Patios ·
The rocking slab: what's underneath a patio that moves
A slab that rocks, a slab that cracked under nothing much, and dark patches that appear after rain and never quite dry are all the same fault. You can diagnose it with your knuckles in about two minutes.
Worth doing yourself
Lifting and re-bedding one or two slabs. It is methodical rather than skilled: the mix matters, the primer matters, and taking your time matters. Most people get a better result than the original.
Not worth doing yourself
A whole patio that rings hollow everywhere. If it was spot-bedded throughout, re-bedding slab by slab is a race you lose — each one you fix leaves the next sitting proud or low. That is a relay, not a repair.
Three complaints turn up together on patios and almost nobody connects them: a slab that tips when you step on the corner, a slab that cracked across the middle under a plant pot, and dark shadowy patches that come up after rain and take days to fade.
They are usually one fault with three faces. There are voids under the slabs, because they were bedded on blobs of mortar rather than a full bed. Here is how to prove it, and how to put a slab back properly.
The tap test
Walk the patio tapping each slab, either with a knuckle or with the handle of a screwdriver. A slab bedded on a full bed of mortar gives a dull, solid note. A slab with air under it rings hollow, and the difference is unmistakable once you have heard both.
Do the whole patio and note where the hollow ones are. If it is one or two, you have a repair. If it is most of them, you know something much more useful — the patio was never bedded properly, and no amount of patching will change that.
Three symptoms, one cause
A void under a slab does three things at once.
It lets the slab pivot, so it rocks — usually on the corner furthest from whatever mortar is actually supporting it. It turns anything heavy standing on the unsupported area into a point load, which is why slabs crack across the middle under a barbecue or a planter that a properly bedded slab would carry without noticing. And it holds water.
That last one produces the staining people find hardest to explain. Water sitting in a void under a porous stone or a low-absorption porcelain slab shows through as a dark, mirror-like patch that appears after rain and lingers long after the surface has dried. It is not a cleaning problem and no sealant fixes it, because the water is underneath.
Full bed versus five blobs
Spot bedding — five blobs of mortar, one at each corner and one in the middle — is quick, and it is why a lot of patios get built that way. It is also not acceptable practice, and manufacturers say so directly.
Marshalls' installation guidance for porcelain states that it is to be laid on a full mortar bed and never on a five spot bed or alternative, with the flags supported on a full wet workable mortar bed of one part cement to four parts sharp sand, between 30 and 40 millimetres thick.
Trade guidance sets the same bar in plainer terms: spot bedding, dot and dab, ring bedding, ribbon bedding or any other variation of mortar with voids in it is not acceptable, the bed must be full, and at least 80 per cent of the bedding should be in direct contact with the underside of the slab. Bed depth between 25 and 40 millimetres — much less and the slab is not properly supported, much more and it settles.
The step nearly every DIY job misses: primer
Porcelain has an exceptionally low water absorption value. That is exactly why it is a good paving material, and exactly why it will not bond to a mortar bed on its own — there is nothing for the mortar to grip.
The answer is a slurry primer brushed onto the back of the slab immediately before it goes down: the base pre-wetted, the slurry applied with a stiff brush at one to two millimetres, coating the reverse fully right out to the edges, with any excess wiped off straight away. It has a working life of roughly an hour, so mix what you will use.
Manufacturers recommend it for natural stone too, both to improve adhesion and to reduce the staining that comes up through the slab from the bed. If you lay porcelain without it, the slab will feel fine for a season and then start to lift.
Falls, and the line you cannot cross
While a slab is up, check two things that are much harder to correct later.
First, the fall. A patio has to shed water away from the house. Porcelain guidance gives a minimum longitudinal fall of 1 in 80 and a cross fall of 1 in 40; warranty guidance for paths and drives sets a minimum of 1 in 80 and a maximum of 1 in 12. A patio laid dead flat holds water on every slab, and in winter that water freezes in the joints.
Second, the damp proof course. Approved Document C requires the damp proof course of an external wall to be at least 150 millimetres above the adjoining ground, and manufacturers repeat the same figure for finished paving. That is roughly two brick courses, though the regulation is stated in millimetres, not courses. This is the rule quietly broken every time somebody lays a new patio straight on top of an old one — each layer takes you closer to the DPC, and the consequence is damp inside the house, not a problem on the patio.
Lifting and re-bedding a slab
Assume the slab is heavier and more fragile than it looks, and that the neighbouring slabs are the thing you are most likely to damage.
Step 1
Open the joint first
Rake or cut the jointing compound out of all four joints before you try to lift anything. A slab still bonded to its neighbours by the joint will take a corner off one of them on the way up.
Step 2
Lift from the joint, not the face
Work a bolster or a thin pry bar into the open joint and lever gently, packing under the raised edge as it comes up. Suction lifters make this much easier on porcelain and large-format stone. Two people and a knee pad beats one person and a bad back.
Step 3
Clean both faces back to something sound
Chip the old mortar off the bed and off the back of the slab. You want the sub-base level and firm, and the underside of the slab clean and free of dust — anything left there is a bond breaker.
Step 4
Dry-run the depth
Sit the slab in the empty bed on a couple of offcuts and check where its surface lands against the slabs around it. Work backwards from that to the bed thickness you need, aiming to land between 25 and 40 millimetres of mortar. Adjust the sub-base, not the mortar depth, if you are outside that.
Step 5
Mix, prime, and lay in one movement
One part cement to four parts sharp sand, mixed wet and workable — not a dry crumbly mix you can walk on. Lay the full bed, screed it roughly level, then brush the primer slurry onto the back of the slab and place it straight away while both are wet. Do not prime a slab and then go and make tea.
Step 6
Tap down to level, checking the fall
Bed it down with a rubber mallet, working from the middle outwards so air is pushed to the edges. Lay a level across it and onto the neighbouring slabs in both directions, and confirm the fall still runs away from the house. Mortar should squeeze slightly at the joints — if nothing squeezes anywhere, your bed is not full.
Step 7
Leave it alone, then joint it
Keep off it for a day before jointing, and longer in cold weather. Joint it with the same product as the rest of the patio, at the joint width that product specifies.
What you need
- A bolster and a thin pry bar, or suction lifters
- Joint rake or an old chisel
- Cement and sharp sand, and something to mix in
- Slurry primer and a stiff brush
- A spirit level, ideally a metre one, and a rubber mallet
- Knee pads, gloves and eye protection
Allow half a day for one slab if you have never done it, including the mixing and the cleaning up.
Check services
Before you put a spade or a bar in the ground
Water, gas, electricity and telecoms runs from the house often sit shallower than people expect, particularly around new estates and anywhere a garage or extension has been added later. Find the service entry points, work out where the runs are likely to head, and dig the first spit by hand. Never swing a mattock or drop a breaker into ground you haven't proved.
Two-page PDF covering the checks to make before any digging starts.
PDF · 2 pages
When it is the whole patio
If the tap test comes back hollow across most of the area, stop. You are not looking at slabs that need re-bedding; you are looking at a patio that was built on spot beds, and every one of them will eventually do the same thing.
The honest options are to live with it and fix them as they fail, or to take it up and relay it on a proper bed. What is not worth doing is paying somebody to re-bed a dozen slabs on a patio where the other sixty are queued up behind them.
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Find out what your patio is actually sitting on
One slab lifted tells you everything: the bed, the depth, whether there is a sub-base under it at all. That is a short visit and it decides whether you are repairing or relaying.
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