Repairs ·
The sand in the joints is the structure
Weeds in a block driveway aren't really a weed problem, and jet washing is what causes most of them. The sand between the blocks is what lets small blocks carry a car — here's how to put it back.
Worth doing yourself
Re-sanding. It is a dry Saturday, a soft broom and a few bags of kiln-dried sand, and it is the one piece of driveway maintenance that genuinely pays for itself.
Not worth doing yourself
Anything where the blocks rock underfoot, the edge course has moved, or there is a rut where the wheels sit. Sand will not fix a failed edge restraint or a sub-base problem, and running a plate compactor over one will make it worse.
Ask most people what the sand between block pavers is for and they will say it keeps the weeds down. It is the other way round. The weeds are what you notice once the sand has gone.
That sand is doing structural work, and understanding why changes how you look after a driveway — starting with the pressure washer in the garage.
What the sand is actually doing
Block paving is not a slab. It is a few thousand small units that carry a car only because they are locked against each other, and the thing doing the locking is the sand in the joints.
Trade guidance puts it plainly: jointing sand is not there simply to fill the gaps between adjacent blocks. It provides the friction that generates the horizontal and vertical interlock which allows small blocks to carry relatively big loads. Without it, the pavement would literally fall apart.
Joints are meant to be between 2 and 5 millimetres wide and full to the top, filled with a fine, dry, free-flowing silica sand — which is what kiln-dried sand is. Dry matters: damp sand bridges partway down the joint and never reaches the bottom.
Why the weeds turn up
Weeds in block paving almost never grow up from underneath. Seed lands on the surface and germinates in whatever is sitting in the joint.
A joint packed full of sharp, dry silica sand is a poor place to be a seedling. A joint that is half empty and holding dust, moss and leaf litter is an excellent one — it holds water and organic matter, which is precisely what a seed needs.
So weeding is downstream of re-sanding. Kill and clear what is there by all means, but if you stop at that, you are back to the same thing next spring.
Jet washing is usually how the sand left
Pressure washing block paving is not forbidden, and a driveway that has gone green does need cleaning. The problem is that a lance strips the joints, and most people never put the sand back.
The trade guidance is specific about technique. If a power hose is used, care must be taken to avoid removing the jointing material; the lance should be held at no more than 30 degrees to the horizontal and worked diagonally across the joints rather than along them; and afterwards the pavement should be inspected and the jointing material replaced as required, once it is dry.
That last clause is the half everybody skips. Cleaning a driveway and not re-sanding it is not maintenance — it is quietly taking the structure out of it.
Why polymeric jointing compound is the wrong product here
The polymeric jointing compounds sold for patios look like the permanent answer to weeds. On block paving they usually are not, and the reason is in the manufacturers' own instructions.
These products specify a minimum joint width — 3 millimetres for some, 5 millimetres for others — and a minimum joint depth, typically 20 to 30 millimetres, with the full depth required in trafficked areas. They also require a stable, free-draining base.
Standard block paving joints are narrower than that by design. Forcing a compound into a joint below its stated minimum is outside what the manufacturer supports, and you lose the ability to top the joint up later. On a block driveway, kiln-dried sand remains the right material.
The method
Nothing here is difficult. The only things that matter are that everything is dry and that you do it more than once.
Step 1
Pick your weather
Dry paving, dry sand, and ideally a couple of dry days afterwards. This is the single biggest difference between a joint that fills and a joint that looks filled. If the blocks are damp, wait.
Step 2
Deal with the weeds first
Pull or treat them and let them die back before you start. Brushing sand over living weeds just buries them, and they come straight back through the fresh joint.
Step 3
Clear the joints, don't excavate them
Take the moss, silt and debris off the top of the joint with a stiff brush or a joint knife. You are not trying to empty the joint — any sound sand still down there is doing its job and should stay.
Step 4
Spread and brush, in several directions
Tip the sand out in small heaps rather than one pile, and work it in with a soft-bristled broom. Brush along the joints, across them, and diagonally. Sand finds its way down when it is pushed from more than one angle.
Step 5
Sweep the surface clean
Leave nothing loose on top. Sand left lying gets walked into the house and washes into the gullies, and a driveway with a film of sand on it looks worse than it did before you started.
Step 6
Do it again in a fortnight
Traffic and rain settle the sand down into the joint. One pass never fills it. A second pass a couple of weeks later, and often a third, is what gets a joint properly full — after which it needs topping up perhaps once a year.
What you need
- Kiln-dried jointing sand — buy more than you think, since an open joint takes far more than a tight one
- A soft-bristled broom for brushing in
- A stiff brush or joint knife for clearing the tops
- Weed treatment, or a knife and patience
- A dry forecast
Allow a couple of hours, twice, a fortnight apart.
When it isn't the sand
Re-sanding fixes open joints. It does not fix the ground.
If blocks rock underfoot, if there is a rut where the wheels always sit, if the edge course has crept outwards or there is a dip in front of the garage, the problem is the sub-base or the edge restraint and no amount of sand will hold. Adding sand to a driveway that is moving simply gives it somewhere to disappear to.
One safety point while you are down there: do not run a plate compactor over paving with cracked blocks or a failed edge. Without a sound restraint at the perimeter, compaction pushes the whole field outwards.
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If it's moving, sand isn't the answer
Open joints are maintenance. Rocking blocks, ruts and a sinking edge are the build underneath, and the only way to know which you have is for someone to open it up and look.
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