Inspired by a clear step-by-step guide. This walk-through draws on the well-regarded YouTube video "How to Repair Loose Patio Slabs — a Step by Step DIY Guide", which demonstrates the full process cleanly and shows exactly how to achieve a level result without expensive equipment. Worth watching alongside this guide before you start work.
1. Find the problem slabs
Walk across the patio slowly and listen for the hollow knock of a slab that has lifted away from its bed, or feel for the telltale rock of one that has sunk on one side. Mark any problem slabs with chalk.
Check the surrounding joints too. If the pointing has crumbled or opened up completely, that is often what lets water in and gradually undermines the bed. Both need fixing.
2. Remove the joint pointing
Use a plugging chisel or a flat cold chisel and club hammer to rake out the existing pointing around the slab you need to lift. Go down at least 20mm if you can — the deeper the better, as it gives you more room to work the slab free without cracking it.
A grout rake (a small oscillating tool attachment) makes this job much faster. For a single slab, doing it by hand is perfectly manageable.
3. Lift the slab
Slide a flat bar or wrecking bar into the joint on one side of the slab. Place a piece of scrap timber flat beside it to protect the adjacent slab edge, then lever gently. The slab should start to rise on the levered side.
Once it has lifted a few millimetres, slide a second flat bar in on the opposite side and alternate the lifting until you can get your fingers underneath. Large slabs are heavy — get help with anything over 600mm square. Lower it carefully onto a couple of batons to keep it off the ground.
4. Remove and assess the bed
Scrape out the old mortar bed with a bolster chisel. It is likely to be crumbling, missing in places, or sunken on one side — which explains why the slab was unstable. Bag up the old material and dispose of it.
Check the sub-base below. If the hardcore or compacted sub-base itself has dropped or washed away, fill it back with compacted hardcore before you lay the new bed. Putting a perfect mortar bed on a weak sub-base is a waste of time.
5. Clean the underside of the slab
Brush the underside of the slab clean with a stiff brush. Old mortar sticking up in lumps will stop it sitting flat. If there are high spots, knock them off carefully with a hammer — just enough to get a flat surface.
6. Lay the new mortar bed
Mix a sharp sand and cement mortar at roughly 5 or 6 parts sand to 1 part cement. It should hold its shape when squeezed but not be sloppy. Alternatively, a semi-dry mix (sometimes called a dry or dusty mix) works well — just damp enough to hold together, laid as five blobs in a cross pattern under the slab.
Both methods work. The semi-dry mix is more forgiving because you can tap the slab down gradually without mortar squeezing out and spoiling adjacent slabs.
7. Lower and level the slab
Lower the slab back onto the new bed, checking that it aligns correctly with its neighbours. Tap it down with a rubber mallet, checking the level in both directions and across to the adjacent slabs with a long spirit level.
The slab should match the height of its neighbours and maintain the original fall away from the house for drainage. If it is too high, remove it and take some mortar out. Too low, add a bit more and re-lay.
8. Re-point the joints
Leave the slab for 24 hours before re-pointing so the bed sets properly. Then mix a stiffer mortar (4:1 sand to cement) and press it firmly into the joints with a pointing trowel or a piece of dowel. Work it tight and finish with a slightly concave profile, which sheds water better than a flat or convex joint.
Brush away any mortar smears with a dry brush before it sets hard. Clean any remaining marks with a damp cloth.
9. Keep off it for 24 hours
Mortar sets, it does not dry. Give the pointing at least 24 hours before walking on it, and 48 hours in cold or damp weather. Stepping on fresh mortar will squeeze the slab down unevenly.
When to call a handyman
One or two slabs is a straightforward DIY repair. If several slabs across the patio are affected, or if the sub-base has dropped significantly, you might be looking at a bigger problem with drainage or ground movement beneath. Call Richard if the scope is growing or if the patio is close to the house and you want to make sure the falls are correct.
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The Sandwich Handyman can help with patio repairs, repointing, path maintenance, and a wide range of outdoor property jobs across Sandwich and East Kent.
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