Kiln-dried sand: what it does and when joints need topping up

Most people look at a block-paved driveway and see the blocks. The part that actually holds it together is the thin line of sand between them. Each block is laid tight against its neighbours, and the sand packed into the joints is what stops them shifting, rocking and spreading apart under the weight of a car. When that sand goes, the driveway starts to fail, even if every block is still in perfect condition.

What the joint sand actually does

Block paving is a flexible surface. The blocks sit on a bed of sand over a compacted sub-base, and they are not glued or mortared together. What locks them is friction. Sand packed tight into every joint means a load on one block is shared with the blocks around it, so the surface carries a car as one piece rather than as hundreds of loose bricks. Take the sand away and each block is on its own. It can tip, sink or creep, and the pattern starts to open up.

Full joints do two other jobs. They leave no room for soil and seeds to settle, which is where weeds in the joint lines come from. And they slow the amount of rain running straight down between the blocks into the bedding layer, which is what washes the bed out and leaves dips in the surface over time.

Why kiln-dried, not building sand

Kiln-dried sand is a fine sand that has been dried in a kiln until there is no moisture left in it. That matters because block paving joints are only a few millimetres wide. Dry sand flows like water when it is brushed across the surface, dropping down into the narrowest gaps and settling to the bottom of each joint, so it fills them from the base upwards rather than bridging across the top.

Building sand or sharp sand from a bag holds moisture. It clumps, sits in the top of the joint and leaves a hollow underneath that collapses the first time it rains. It can also stain lighter blocks. That is why kiln-dried sand is the standard for block paving joints, and why Whales uses it on every re-sand.

Why joints empty out

• Cleaning. Any pressure clean lifts some of the jointing sand along with the moss and dirt. Done properly, with a rotary surface cleaner and controlled pressure, the loss is limited, but it is never zero. Our guide on whether pressure washing is safe for block paving covers what happens when a focused jet is used instead.
• Rain and run-off. Water running across the drive, especially from a downpipe or a slope, carries sand out of the joints a little at a time.
• Weeds and moss. Roots growing in a joint break the sand up, and when the weed is pulled out a plug of sand often comes with it.
• Traffic and sweeping. Tyres turning on the spot, and stiff brushing, slowly work sand loose from the top of the joint.

Signs your joints need topping up

• You can see a clear gap at the top of the joints, or run a fingernail into them without touching sand.
• Weeds or moss are growing along the joint lines rather than on the face of the blocks.
• Blocks rock or click when you walk on them.
• Blocks are sitting lower than the kerb or edging, or the surface has a dip where water pools.
• Sand is washing across the surface or collecting at the bottom of the drive after rain.

Topping up early is the cheap fix. Once blocks have started to move or sink, the bedding underneath has usually been disturbed, and that is a lifting and relaying job rather than a re-sand.

How a re-sand is done

Re-sanding only works on a clean drive. Brushing fresh sand into joints still full of moss, weeds and old silt just traps the growth in place. So the drive is cleaned first: the moss and black spot are pre-treated with biocide, then the surface is cleaned with a rotary surface cleaner run off a Honda GX390, which takes the weeds and moss out of the joints at the same time as cleaning the blocks.

The joints then have to be dry. Kiln-dried sand brushed into wet joints sticks to the sides and stops flowing, which defeats the point of using it. That is why, on some jobs, the clean is one visit and the re-sanding follows on a later, dry day. Your quote says which. Once dry, the sand is brushed into every joint until full, and the excess swept off.

Making the sand stay put

Fresh sand on its own will slowly work loose again for all the reasons above. Sealing over the top is what holds it. A sealer bonds the sand at the top of the joint, so rain, sweeping and weeds have far less to work on, and it protects the blocks from staining and UV fade at the same time. A quality sealer protects for 2–3 years, with busy driveways at the lower end of that range. Our guide on when to seal a driveway sets out when it is worth it and when it is not.

Slabbed patios with mortared joints are a different job: failed joints are ground out and repointed with Nexus Pro compound. See repointing restoration.

What it costs

Driveway cleaning starts from £3.50/m². A standard single driveway, around 25m², is usually £90–£160, and a double, around 50m², £175–£320. Re-sanding and sealing are not included in the cleaning price and are quoted separately, because how much sand a drive takes depends on how empty the joints are, and that only shows once the drive is clean.

If you are not sure whether your joints need doing, send James two photos: one of the whole drive and one close-up of the joints at the worst spot. He will say whether a clean is enough, whether it needs re-sanding and sealing, and what it costs before anything is booked.