Why Cracks and Rocking Slabs Need Attention

A slab that moves underfoot is more than a cosmetic nuisance. Every time it rocks, water is finding its way beneath it, and once that happens the damage accelerates quickly. In winter, trapped moisture freezes, expands and lifts the slab further, which is why a problem that seems minor in July can look considerably worse by February.

Left alone, the consequences stack up:

• Trip hazards — a slab standing proud by even 10mm is enough to catch a toe, and a paving surface is one of the most common places for falls at home.
• Progressive failure — a single loose slab lets water reach the bedding and sub-base, undermining its neighbours in turn.
• Weeds and ants — gaps and voids become a seed bed, and the resulting growth prises joints open further.
• Frost damage — any slab that holds water on its surface will spall, flaking away its top layer season by season.

The good news is that repair work is well within the reach of a competent DIYer. A couple of days, a few basic tools and a bag of sand and cement will restore a surface that is safe, level and tidy.

Assessing the Damage and Gathering Your Tools

Before lifting anything, walk the whole area and note what moves. A gentle press with your foot on opposite corners will show which slabs are rocking, and tapping them with a hammer handle gives a hollow ring where the bedding has failed. Cracks that run across the face of a slab usually mean the slab itself is finished and will need replacing, while cracks that follow the joint lines are simply failed mortar and can be raked out and repointed.

For a typical repair you will want:

• A club hammer and bolster chisel, plus a slab lifter or two sturdy crowbars
• A spade and a hand trowel for excavation and bedding
• Sharp sand, cement and clean water, with a bucket for mixing
• A rubber mallet, a spirit level and a length of straight timber at least a metre long
• Safety glasses and gloves — lifting slabs is heavy work and edges bite

Lifting the Slab Without Making Things Worse

Work from a sound slab rather than the loose one, sliding a bolster into the joint and prising gently. Take your time: levering too hard against a neighbouring slab will crack it. Once the slab is free, lift it with both hands close to your body and stack it face to face with another slab to protect the surface. Never drag a slab across the paving.

Dig out the old bedding to a depth of about 75mm and check the sub-base underneath. If it is soft, muddy or full of roots, it needs replacing rather than patching. Remove any loose material and any offending roots, cutting them back cleanly rather than tearing them.

Preparing a Firm, Well-Draining Base

Reinstate the sub-base with MOT Type 1 or a similar compactable aggregate, laid in two layers of around 40mm. Compact each layer thoroughly with a plate compactor or a heavy hand tamper — this is the step most often skimped, and it is the reason repairs fail again within a year. The finished sub-base should sit about 40mm below the final slab level.

Give the surface a fall of roughly 1:60 away from the house, so water runs off rather than pooling at the wall. Check this with your spirit level on the straight timber, packing sand beneath one end until you have the slope you want. A very common cause of recurrence is a fall that runs back towards the building.

Resetting the Slab and Checking Levels

Mix a bedding mortar of four parts sharp sand to one part cement, adding water until it holds together when squeezed but does not glisten. Sharp sand gives grip and drainage; builder's sand is too soft for this job. Lay the mix about 30mm deep, slightly proud of the surrounding paving so the slab beds down into it.

Position the slab and press it down firmly, then tap it level with a rubber mallet on a scrap of timber to spread the blows. Check across to its neighbours with the straight edge and level, aiming for joints of 10 to 12mm. Use string lines for long runs so the joints stay straight, and keep your fingers clear as you adjust. Leave it undisturbed for at least 24 hours before pointing.

Repointing Joints and Final Aftercare

Rake out any loose or crumbling mortar along the surrounding joints to a depth of about 20mm and brush the dust away. Mix a fresh batch of pointing mortar — three parts sharp sand to one part cement is a good, durable ratio — and work it into the joints with a pointing trowel, pressing firmly so it fills the full depth rather than sitting on top. Finish with a jointer or the rounded handle of a finger dipped in water for a smooth, slightly recessed profile.

Once it has firmed, tidy the surface with a soft brush, avoiding the temptation to wash it down while the mortar is green. Keep foot traffic off the area for 48 hours and protect new work from frost with a sheet of polythene or old carpet if a cold snap is forecast. When paving is sound, level and properly jointed, a few hours of work will hold for many years — and if you find several slabs affected, or water pooling against the house, it is worth asking a paving specialist to look at the drainage before you lay anything else.