What causes a slab to rock
Slabs are set on a bedding layer — traditionally a semi-dry mortar mix, sometimes a full mortar bed. A rocking slab means there's a void underneath it, usually because the bedding wasn't fully compacted under the slab when it was laid, leaving a hollow at one corner or edge.
Over time, water finds its way into that void and washes more material out, which is why a slight rock when new often gets progressively worse rather than better.
Other causes worth ruling out
Frost heave — water in the bedding freezing and expanding — can lift a slab slightly during winter and it may settle back or may not, depending on how much material was disturbed.
Tree and shrub roots growing beneath a patio can lift slabs steadily over months or years, usually starting nearest the planting and spreading outward.
Checking the damage yourself
Walk the whole patio in shoes with a hard sole so you can feel the difference between a solid slab and a hollow-sounding one. Note how many slabs are affected and whether they're clustered together or scattered randomly.
A single rocking slab, or a small cluster, is usually a straightforward localised repair. Rocking spread evenly across most of the patio suggests a widespread bedding or base issue rather than a one-off.
- One or two rocking slabs, isolated: localised bedding void, easy fix
- Cluster of rocking slabs near a tree or large shrub: check for root movement
- Rocking spread across the whole patio: widespread bedding or base issue
- A slab noticeably higher than its neighbours: heave or root lift, not a void
How a proper repair is done
The slab is lifted, the old bedding cleaned off both the slab and the base, and a fresh, fully compacted bedding layer laid before the slab goes back down level and firm — not just re-pointed around the edges, which does nothing for the void underneath.
Where roots are the cause, the root needs addressing (or the slab repositioned clear of it) or the lift returns within a season or two.
When it points to a bigger job
If most of the patio is affected, it often means the original bedding was inconsistent across the whole job — in which case a full re-lay, rather than repeatedly chasing individual slabs, is the more sensible long-term route.
Frequently asked questions
Is a rocking slab dangerous?+
It's a genuine trip hazard, especially outside at night, so it's worth dealing with rather than leaving it.
Can I just re-point around a rocking slab?+
No — pointing doesn't fill the void underneath, so the slab will keep rocking and the new pointing will crack again quickly.
Will lifted slabs need replacing?+
Usually not — most slabs can be lifted, the bedding corrected, and the same slab relaid without any visible difference.
How do I know if a tree root is the cause?+
Rocking or lifting concentrated near a tree or large shrub, spreading outward over time, is the typical pattern.
Is patio repair covered by your guarantee?+
Yes, repair work carried out by us is covered under our 10-Year Guarantee.
Get your rocking slabs sorted properly
A quick call or a couple of photos is usually enough for us to scope the repair — 0191 743 3072 or email us anytime.