Repairs · · updated
Why driveways sink, and whose fault it is
Four causes, and how to tell which one you've got from the shape of the failure — plus when a lift and relay fixes it and when the whole drive has to come up.
Worth doing yourself
The whole diagnosis. Lifting one block, measuring the sand and probing the sub-base, checking the falls with a string line and a spirit level, and pushing on the edge course all take an hour, cost nothing, and tell you more than most quotes will.
Not worth doing yourself
The fix. A lift and relay means taking up every block, re-grading and re-compacting the base with a whacker plate, re-setting the edge in concrete and screeding fresh sand. It is plant, a skip, and two people for several days — not a weekend.
A driveway that has sunk almost never sank because of the blocks. The blocks are the last thing anybody looks at and the first thing everybody blames. What has moved is underneath them, and the shape of the dip tells you which layer let go.
This is the diagnostic post. Read the pattern of your failure, work out the likely cause, and you will know before anyone quotes whether you are buying a repair or a rebuild.
The four things that actually cause it
Every sunken drive we open up comes back to one of four things, or a combination of two of them.
- Sub-base too thin. Stone was laid at 50–75mm where a driveway needs around 150mm of compacted MOT Type 1, more on clay. It carried a car for a couple of winters and then it didn't.
- Not compacted in layers. The right depth of stone tipped in one go and rolled once is not a compacted sub-base. It consolidates under traffic instead, which is another way of saying it sinks.
- No edge restraint, or an edge that has let go. Block paving works by interlock. Without an edge haunched in concrete, the whole field creeps sideways, joints open and the surface drops.
- Water with nowhere to go. Water in the sub-base is the one that destroys everything else. It washes fines out of the stone, softens the formation and freezes in the joints.
Read the pattern: what the shape of the dip is telling you
Stand at the top of the drive after rain and look at where the water sits. The outline of the puddle is a diagram of the failure.
- A dip in the middle of the drive, roughly circular. Localised sub-base failure or something buried — a backfilled trench, an old soakaway, a service run that was never compacted properly on reinstatement.
- Sinking along the edges, with blocks tilting outwards. Edge restraint failure. The paving is spreading and the perimeter is dropping as it goes. This one gets worse quickly.
- Two parallel troughs in the wheel tracks, the full length of the drive. Structural: the sub-base is too thin or under-compacted across the whole drive. A patch repair here is money you will spend again.
- A dip along one straight line. A trench. Drain run, water main, duct or an old wall footing removed and backfilled with spoil rather than stone.
- Sinking against the house, the garage threshold or a manhole. Almost always water — a downpipe discharging onto the surface, or no channel where the drive meets the building.
When a lift and relay fixes it
A lift and relay is exactly what it sounds like: the blocks come up, the layer underneath is put right, the blocks go back down. It works when the failure is local and the cause is correctable — a settled patch over a trench, a dropped area by a cover, one section of edge that has given way.
It costs £75–£120/m². The low end is a contained area where the sub-base underneath turns out to be sound and only the laying course and joints need putting right. The top end is a full lift with sub-base correction, new haunched edges and re-sanding throughout.
The test is honest and simple: can we identify the cause and reach it without taking up most of the drive? If yes, relay. If the answer is that the stone is thin everywhere, relaying is a repair to the symptom.
When the whole thing has to come up
Rutting in both wheel tracks along the full length. Movement across more than about a third of the area. Blocks that rock everywhere rather than in one patch. Cracked imprinted concrete, or resin that has lifted off a base that is itself failing. In each of those the surface is reporting a fault in the layer that carries the load, and there is no version of relaying that fixes a layer which was never thick enough.
A rebuild costs more than a relay, obviously. It costs less than a relay followed by a rebuild two winters later, which is the actual alternative.
Diagnose it yourself in an hour
You need one block out, a screwdriver, a tape, a length of string and a spirit level. Do it on a dry day so you can tell settled sand from wet sand.
Step 1
Lift one block without breaking it
Start at a cut edge or next to a channel drain, where there is already a joint to work into. Push two long flat-bladed screwdrivers down either side of a whole block — not a cut one — lever them together and lift straight up. Block extractors are about a tenner and do it in one go without chipping the arrises. Lift a block from the middle of the dip and, for comparison, one from a part of the drive that has not moved.
Step 2
Measure the laying course
Sand under a block should be 30–40mm after compaction, and even. If your tape reads 60, 80 or 100mm in the dip and 35mm on the sound area, nobody built up the sub-base properly — they buried the shortfall in sand, and sand migrates.
Step 3
Probe what's under the sand
Push a long screwdriver or a thin steel bar straight down through the sand into the layer beneath and lean on it. Properly compacted Type 1 will stop a bar dead within 10–20mm. If it slides in 100mm with hand pressure, or you hit soft clay or topsoil almost immediately, you have found the reason. Note the depth it stops at — that is roughly your sub-base thickness.
Step 4
String-line the falls
Peg a taut string from the wall of the house to the outer edge of the drive, level it with a spirit level, and measure down from the string to the paving at both ends. Over a 6m run you want at least 100mm of drop away from the house — that is 1:60. Anything flatter, or falling towards the building, and water is sitting on the drive rather than leaving it.
Step 5
Push on the edge
Put a foot against the outside face of the perimeter course, low down, and shove hard. It should not move at all. If it rocks, tilts outwards or you can see an open joint behind it, the edge restraint has gone. Everything inboard of a failed edge will keep creeping outwards no matter what you do to the middle.
Step 6
Read the shape of the failure
Two parallel dips where the car sits means the sub-base is too thin for the load. One straight dip across the drive usually means a trench — a service run backfilled loose. Sinking all round the perimeter is edge restraint. Water standing in the middle with no dip at all is falls, not settlement.
What you need
- Two long flat-bladed screwdrivers, or block extractors (about £10)
- A thin steel bar or a long screwdriver for probing
- Tape measure, spirit level, string line and two pegs
- A dry day, and a phone to photograph each measurement
Bed the block back on the same sand and tap it down level with a rubber mallet, then brush kiln-dried sand into the joints. Allow an hour.
One caution on lifting blocks
Lifting a block or two is harmless. Lifting a run of them next to a driveway edge, a kerb line or a retaining course is not — take the restraint away and the whole field can spread. Take one block, take your measurements, and put it back the same day.
Why the fix isn't a weekend job
Once you know the sub-base is thin or the edge has gone, the honest answer is that the repair is not proportionate to DIY. A lift and relay is: every block up and stacked, the sand off, the base re-graded and brought up to depth in compacted layers, the edge course re-bedded in concrete, then the whole field re-laid, re-screeded and vibrated in.
That needs a plate compactor, a way of moving several tonnes of Type 1 and several tonnes of arisings, and two people who can lay a field flat and keep the courses straight. It is three to five days on a typical drive. The measurements above are what tell you whether you are buying that, or a rebuild.
If it's still under someone else's guarantee
Before you pay anyone, go back to whoever laid it. Put it in writing, attach dated photographs, and ask four specific questions.
- Does the guarantee cover the sub-base, or only the paving? This is the question that decides everything. A great many driveway guarantees cover the blocks and the laying — that is, the part that hasn't failed.
- Is it insurance-backed, or is it the company's own promise? A company promise is worth exactly as long as the company lasts.
- What is the specified build-up you were quoted — excavation depth, sub-base depth, edge detail, falls? Ask for it in writing and compare it to what you were given.
- What is the remedy: repair, relay, or rebuild — and who decides which?
Get the failure diagnosed before you pay for a repair
Send photographs of the dip and of the drive after rain, and we'll tell you which of the four it looks like and whether it's a relay or a rebuild. If it's under someone else's guarantee, we'll say that instead of quoting.
Read next
- Laying turf on Aylesbury Vale clay: what has to happen first
Why turf laid straight onto clay dies, what the failure looks like at three weeks and at three months, and the preparation that actually works.
- Shed, garden room and hot tub bases: the spec you actually need
Start with the arithmetic — water weighs a tonne per cubic metre — then work out whether you need compacted Type 1 and slabs, or reinforced concrete.
- Driveway prices across Buckinghamshire
What a proper rebuild costs, broken down by surface, so you can sanity-check a repair quote.
- Driveway drainage: the questions people ask
Where the water is supposed to go, and what happens to the base when it doesn't.
- How we build
The seven stages, and what each one is preventing.
- Our guarantee
Five years on workmanship, sub-base included — the wording in full.