Why patios sink and crack
A patio rarely fails all at once, and it rarely fails evenly. How it has gone tells you what is wrong underneath, which is what decides whether it needs relaying or rebuilding.
People choose a patio by the slab. The slab is the part you can see in a showroom, and it is the part that least decides how long the patio lasts. Almost everything that goes wrong is underneath, finished and buried before the paving arrives.
The useful thing is that the failure is legible. A rocking slab, a sunken corner and a cracked slab are three different faults with three different causes.
A slab that rocks
Step on one corner and the opposite corner lifts. That means there is a void under it.
Slabs should be laid on a full wet mortar bed, with mortar under the whole of the underside. The quicker method, and the one behind a lot of early failures, is spot bedding: five dabs of mortar, one at each corner and one in the middle, with nothing in between. It goes down faster and it uses less material.
The problem is what fills the gaps. Water gets under the slab and has nowhere to go, because the slab caps it. In winter it freezes, and freezing water expands. That is enough to break the mortar dabs away from the slab and to crack the slab itself from underneath. A patio bedded this way can look immaculate for two winters and start rocking in the third.
You can usually tell by tapping across the patio with a mallet or the heel of your hand. A slab on a full bed sounds dull and solid. A slab over a void sounds hollow, and the sound changes as you move across it.
One area sunk, the rest sound
When a patio dips in one place and the rest of it has not moved, the problem is usually below the sub-base rather than in it.
Common causes are a soft spot that was never dug out, a service trench that was backfilled loosely and then paved over, or made ground from an earlier building that was never compacted. The sub-base bridges it for a while and then settles into it.
This is the most repairable of the faults. If the rest of the patio genuinely tests solid, that section can be lifted, dug out properly and relaid.
The whole patio has dropped
Even settlement across the whole area points at the sub-base itself: laid too thin, or tipped in and levelled without being compacted in layers.
Compacting matters more than people expect. A sub-base is not compacted by the weight of the slabs on top of it or by walking on it. It is done in layers with a plate compactor, because a thick layer compacted in one pass is only firm at the top. Around 100mm of compacted hardcore is normal practice for a domestic patio that only takes foot traffic, and more than that where the ground is poor.
There is a British Standard for this, BS 7533, which covers paving design and construction. It is worth knowing the name of, because it is the benchmark a contractor should be working to and the document that gets referenced if a patio is ever disputed.
A cracked slab, not a loose one
A slab that has cracked across the middle while staying put has usually been unsupported underneath and then loaded. Spot bedding causes this, and so does a hollow left where bedding sand has washed out.
Cracks that run in a line across several slabs are a different matter, and usually mean the ground below is moving rather than the bedding failing.
Why Norfolk ground changes the answer
Norfolk is not one ground type, and that matters more than it sounds.
There is heavy boulder clay through the south of the county, free-draining sand over the Brecks and parts of the coast, and peat in the Fens and the Broads. These behave completely differently under paving. Clay shrinks in a dry summer and swells again over a wet winter, and that seasonal movement is exactly what opens up joints and tips slabs out of level. Sand does not do that, but it moves if water is running through it and nothing is holding it in place.
So the right build-up follows the ground rather than a standard recipe. A specification that is perfectly sound on sand is under-built on clay. It is the main reason two patios laid the same way in the same year can behave quite differently a few miles apart.
Where the water was supposed to go
Standing water after rain is its own diagnosis. A patio should be laid with a slight fall so water runs off it, away from the house.
If water sits in the middle, the levels are wrong. If it sits against the wall of the house, that is worth dealing with sooner rather than later, and not only for the patio’s sake. Water that has nowhere to run will also find its way under the slabs, wash out the bedding, and start the sequence at the top of this page.
What is worth saving
Be honest about the arithmetic. Lifting and relaying one failed section is sensible work when the rest of the patio is sound and the slabs come up cleanly.
If the base was thin or uncompacted across the whole area, patching the worst part does not fix anything. The patio will keep failing in whichever section is next weakest, and doing it in pieces over several winters costs more than doing it once.
The test is not how bad the sunken part looks. It is whether the rest of it is genuinely solid when you tap across it.
If you want someone to look at it
We lay patios across Norwich and the surrounding villages, and we get called out to plenty that somebody else laid.
If yours is rocking, dipping or holding water, request a free quote and we will come and have a look at what is actually going on underneath. There is more about how we build them on the patios page.
Not answered here?
Ring Danny and ask. He would rather talk it through than have you guessing.
Questions we get asked about this
01. Can you relay just the part that has sunk?
Often yes, and it is worth asking about before agreeing to a full rebuild. If the rest of the patio tests solid and the failure is one area, that area can be lifted, the base put right and the same slabs relaid. The honest caveat is that the repair is only as good as the ground under it. If the base was thin across the whole patio and one part simply gave way first, patching it buys a year or two and the next area goes.
02. Is it worth lifting the old slabs and reusing them?
It depends on how they were bedded. Slabs laid on a full mortar bed usually come up with mortar stuck to the back, and cleaning them off is slow work that can cost more than the slabs are worth. Slabs on spot bedding often lift cleanly because most of the underside was never in contact with anything. Natural stone is more likely to be worth saving than a thin concrete slab.
03. My patio is only a few years old. Should it have moved already?
No. A patio built on a properly dug and compacted base should not be rocking or sinking within a few years. Movement that early points at the build rather than the ground. It usually means a base that was too thin, one that was never compacted in layers, or one laid straight over soft ground that should have been dug out first. Ground conditions make a difference, but they do not explain a patio failing that quickly.
Get a free quote for your garden
Tell us what you need doing and Danny will come out, take a look and give you a price. No charge for the visit and no pressure afterwards.