Why block paving sinks, and what it takes to put right
Block paving does not sink at random. Where it fails tells you why, and why tells you what it will take to fix.
The blocks are almost never the problem
When a block paved driveway goes uneven, the instinct is to look at the surface. Almost always the answer is underneath it, and the blocks themselves are in perfectly good condition.
That is genuinely good news, because it means the expensive-looking part of your driveway is reusable. The question is not whether you need a new driveway. It is how much of the base has to be rebuilt, and that depends on what caused it.
The useful thing is that block paving fails in a small number of recognisable patterns, and you can diagnose most of them yourself from the pavement.
Read the pattern
Find the one that matches yours. The right-hand column is what it usually takes.
| What you can see | What it usually means | What it takes |
|---|---|---|
| Two parallel troughs where the wheels sit | Sub-base too thin, or never properly compacted. The load path has consolidated | Lift the affected area, rebuild the base to depth, re-lay the same blocks |
| One isolated dip, often near a manhole or across a line | A service trench backfilled loosely, or ground disturbed for a drain or water main | Localised repair. The cheapest kind there is, often a day |
| Settlement along the front, at the kerb | The most heavily trafficked strip, usually built over ground disturbed by the crossover | Rebuild the front metre or two, including the edge restraint |
| Joints opening, edges creeping outwards | Edge restraint has failed or was never haunched. Interlock is being lost | Rebuild the restraint, re-lay the perimeter, re-joint |
| The whole drive is uneven, ponding in several places | The original construction was inadequate throughout | Full lift and re-lay. Still much cheaper than a new driveway |
Why it happens in the first place
Not enough sub-base
The most common cause by a distance. A driveway needs meaningfully more sub-base than a path, because it has to spread a concentrated wheel load out across the ground beneath. Build a driveway to footpath depth and it will carry a car for a while, then it will not.
Compacted all at once
Sub-base has to be compacted in layers. Tipped in one go and rolled over the top, the lower part is never densified. It looks identical on the day and it consolidates gradually for the next two or three years, which is why so many driveways look fine for a season and then go.
Too much sand under the blocks
A laying course is meant to be a consistent screeded thickness of 30 to 50mm. Where it has been used to make up levels because the sub-base was not brought up properly, you get thick sand in some places and thin in others. Thick sand consolidates more. That is a dip, and it appears exactly where the sand was deepest.
Water where it should not be
Water is the accelerant for everything above. It washes fines out of the base, softens clay subgrade, and turns slow settlement into fast settlement. A leaking gully, a blocked or silted soakaway, a downpipe discharging onto the paving, or falls that were never set properly will all do it. This is why standing water is worth taking seriously rather than living with.
Jointing sand that has gone
Less obvious but genuinely structural. Block paving carries load by transferring it sideways between blocks through the joints. Empty joints mean no transfer, so each block works alone, rocks, and grinds its neighbours. Pressure washing the joints out and not replacing the sand is one of the most common ways people damage their own driveway.
Roots and clay
On London Clay, which underlies a good deal of this area, a mature tree close to the frontage draws water out of the ground in summer and the clay shrinks. In winter it takes water back and swells. That seasonal cycle moves paving, and no amount of sub-base entirely prevents it. It can be designed around, but it has to be recognised first.
Diagnose it yourself in five minutes
- Look after rain. Where does water sit? That is where the base has consolidated most.
- Check the joints. Run a screwdriver into one. If it goes in easily to any depth, the jointing sand has gone.
- Walk the perimeter. Does anything rock? Have the edge blocks tipped or moved outwards?
- Lift one block at the edge of the dip. A bolster and a bit of patience. What is under the sand: firm compacted stone, or soft ground?
- Look for a straight line. Settlement in a straight line across the drive is almost always a buried service trench.
What you find under that one block tells you most of what you need to know.
If the joints are simply empty and nothing is moving, a bag of kiln-dried jointing sand and half an hour will genuinely sort it, and it may prevent the problem you would otherwise be calling us about in three years. How to do it properly.
Sinking and settlement questions
Is a small dip actually a problem?
A dip that has stopped moving and does not hold water is mostly cosmetic. A dip that holds water is not, because standing water works into the base, softens it and accelerates the settlement. Water is what turns a small problem into a large one.
The practical test is whether it puddles. If it does, deal with it. If it does not, keep an eye on it and see whether it changes over a year.
Can I just lift the blocks and put more sand under them?
You can, and if the sub-base underneath is genuinely sound it is a reasonable fix. It is a normal thing for a competent DIYer to do on a small area.
But if the sub-base is the problem, adding sand makes it worse, because a thick sand layer consolidates more than a thin one. Lift a block at the edge of the dip and look. If you find firm compacted stone, sand will do. If you find soft ground, loose fill or nothing much at all, the base needs rebuilding.
Why does it always sink near the entrance?
Three reasons that compound. Every vehicle crosses that strip, so it takes the most repeated loading of any part of the driveway. Vehicles are usually turning as they cross it, which adds twisting force. And the ground there was often disturbed when the crossover was built, then backfilled and compacted less thoroughly than undisturbed ground.
It is the single most common place for a driveway to fail, and it usually needs the front metre or two rebuilding properly rather than packing up.
Does a heavier car cause it?
It brings forward a failure that was already coming. A correctly built domestic driveway carries a large family car or a van without difficulty. If a driveway is sinking under a normal vehicle, the base was inadequate rather than the car excessive.
The genuine exceptions are repeated deliveries by heavy lorries, or a skip placed directly on the paving, which is capable of destroying an otherwise sound driveway in an afternoon.
How urgently does it need doing?
If water is ponding, or the movement is close to the house, sooner rather than later. Water in the base is the mechanism that spreads the failure, so a localised problem left alone through a wet winter tends to become a larger one.
If it is dry, stable and away from the building, it can reasonably wait until it suits you. We will tell you which of those it is.
Tell us what yours is doing
Describe where it has gone and roughly how long it has been getting worse. We will tell you honestly whether it is a repair, a re-lay, or something you can put right yourself.
We normally reply within 24 hours.