonally to the joints. Then run a plate compactor with a rubber mat over it — the vibration settles the sand far deeper than brushing can. Top up and repeat two or three times until the joints stop taking sand.
5. Sweep the surface clean before the last pass.Loose sand under the plate grinds into the block faces and dulls them.
6. Decide whether to seal.A block paving sealer binds the top of the jointing sand and locks it in place, and it is the standard answer to joints that empty repeatedly. It needs a dry, clean, fully jointed surface, a product suited to the block, and re-application every few years.
7. Consider a polymeric jointing sand as the alternative.A sand with a binder that sets when wetted, brushed in dry and then activated with a fine spray. Once cured it resists washing, brushing and weed growth. It is more expensive, less tolerant of application errors, and much harder to remove if you need to lift blocks later.
8. Change how you clean it.If pressure washing emptied the joints, going back to pressure washing will empty them again. A stiff brush and a patio cleaner with a biocide deals with the green film without stripping the joints.

Tips

• Buy more sand than you think. A joint takes far more than it looks like it should, and running out halfway leaves you with half a locked pavement.
• Do this in late summer. You need dry paving and dry sand, and you want the joints full going into the wet months.
• A leaf blower on a low setting is very effective at clearing loose sand off the surface without disturbing the joints.
• If the paving is new and the joints were never filled properly, go back to the installer. Full joints are part of laying block paving, not aftercare.

Common mistakes

• Brushing sand into damp joints — It bridges at the top and leaves the joint hollow underneath. It looks filled and it is not, and the first rain takes it back out.
• Using building or play sand — Both hold moisture and neither flows into a narrow joint. Kiln-dried sand is dried specifically so it will run to the bottom of the joint.
• Pressure washing every spring and refilling every autumn — It is a cycle that slowly destroys the pavement — each wash loosens the blocks a little more and scours the block surface. Brush and treat instead.

If it doesn't work

Questions people ask