How to Fix Sunken Patio Slabs – DIY Guide
Fix sunken patio slabs by lifting the affected slabs, removing the old mortar bed, laying a fresh sand and cement mix to the correct level, and re-seating the slabs with even joint spacing. Allow 24–48 hours to cure before walking on the surface.
- Bolster chisel
- Club hammer
- Brick trowel
- Rubber mallet
- Spirit level
- Pointing trowel
- Stiff-bristle brush
- Safety goggles and gloves
- Sharp sand
- Cement (grey, general purpose)
- Dry mortar mix (5:1 sand to cement)
- Patio jointing compound or dry mortar
- Weed-suppressing membrane (if required)
- Water
Step-by-Step Guide
Identify and Mark the Sunken Slabs
Walk the patio and press down on every slab — any that rock, dip, or sound hollow when tapped with your foot need lifting. Use chalk or a marker to outline each problem slab before you start, so you have a clear picture of the repair area. Check with a long spirit level across neighbouring slabs to confirm the drop and the direction of the fall.
Lift the Slabs Safely
Insert a bolster chisel into the mortar joint at one edge of the slab and strike firmly with a club hammer to break the seal. Work around two adjacent sides before trying to lever the slab up — forcing it from one point cracks the slab. Once the joint is broken, use the chisel or a flat bar to lift one corner, then slide the slab clear. Stack lifted slabs on edge, not flat on top of each other, to avoid cracking. Wear safety goggles throughout — cleaning your patio beforehand makes it easier to see joint lines clearly.
Remove Old Mortar and Prepare the Sub-base
Chip away all old mortar from the underside of the lifted slabs and from the exposed sub-base using the bolster chisel and club hammer. Brush the area clean. If the sub-base is loose, compacted hardcore has shifted, or you can see standing water pooling in the void, you must address the root cause — add compacted hardcore or sharp sand to restore a stable, well-drained foundation before laying new mortar. Poor drainage is the most common reason slabs sink in the first place; if this is a recurring problem on your patio, also see our guide to fixing a waterlogged garden for drainage solutions.
Mix and Lay the Fresh Mortar Bed
Mix sharp sand and cement in a 5:1 ratio (five parts sand to one part cement) to a firm but workable consistency — often called a ‘damp’ or ‘semi-dry’ mix, which holds its shape when squeezed in your fist but does not slump. Spread the mortar bed across the sub-base using a brick trowel to the depth required to bring the slab up flush with its neighbours — typically 25–40 mm. Tamp the bed level and check across multiple directions with a spirit level. For a patio laid to drain correctly, maintain a slight fall of around 1:60 away from the house. If you plan to lay a patio on a budget, the same bedding principles apply.
Re-seat the Slab and Check the Level
Lower the slab carefully back onto the mortar bed, aligning it with the surrounding slabs and maintaining even joint gaps on all sides — use wooden spacers or pieces of tile as temporary distance guides. Press the slab down firmly with both hands, then tap across the entire surface with a rubber mallet to bed it in evenly. Check immediately with a spirit level in both directions and across to adjacent slabs. If one corner is high, tap it down; if it is low, lift the slab and add a small amount of mortar beneath that point before re-seating.
Point the Joints and Allow to Cure
Once all slabs are re-seated and level, fill the joints. For a traditional finish, brush a dry mix of 3:1 sand and cement into the joints, compact it lightly with a pointing trowel, and lightly mist with water. Alternatively, use a proprietary patio jointing compound for a cleaner, more weed-resistant result. Remove spacers and brush away surplus material before it sets. Keep foot traffic off the repaired area for at least 24 hours — 48 hours in cold or damp weather. For best results with joint finishing, see our full guide on how to repoint patio slabs .
Frequently Asked Questions
Why do patio slabs sink in the first place?
The most common causes are poor drainage causing the sub-base to wash away, frost heave in winter that shifts the mortar bed, tree root movement, or an inadequate hardcore base laid when the patio was originally installed. Identifying the root cause before re-bedding is essential — otherwise the slabs will sink again.
Can I re-use the same slabs once lifted?
Yes, in most cases. Clean the old mortar off the underside thoroughly using a bolster chisel and check for cracks before re-laying. Hairline cracks in sound slabs are usually fine; anything cracked through the body of the slab should be replaced to avoid it breaking under load.
Do I need to lift all sunken slabs at once or can I do them one at a time?
You can work one slab at a time, but it is more efficient to lift all sunken slabs in one session so the sub-base work and mortar mixing can be done in a single batch. Doing them individually risks slight level discrepancies between each repair.
What is the correct fall for a patio to drain properly?
A fall of approximately 1:60 (roughly 15–17 mm per metre) away from the house is standard for UK patios. This is enough to shed rainwater without being noticeable underfoot. If you are planning a full patio installation, incorporate this fall from the start.
How long will the repair last?
A properly prepared repair — with good drainage, a compacted sub-base, and a well-mixed mortar bed — should last ten or more years. The longevity depends almost entirely on whether the underlying drainage issue has been resolved.
Before re-bedding, dry-lay the slab on top of the fresh mortar and mark the high points with chalk dust — then scrape the mortar down precisely at those points rather than guessing how much to remove. This gives you a far flatter, more consistent bed first time and eliminates the back-and-forth of tapping and re-checking.
Sources
- RHS — Patio construction and maintenance advice — rhs.org.uk
- HSE — Manual handling and safe working with heavy paving slabs — hse.gov.uk
- gov.uk — Planning permission for patios and hard standing — gov.uk
This guide is for general information only. Always work safely and follow manufacturer instructions. DIYnut accepts no liability for injury or damage arising from DIY work.
