Why Patios Sink, Crack or Lift, and How to Put Them Right

A patio that rocks underfoot, holds a puddle by the back door or has a crack running across a slab is annoying at best and a trip hazard at worst. The good news is that the causes are usually easy to identify once you know what to look for, and not every problem means starting again.

Below we go through the common faults, what tends to cause each one, and the honest range of fixes. If you would rather someone looked at it for you, our patio and paving team repairs and relays patios as well as laying new ones.

Slabs that rock or sound hollow

Tap a slab with the handle of a broom. If it sounds hollow, or rocks when you step on one corner, it is not fully supported underneath. The most common reason is that it was laid on a few dabs of mortar rather than a full bed, leaving voids beneath. Over time water gets into those voids, washes out fine material and, in frosty weather, freezes and pushes the slab about.

A single loose slab can usually be lifted, the old mortar cleaned off, and the slab relaid on a full bed and repointed. If most of the patio sounds hollow, the same fault runs right through it, and relaying the whole area on a proper bed is usually the lasting answer.

Sinking and dips

When part of a patio drops, the cause is almost always below the bedding. The usual suspects are a sub-base that was too thin or never properly compacted, soft ground that was not dug out, or an old trench, such as a drain run or a filled-in pond, that has slowly settled. Clay soils, which are common across much of South Wales, add to the problem because they shrink in dry spells and swell again when wet.

• A small dip in one area often points to a local soft spot or a settled trench. Lifting that section, rebuilding the base and relaying can be enough.
• A patio sinking all over suggests the whole sub-base is inadequate. Patching will only buy time; taking it up and rebuilding the base is the real fix.
• Sinking next to the house can mean a leaking drain or downpipe washing out the ground. That needs checking before any paving goes back down.

Cracked slabs

Slabs usually crack for one of three reasons. They were unsupported underneath, so they flexed under load and snapped. They have carried weight they were not made for, such as a car, a skip or a heavy hot tub on paving meant for foot traffic. Or the ground beneath them has moved. A fine crack in natural stone can also follow a natural vein in the slab.

A cracked slab can normally be cut out and replaced, although matching colour can be tricky with natural stone that has weathered, and older concrete ranges may no longer be made. It is worth keeping a few spare slabs from any new patio for exactly this reason. If cracks keep appearing, look at what is going on underneath before replacing more.

Lifting and heaving

Paving that rises, rather than sinks, is most often caused by tree or shrub roots growing beneath it. Roots thicken each year and can lift slabs, tilt edging and push blocks out of line. Frost can also heave paving on wet, poorly drained ground, where water trapped in the base freezes and expands.

Roots need careful thought. Cutting large roots can harm or destabilise a tree, and if the tree is protected by a tree preservation order or sits in a conservation area, you may need the council's consent before doing any work to it. Our guide on permission for tree work in Wales covers the basics, and our tree surgery team can advise on the tree itself. Sometimes the best answer is to reshape the paving around the tree rather than fight it.

Puddles and water by the house

Standing water means the patio has too little fall, the wrong fall, or a dip. Water pooling against the house wall is the one to take seriously, especially if the paving is close to the damp-proof course, because it can lead to damp inside. Sometimes a channel drain or a small alteration at the edge will cure it; in other cases the patio needs lifting and relaying to a proper fall away from the building.

Water that has nowhere to go on a wet site may be a wider drainage problem. Our guide to stopping a garden waterlogging explains the options.

Crumbling joints and loose edges

Joints that have cracked, crumbled or been blasted out by a pressure washer let water into the bedding and give weeds a home. Raking them out and repointing with a suitable mortar or jointing compound is one of the most worthwhile small repairs you can make. Loose slabs along the outside edge usually mean there was no concrete haunch or firm border holding them in; that can be added when the edge slabs are relaid.

Block paving has its own version of this: sand washing out of the joints and blocks creeping at unrestrained edges. Topping up the jointing sand and fitting proper edging normally sorts it. Our article on block paving driveways explains the edge restraint in more detail.

Repair or relay?

A useful rule of thumb: if the problem is confined to a few slabs and the rest of the patio is sound, repair it. If the faults are spread across the whole area, the base is the problem, and repairs will keep coming back. Relaying does not always mean new slabs; good natural stone and porcelain can often be lifted, cleaned and reused on a proper base, which can make it a more modest job than people expect. Our guide to how a patio should be laid shows what the finished base should look like.

Tony and the team will look at a failing patio and tell you plainly which way we would go. We cover the whole region, from steep terraced gardens in Tonypandy to larger plots around Chepstow. Ask for a free quote and we will come and take a look.

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