Your old patio has been pooling water against the back door again. The slabs aren't broken — the fall is wrong, the mortar bed has crumbled in places, and the sub-base was probably never deep enough in the first place. A British patio that lasts for fifteen years isn't about expensive stone. It's about the four inches of MOT Type 1 underneath, the mortar mix you actually trowel out, and the 1-in-80 slope that takes water away from the house.
Spring 2026 is the moment to put one in properly. Slab prices have settled — porcelain 600×600 averages £38–£55 per square metre at Wickes and Travis Perkins; riven sandstone is back to £30–£42 per square metre after the Indian quarry shortages eased last year. Sand and cement haven't moved much: a 25 kg bag of sharp sand is around £4.20, OPC cement £6.80. The labour you'd pay a landscaper for a 20 m² patio still sits between £1,800 and £2,800. Doing it yourself, materials in, you'll spend £900–£1,400 — which is why the queues at Travis Perkins on a Saturday in May are mostly homeowners with hire trailers.
Get the planning and measurements straight before you order anything
Patios under 30 m² and below 300 mm above ground level don't need planning permission in England, Wales or Scotland. There is, however, a quiet rule that catches people: if you're replacing a permeable surface (lawn, gravel) with an impermeable one and the area is over 5 m², you need either a soakaway, a permeable jointing system, or planning permission. The 2008 update to Permitted Development is still in force in 2026, and councils enforce it when neighbours complain about run-off. The simple fix is to lay the slabs with a 5 mm permeable jointing compound (GftK VDW 850 is the trade default) — that gets you compliant without a soakaway.
Measure twice and add 10% to your slab order for cuts and breakages. A 4 m × 5 m patio is 20 m², so you order 22 m². For 600×600 slabs that's 62 slabs, plus mortar quantities of roughly one 25 kg bag of sharp sand and one 25 kg bag of cement per 1.5 m² of bedding (a 4:1 mix is standard). A typical 20 m² job needs around fifteen bags of sharp sand, four bags of cement, and 1.5 tonnes of MOT Type 1 for the sub-base — which is a builder's bag and a half delivered.
Tools you actually need (and one that's not optional)
- A hire-a-plate compactor (£35–£45 per day from HSS or Brandon Hire). Don't try this with a hand tamper — sub-base settling is what kills patios.
- A spirit level — at least 1.2 m long. A 600 mm level lies to you on a patio.
- String line and pegs for setting your falls.
- A rubber mallet, a brick bolster, a club hammer, and a wet-cut tile saw or 9-inch angle grinder with a stone-cutting disc.
- Knee pads. Trust me.
Excavation: the depth most homeowners get wrong
The single biggest mistake is digging too shallow. For a domestic patio that won't take vehicles, you need 150 mm below the finished slab level, broken down as: 100 mm of compacted MOT Type 1 sub-base, 30–40 mm of mortar bed, and the slab thickness itself (usually 22 mm for porcelain or 25–30 mm for sandstone). If the patio sits next to the house, the finished surface must be at least 150 mm below the damp-proof course — measure your DPC (the black line in the brickwork) and work down from there.
Strip the topsoil and any soft material until you hit firm sub-grade. Old lawn, root zones and made-up ground all need to come out. If the ground bounces under your boot when you walk on it, dig deeper. South of the M25 you'll often hit clay — that needs a geotextile membrane (Terram 1000 or equivalent, £45 for a 4×25 m roll) laid over the sub-grade before the MOT goes in, otherwise the clay will heave the patio over a couple of winters.
Setting the falls — the bit that separates a £900 patio from a £900 puddle
Water has to move away from the house at a fall of 1 in 80 minimum. That's 12.5 mm of drop per metre. On a 4 m patio running away from the house, the far edge sits 50 mm lower than the wall edge. Mark this with two tight string lines pegged at the correct heights — never trust your eye, and never trust a level held against a 600 mm slab. The fall isn't optional and it isn't subtle. If you set 1 in 100 because "it looks flat", you'll regret it the first time the gutter overflows in November.
And here's where most DIY guides go quiet: the fall should run across the patio, not just away from the house. If the patio is wider than it is deep, you want a slight cross-fall too, towards a planted bed or a gravel strip — somewhere the water can soak away. Otherwise it pools at one end of the slope.
Sub-base: the bit nobody sees and everybody gets wrong
MOT Type 1 is graded crushed stone, 0–63 mm, designed to interlock when compacted. Spread it in two layers of 50 mm each, never one 100 mm layer — the compactor only reaches 75 mm into a single layer, and the bottom 25 mm stays loose. Each layer gets at least four passes with the plate compactor. You'll know it's done when the surface looks tight and the compactor stops sinking.
Wet the sub-base lightly between passes if it's bone dry — a fine spray with a hose, not a flood. Damp Type 1 compacts better than dry, but waterlogged Type 1 turns to soup and you'll have to wait days for it to dry. A common builder's trick: borrow a wacker plate on Saturday morning and you can do all the compacting in two hours. The lawn pattern will recover by August.
Bedding the slabs: full mortar bed, not five spots
The five-blob method — five lumps of mortar, slab pressed down — has killed more patios than frost. It leaves voids underneath where water sits, freezes and lifts the slab. For domestic patios in 2026, the British Standards code BS 7533-101 essentially requires a full mortar bed, and the porcelain slab manufacturers (Marshalls, Bradstone, London Stone) all void warranties without it.
Mix a 4:1 sharp sand to cement, with enough water to make a stiff, ball-shaped mix that holds its shape when squeezed. Too wet and the slab sinks unevenly. Too dry and it won't bond. Trowel a 30–40 mm bed slightly larger than the slab, butter the back of porcelain slabs with a slurry primer (SBR-modified — £18 for a 5-litre bottle from Toolstation), and bed the slab with a rubber mallet against your string line. Check the level along the slab and to the next slab. Adjust by tapping down high corners — never lift and re-bed unless the level is way off.
Joints: the difference between a patio and a weed garden
Leave 5 mm joints between slabs — uniform, deliberate. Use 5 mm tile spacers if you don't trust your eye. Do not butt slabs together. Tight joints crack at the edges within two winters because the slabs need a fraction of room to move with temperature. After 24 hours of cure, sweep a brush-in jointing compound across the joints — Sika FastFix, GftK VDW 850 or Marshalls Weatherpoint 365 are the ones that actually last. Skip "kiln-dried sand" unless you enjoy weeding — it washes out by year two.
The cuts you can't avoid
Every patio has at least one tricky edge. If you're going against a curved bed or around a manhole cover, the cuts are unavoidable. A wet-cut tile saw (£90 for the day from HSS) gives you straight, chip-free edges on porcelain. For sandstone, a 9-inch angle grinder with a diamond stone disc works fine, but expect dust — wear a P3 mask (not the cheap P1 paper kind) and goggles, not glasses. The 2026 HSE guidance on respirable crystalline silica means wet-cutting is now strongly recommended even for hobby use.
For the manhole cover, don't slab over it. Use a recessed tray cover (£75–£140) that takes a slab on top, so you keep access without breaking the visual. The drainage company won't thank you for sealing their cover under mortar.
The first 48 hours, and the rookie mistake
Do not walk on the slabs for at least 24 hours after bedding. Don't even let the dog on. The mortar needs time to set; standing on it now leaves invisible voids that show up as rocking slabs in eighteen months. After 24 hours, you can sweep on the joint compound. After 48 hours, you can put the garden furniture out. Do not hose the patio for the first week — let the mortar cure naturally.
The rookie mistake people make on hot bank holiday weekends: laying slabs in direct 22°C sun, with mortar drying on the trowel before it touches the slab. Either work in the shade of the house, lay early and late, or wet the slabs and the bed lightly before bedding. A skin-dry mortar bed will not bond and the slab will eventually rock.
What this actually costs in May 2026
For a typical 20 m² rear patio, here's the realistic spend at Travis Perkins or Wickes prices in May 2026: porcelain slabs at £40/m² is £880 (with 10% extra). MOT Type 1 sub-base for 1.5 tonnes is £85 delivered. Sharp sand fifteen bags is £63. Cement four bags is £27. Permeable jointing compound is £55. SBR primer is £18. Compactor hire two days is £80. Plus skip hire (£180) if you're shifting old paving. Total: roughly £1,400. A landscaper would have charged you £2,400 plus VAT. Saved labour: about £1,000, plus the satisfaction of knowing what's under there.
If you skipped the geotextile membrane on clay ground because it "felt firm", come back here next March and tell me how the slabs are sitting. The patio you build in May 2026 should still be flat and draining in May 2041 — and the only way to get there is the unglamorous bit underneath.