Loose or Rocking Patio Slabs: Why They Move and How to Relay Them

The most common cause we find is spot bedding. Slabs laid on five dabs of mortar, one at each corner and one in the middle, look fine on the day. Water gets into the voids between the dabs, freezes, and over a few winters the dabs crack away from the underside. The slab then rocks on whichever dab is still holding.

The second cause is a bed that was too dry or too weak when it went down. Mortar mixed dry enough to be easy to work with often has too little cement paste to grip the back of the slab, especially with dense porcelain or riven natural stone that has an uneven underside.

The third is water tracking under the patio from a downpipe, a leaking gully or a border that sits higher than the paving. Moving water washes fines out of the bed, leaving a hollow that the slab drops into. If one corner of your patio is always the loose corner, look at where rainwater goes first.

Walk the patio slowly and tap each slab with the heel of your shoe. A well bedded slab gives a dull thud. A hollow ring means there is air under it, and a slab that has already started to rock will usually be joined by two or three of its neighbours doing the same thing quietly.

Then look at the joints. Isolated movement leaves the pointing largely intact except right around the loose slab. Widespread bed failure shows up as pointing that has cracked out in long runs, slabs that have drifted apart slightly, and edges that are no longer in line.

Finally, check the falls with a spirit level. Paving should shed water away from the house. If puddles now sit where they never used to, the substrate has settled rather than just the bedding letting go.

This is a reasonable job for a confident homeowner. Rake out the pointing around the slab with a plugging chisel or an oscillating tool, then lift the slab with two suction pads or by levering gently at one edge with a bolster and a timber offcut so you do not chip the arris.

Clean the old mortar off the back of the slab and out of the recess, then lay a full bed of 4:1 sharp sand and cement, wet enough to hold a shape but not sloppy. Prime the back of the slab with a slurry primer, especially with porcelain, which will not bond reliably to mortar on its own. Tap the slab down to the level of its neighbours and check with a straight edge across three slabs, not just one.

Leave it to set before pointing, and use the same jointing compound as the rest of the patio so the repair does not stand out. If you are unsure about the primer or the mix, that is the point at which it is worth a call rather than a second attempt.

If more than a fifth of the slabs are loose, or the sub-base has settled and the falls are wrong, patching is a poor use of money. We lift the paving, take the base back to a compacted stone layer, set the correct falls away from the house, and relay on a full wet bed with a primer suited to the slab type.

Most slabs can be reused. Porcelain and dense natural stone usually come up cleanly. Older riven sandstone with a thick mortar back is more likely to break on lifting, so we allow for a small percentage of replacements and tell you before we start rather than after.

Every relay and rebuild we do is covered by our ten year guarantee, which applies across all our work.