Paving Slabs: The Complete UK Guide to Choosing, Sub-Basing, and Laying a Post-Extension Patio

Your builder's quote ends at the rear door threshold. The external concrete slab is poured, the doors are in, the plasterers have finished inside. Then the quote you accepted six months ago turns out not to include the patio, because "patio is landscaping, we priced the build." So you're now standing on a concrete raft at 60mm below the finished door level, looking at a churned-up garden, and somebody needs to decide what paving to buy, on what sub-base, with what falls. Get those decisions wrong and the patio rocks within a year, the door threshold sits in a puddle every time it rains, or the whole thing fails building control because the paved surface sits 50mm above the DPC.

Paving is the part of a post-extension build that homeowners are most likely to source and manage themselves, and the part where the sub-base costs more than the slabs on top. Eighty percent of what separates a patio that lasts thirty years from one that fails in eighteen months happens below ground, before the first slab goes down.

What it is and what it's for

Paving slabs are the finished surface layer of a patio, path, or external hard-standing. In a UK post-extension context, a patio typically runs from the back of the extension out into the garden by 3-6m and along the full width of the rear elevation. Material choice splits three ways: concrete (the default, widest range, lowest price), natural stone (Indian sandstone, limestone, slate, granite, Yorkstone), and porcelain (outdoor-grade 20mm vitrified ceramic, increasingly popular since about 2020).

Slabs don't work in isolation. A patio is a build-up of four engineered layers:

1. Sub-base (typically 100-150mm of compacted MOT Type 1 hardcore, BS EN 13285). This carries the load and prevents settlement.
2. Bedding layer (a full mortar bed, 30-40mm of 4:1 sharp sand and cement; or a sharp sand bed for block paving only).
3. Paving slab (20-40mm thick depending on material).
4. Jointing compound (kiln-dried sand for block paving, or a resin or cement-based brush-in compound for slab joints).

The NHBC Standards for drives, paths and landscaping (Chapter 10.2) set the baseline: 100mm of compacted, non-frost-susceptible crushed rock sub-base with 25mm of sand blinding on top for typical soils with CBR (California Bearing Ratio, a soil strength test) between 7 and 20 percent. Clay soils with CBR below 5 percent need 150mm-250mm. For any garden paving within 2m of the house, Building Regulations require the finished surface to sit at least 150mm below the damp proof course and fall away from the building at a minimum 1:80 gradient. These aren't style choices. They're what keeps water out of your wall cavity and what stops the surface ponding in winter.

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Concrete, natural stone, or porcelain: pick the material before anything else

The three main material categories behave differently on site, cost differently at the merchant, and demand different installation technique. Pick one before you price sub-base or book a landscaper, because the slab depth affects the threshold detail and the cutting method affects how long the job takes.

Concrete paving

Concrete slabs are the workhorse. A 600x600x38mm Marshalls Richmond at Wickes lands at around £25/m². Saxon textured at £42/m². Premium Heritage Yorkstone-effect concrete pushes £84/m². Every major UK merchant (Wickes, Travis Perkins, Jewson, B&Q) carries concrete paving under multiple brand names (Marshalls, Bradstone, Stonemarket). You can walk in and buy a pallet today for most standard ranges.

Two weaknesses to know about. Concrete blooms efflorescence (a white calcium-carbonate haze that leaches out as the slab cures through its first summer). This is not a product fault and it is not permanent. Dry-brush it off at 3-6 month intervals during the first year. Wet-scrubbing it redistributes the salts deeper into the pores and makes it worse. Pavingexpert is categorical on this: "Do not wet scrub efflorescence." A penetrating sealer applied after 12-18 months once the efflorescence has fully exhausted itself will prevent further blooms.

The second weakness is colour. Pigmented concrete fades under UV, noticeably after 5-8 years. Through-coloured concrete (where the pigment runs the full depth of the slab) fades less obviously than surface-pigmented budget lines. For a patio that has to look good for 20 years, pay the extra for Marshalls Saxon, Drivesett, or equivalent rather than bottom-of-range builders-merchant concrete.

Natural stone (Indian sandstone, limestone, slate, granite)

Indian sandstone dominates the UK natural stone market. Calibrated to 22mm thickness, sourced from Rajasthan, sold through London Stone, Stonemarket, Paving Slabs UK, Pavestone, and hundreds of online retailers. Supply pricing starts around £22/m² for entry Kandla Grey or Raj Green, rises to £45-55/m² for premium calibrated riven Fossil Mint or Autumn Brown, and the very top Marshalls Antique Sandstone or London Stone Cottage range hits £55-70/m². Older guides cite £40-95/m² because they were quoting installed costs, not supply-only figures; the current supply-only range is substantially lower than older sources suggest.

Limestone and slate sit at similar price points to mid-range sandstone. Granite 30mm is £35-45/m² for paths and patios, £50-60/m² in 50mm for driveways.

The three real pitfalls with cheap Indian sandstone:

• Uncalibrated stock. Some budget retailers sell uncalibrated stone with thickness varying 20-35mm across a single pallet. Uncalibrated stone takes twice as long to lay because the bedding mortar has to be thickened or thinned slab by slab. Specify calibrated 22mm when ordering.
• No declared slip rating. Reputable retailers declare a Pendulum Test Value (PTV) or an R-rating. Cheap stock often declares neither, and the HSE minimum PTV of 36 in wet conditions is not guaranteed on bargain pallets. Riven finish provides natural slip resistance; sawn or honed finishes are slippery when wet. For a patio used in UK weather, specify riven and ask for the PTV.
• Batch variation. Natural stone colour varies batch to batch. Two pallets bought six weeks apart may not match. Order all the stone for a single patio from the same batch, on the same delivery, and mix slabs across pallets as you lay to average out any within-batch variation.

Sandstone does not need sealing immediately. Leaving it unsealed for the first 12 months lets any efflorescence and any iron-spot deposits work themselves to the surface, where they can be brushed off. Apply an impregnating (not topical) sealer after 12 months if you want reduced staining risk. Topical film-forming sealers peel and blister, don't use them outdoors.

Porcelain (20mm outdoor-grade)

Porcelain is the fastest-growing category and the most unforgiving to install. The 20mm outdoor-grade spec exists because 10mm indoor porcelain will snap under a patio point load, never use indoor porcelain outside. At the bottom end, online retailers (Paving Superstore, Marble Mosaics) sell white or grey 900x600x20mm porcelain from £19-25/m². Marshalls Arrento, Ardara, and Symphony branded ranges retail at £38-50/m². Italian and Spanish designer porcelain pushes £60-80/m².

Porcelain's non-porous body is its strength and its weakness. It won't stain, won't absorb rain, won't frost-damage, and won't suffer efflorescence. But the same property stops adhesive bonding the way it would to stone or concrete. Every porcelain manufacturer mandates an SBR primer slurry (Instarmac Pro-Prime, Dunlop Grip-It, Larsen ProPrime) painted onto the slab underside immediately before bedding. Skip the primer and the porcelain will start rocking within 6-12 months, requiring full lift-and-relay at £1,500-£2,500 for a typical 20m² patio.

Porcelain also cuts differently. A standard angle grinder with a general-purpose diamond blade chips the glazed edge so badly that the cut never grouts cleanly. The correct tool is a wet saw (water-cooled bench tile saw) with a porcelain-rated continuous-rim diamond blade. Hire one for the day; don't try to cut porcelain with a 9-inch grinder.

Slip ratings: R-rating vs PTV, and what to actually ask for

UK paving uses two overlapping slip-resistance systems, which creates confusion when you're buying:

• R-rating (DIN 51130): German standard, measured on an oiled ramp. R9 is dry only. R10 is standard kitchen and bathroom floors. R11 is outdoor wet zones. R12-R13 is industrial. Most UK-sold exterior paving is R11.
• PTV (Pendulum Test Value, BS 7976-2): the UK-preferred standard, measured by a pendulum with a rubber slider. HSE minimum for wet conditions is PTV 36. Above 75 is excellent grip.

The two systems do not map one-to-one, which is why reputable retailers quote both. For a UK patio used in rain and winter, specify R11 minimum and PTV 36 minimum. Don't accept paving sold without either rating declared. Riven-finish natural stone and structured-finish porcelain clear this bar comfortably; sawn, honed, or polished finishes typically do not.

Sub-base: 80 percent of your patio cost, 100 percent of whether it lasts

A patio fails at the sub-base before it fails anywhere else. The single most repeated warning across DIYnot, BuildHub, and every paving trade forum is that sand-only bedding under a slab patio fails within 8-18 months. Contractors who propose laying slabs on a sand bed without compacted hardcore beneath, or who propose dot-and-dab mortar instead of a full bed, are either cutting corners on purpose or don't know what they're doing. Either way, the job fails and they've usually moved on by the time it does.

The correct sub-base build-up for a garden patio on typical UK ground:

1. Excavate to a depth that allows 100-150mm of sub-base plus bedding plus slab thickness plus the required 150mm DPC clearance. For a 22mm Indian sandstone patio on a 40mm mortar bed over 150mm MOT Type 1 with 25mm blinding, dig 237mm below finished slab level.
2. Geotextile membrane on clay soils. A Terram 1000 or similar non-woven membrane laid over the excavated subgrade stops clay particles migrating up into the sub-base, which would reduce its load-bearing capacity over time.
3. MOT Type 1 hardcore, 100mm for pedestrian patios on good soil, 150mm for clay or light vehicle crossings. Lay in 75mm maximum lifts and compact each lift with a plate compactor (also called a wacker plate) before adding the next. One pass isn't enough, aim for 4-6 passes per lift. MOT Type 1 runs £35–45/tonne delivered; a 20m² patio needs roughly 3-4 tonnes at 100mm depth or 5-6 tonnes at 150mm.
4. 25mm sharp sand blinding rolled or screeded level. This fills micro-voids in the compacted hardcore and gives the mortar bed a stable contact surface.
5. Full mortar bed at 4:1 sharp sand to cement, 30-40mm thick, laid in advance of each slab. Full bed, not spots. BS 7533-4 and the paving trade consensus on natural stone are categorical: spot bedding, dot-and-dab, and five-spot laying all fail. Structural voids under the slab cause rocking, reflective staining (the spot pattern visible through the slab face within 12 months), pest ingress, and eventual cracking.

Plate compactor hire from local independent hire yards runs £35–50/day; national chains (HSS, Speedy, National Tool Hire) charge £60–75/day inc. VAT. For a 20m² patio budget two days of compactor hire to handle two sub-base lifts plus final consolidation.

DPC clearance: the rule that catches most homeowners out

The damp proof course is a horizontal waterproof layer in the wall of your house, usually a black polythene strip you can see in the mortar joint about two brick courses above ground level. It stops rising damp. Any external surface next to the wall must sit at least 150mm below the DPC so that rainwater, splashback from the surface, and any temporary ponding cannot breach it.

In a post-extension scenario this rule routinely gets broken because:

• The builder poured the external concrete slab to a level that suited concrete block paving (35mm total build-up) but the homeowner later chose 20mm porcelain on a 40mm mortar bed (60mm total build-up), raising the finished patio 25mm above the planned level.
• The existing garden sloped back toward the house and the landscaper graded the patio to match, rather than regrading to fall away.
• The new patio met the threshold of level-access bifold doors at door level, putting the paving only 20-40mm below DPC, well short of the 150mm rule.

Pavingexpert (the UK's authoritative paving reference) is clear: 150mm is the requirement. Where 150mm genuinely cannot be achieved (level-access thresholds for disabled access under Document M, for example), the absolute floor is 75mm, and only with enhanced mitigation. The trade fixes for constrained situations are:

• Threshold drainage channel (an ACO-style linear drain) running the full width of the door, piped to a soakaway or surface-water drain. This catches splashback before it reaches the wall.
• Narrow edge course at a 1:12 fall against the wall for the first 300-500mm of paving, then flattening to the standard 1:80 across the patio. The steeper fall drives water away fast.
• Land drainage channel running parallel to the wall 100-150mm out from the brickwork, under the paving, intercepting any water that gets past the first line.

If your builder has poured the external slab at a level that forces paving within 150mm of DPC and you have no space to rebuild, get a threshold drain in. Don't just ignore the rule, building control will flag it at final inspection and insurers will use it as grounds to dispute any future damp claim.

Falls and drainage: 1:80 is the minimum, 1:60 is what to aim for

NHBC Standard 10.2 specifies 1:80 as the minimum gradient for private drives, paths, and garden paving. That's 12.5mm of fall per metre of patio. Pavingexpert cites 1:60 (17mm per metre) as the practical target, not because NHBC is wrong, but because 1:80 on a textured or riven slab can still pond in spots where the surface profile dips slightly below the theoretical plane.

For a 4m-wide patio falling away from the house to the garden at 1:80, the garden edge finishes 50mm below the house edge. At 1:60, it's 67mm lower. Plan this early. A patio that falls only 20mm over 4m will pond in winter, and a patio that falls 100mm may look visually wrong against horizontal bifold thresholds.

Falls must direct water away from the building. The commonest mistake on new extensions, cited in three of seven forum threads reviewed for this guide, is a patio that falls toward the house because the natural garden gradient already sloped that way. Regrade the ground before laying the sub-base rather than inheriting a hostile gradient.

Also on falls: the NHBC rule that surfaces must not be flatter than 1:40 as a cross-fall. In practice this means you can't have a perfectly level patio running parallel to the house. Either you cross-fall into a linear drain or you fall directly away. Flat surfaces will pond regardless of how well the sub-base is compacted.

Jointing: sand, one-part compound, or two-part resin

Once the slabs are bedded, cured, and level, the joints need filling. Four options, in rising order of cost and performance:

1. Kiln-dried sand. £4-7 per 25kg bag at 3-4kg/m² coverage. Brushed dry into open joints between block paving, never between laid slabs. Works only for flexible joint systems (block paving on a sharp sand bed, where joints can lock and shift together). On rigid mortar-bedded slab installations the sand washes out and weeds follow. Don't use kiln-dried sand on a slab patio.
2. One-part brush-in compound (Geo-Fix All Weather, Sika FastFix, Nexus Pro-Joint Fusion). Geo-Fix All Weather retails at £26.34 for a 14kg tub, covering roughly 13-15m² of 10mm-wide joints between 600x600mm slabs. Cement-free, ready-to-use, cured in 4-12 hours, weather- and weed-resistant, water-permeable so joints don't puddle. Applied dry or damp-brushed into the joints then compacted with a jointing tool. The current UK standard for domestic patio jointing. Do not pressure-wash for 14 days post-application. Note that Geo-Fix All Weather is sold in a 14kg tub, not 25kg, common retailer listings sometimes get this wrong.
3. Two-part epoxy resin jointing (a two-part Joint-It system, EASYJoint Pro-Plus). A two-part Joint-It 22kg tubs retail at £69.59 and cover ~18m², working out to roughly £3.87/m². Non-porous, available in buff, grey, or black, takes foot traffic in 24 hours. Suitable for porcelain and natural stone where a perfectly sealed joint is needed. Four times the per-tub price of Geo-Fix but justified for porcelain or premium natural stone where you cannot afford joint staining or weed ingress.
4. Traditional sand-and-cement pointing. Troweled in wet by the landscaper, cheapest materials but skilled labour. Prone to staining the slab face if done carelessly; prone to cracking in joints over 10mm wide. Still used on heritage Yorkstone and traditional installations where the appearance matches the material.

For a typical post-extension patio in Indian sandstone or budget porcelain, one-part Geo-Fix All Weather is the default choice. For premium porcelain (£45+/m²) or where the aesthetic demands a perfectly uniform joint colour, go to two-part resin. For block paving (concrete pavers on a sharp sand bed, forming a driveway or path), kiln-dried sand is the correct choice; two-part resin is wrong because the joint flex is essential to block paving's load spreading.

How much do you need for a 20m² patio

A typical post-extension rear patio runs 4m out from the house along a 5m-wide extension, totalling 20m². Quantity planning for this size, in 22mm calibrated Indian sandstone laid on a full mortar bed over 150mm of MOT Type 1:

• Paving: 20m² + 10% wastage = 22m² ordered. At 22mm calibrated mixed-size riven Indian sandstone (typical project pack of 600x600, 600x300, 300x300, 900x600 in random layout), expect 2.5-3.5 tonnes of stone weight. Allow for pallet delivery only to hard-standing the lorry can reach; grab-lorry offload isn't usually possible for natural stone.
• MOT Type 1 hardcore: 20m² × 0.15m depth × 1.9 t/m³ compacted = 5.7 tonnes. Order 6 tonnes. At £40/tonne delivered: £240.
• Sharp sand (blinding): 20m² × 0.025m depth = 0.5m³, or roughly 0.8 tonnes. One tonne bag at £45-55 delivered.
• Bedding mortar (4:1 sand:cement): 20m² × 0.04m depth = 0.8m³, needing approximately 1.2 tonnes of sharp sand and 6-8 bags of cement (allowing for waste). At £50/tonne sand and £7/bag cement: roughly £100-120.
• Jointing compound: 10mm joints on 600x600mm mixed-size slabs across 20m² needs ~18-20kg of compound. Two 14kg tubs of Geo-Fix All Weather at £26.34 each = £52.68.
• Plate compactor hire: two days at £35–50/day from a local hire yard, or £60–75/day inc. VAT from HSS or Speedy.
• Diamond blade: 230mm angle-grinder diamond blade for sandstone cuts at £25-40. For porcelain, add wet saw hire at £80-120/day for 1-2 days.

Materials sub-total for a 20m² Indian sandstone patio, not counting labour: approximately £1,100-£1,400. Labour at £40-80/m² installed adds £800-£1,600. All-in installed cost £1,900-£3,000. Premium porcelain or Marshalls branded concrete with skilled labour at the higher end of the rate range can push material cost to £1,600-£2,200 and the total to £2,800-£4,200.

Wastage allowance is 10% for standard stretcher-bond or stack-bond layouts and 15% for random-bond or diagonal layouts where cutting waste is higher. Complex random patterns specifying multiple slab sizes (the four-size pack) waste less than single-size random, but both lose more to cuts than basic stretcher bond.

Installing paving: the sequence that doesn't get skipped

The end-to-end sequence for laying a 20m² patio against the rear of a post-extension house:

1. Confirm DPC level and target finished paving height. Mark the 150mm-below-DPC line on the wall with a chalk string. Check that your chosen build-up (paving thickness + mortar bed + blinding + hardcore) fits within the excavation you're about to dig.
2. Excavate to the required depth. A 22mm slab + 40mm mortar + 25mm sand + 150mm hardcore = 237mm below finished paving level, which is 387mm below DPC. Remove spoil to a skip or arrange muck-away. For a 20m² patio at 237mm excavation, expect 4.7m³ of soil, roughly a 6-yard skip load.
3. Regrade the subgrade to a rough 1:80 fall away from the house. Compact the subgrade with a plate compactor before any hardcore goes down.
4. Lay geotextile on clay soils. Overlap joins by 300mm.
5. Place MOT Type 1 in 75mm lifts. Compact each lift 4-6 passes. Check gradient with a long spirit level or a laser level on a tripod.
6. Blind with sharp sand, screeded level, lightly compacted.
7. Set string lines for the perimeter and any major joints. Check the 1:80 fall against the lines with a graded timber batten and spirit level, or a laser.
8. Mix mortar 4:1 sharp sand to cement, workable but not sloppy. Lay one slab at a time, full bed beneath, tap down with a rubber mallet to the string line. For porcelain, apply SBR primer slurry to the slab underside first and lay immediately while the primer is still wet.
9. Check each slab for rocking and level within a minute of placement. Lift and re-bed any slab that isn't sitting solid. Once the mortar starts to go off, the window for adjustment closes.
10. Cure the bedding for 24-48 hours before walking on the patio. Don't walk the slabs while the mortar is still soft, movement at this stage causes hollow-spot rocking later.
11. Point the joints with Geo-Fix or two-part resin per manufacturer instructions. Weather above 5°C, dry conditions. No pressure washing for 14 days.
12. Install edge restraint where the patio meets soft landscaping (lawn, borders). Concrete haunching behind the outer paving row prevents lateral spread. Without edge restraint the perimeter slabs gradually migrate outward and the field joints open up.

Most of this can be done in a long weekend by two people with basic competence, provided the excavation is done separately by a groundworker or mini-digger hire. Solo installations run 4-6 days for a 20m² patio.

Alternatives

Paving isn't the only viable external hard-standing for a post-extension garden. The main alternatives and when each wins:

• Resin-bound gravel. A 3-stone aggregate bound with UV-stable resin, 15-18mm laid depth over a compacted base. Continuous surface, permeable (so it meets SuDS requirements without a soakaway), low maintenance. Costs roughly £60-90/m² installed. Loses to paving on heavy-use areas near furniture legs and on ability to repair a single damaged section.
• Loose gravel (decorative chippings). £30-50/m² all-in. Fine for paths and low-traffic areas, bad for garden furniture and wheeled items. Migrates, needs topping up every 2-3 years.
• Timber decking. £40-80/m² in treated softwood, £100-160/m² in composite. Warmer underfoot, drains instantly, easy to level across sloping ground. Loses to paving on lifespan (softwood rots in 10-15 years even treated) and maintenance (annual re-treatment, replacement boards).
• Concrete block paving. £25-50/m² supply for blocks, £40-60/m² installed. The only option that correctly uses a sharp-sand bed (the blocks interlock and load-spread as a flexible pavement). Standard for driveways. Works for patios where you're happy with the gridded aesthetic. Loses on visual character against natural stone.

For a typical UK post-extension patio, sandstone or porcelain slabs on a full mortar bed win on durability and appearance eight times out of ten. Resin-bound gravel is the right answer when SuDS compliance matters or when permeable surfacing is a planning condition. Decking wins where the garden level drops dramatically below the door threshold and building up to slab level would be disproportionate.

Planning permission and SuDS: what applies to a rear patio

For a typical rear garden patio under 100m², permission is not usually required and SuDS approval is not mandatory. The relevant rules:

• Front of house (between the building and a public highway): non-permeable hard surfacing over 5m² needs planning permission or a permeable specification. Permeable driveways of any size are permitted development.
• Rear and side gardens: not subject to the front-highway rule. Rear patios are permitted development by default under size limits that exceed what most homeowners would build.
• SuDS in England: Schedule 3 of the Flood and Water Management Act 2010 has been repeatedly deferred. As of early 2026 it is not in force; DEFRA's stated position is implementation no earlier than 2026, and a consultation on further changes closed in March 2026. The government introduced non-statutory national SuDS standards in 2025 and updated the NPPF in 2024 to encourage SuDS delivery, but formal SAB approval is not currently a legal requirement in England. Check current status before assuming any exemption applies to your project.
• Wales: has mandated SuDS approval for hard surfaces via the SuDS Approval Body (SAB) since 2019. This is materially stricter than the position in England.
• Listed buildings and conservation areas have additional controls. Check with your local planning authority before laying any paving on a listed property.

Where you'll need this

• Patio reinstatement - the post-extension phase where the rear patio is laid, typically sourced and managed by the homeowner after the main contract ends at the door threshold
• Snagging checklist - final inspection stage where paving falls, DPC clearance, and joint condition are checked before the build is signed off
• Garden reinstatement - patio interfaces with the reinstated lawn and planting, where paving edge restraint and level transitions to turf matter

Paving is part of every extension, renovation, garden room, and outbuilding project where the external ground level is rebuilt. The rules on sub-base depth, DPC clearance, falls, and jointing apply equally whether you're laying a 10m² path to a shed, a 30m² rear patio, or a 100m² driveway.

Common mistakes

Laying slabs on a sand bed instead of a full mortar bed. The single most-repeated warning across every paving forum. Sand-only slab bedding fails within two seasons before the slabs rock and weeds take over. If a landscaper quotes for "slabs on sharp sand" on a patio (not block paving), refuse the quote.

Dot-and-dab or five-spot mortar bedding. Looks faster and uses less mortar. Causes reflective staining (the mortar spots show through the slab face within a year), voids that collect pests, and eventual cracking when point loads (garden furniture, dropped tools) hit an unsupported zone. BS 7533-4 requires full mortar bed contact across at least 80% of the slab underside. Check this by lifting a random freshly laid slab during installation and confirming full mortar contact.

Ignoring DPC clearance. Paving within 150mm of the DPC is non-compliant and will fail snagging inspection. In constrained post-extension scenarios, fit a threshold drainage channel and ensure a 1:12 fall away from the wall for the first 300-500mm of paving.

Falling the patio toward the house. Happens when the garden already slopes toward the building and the landscaper inherits the gradient. Regrade the subgrade before laying any sub-base. Falls must go away from the wall at 1:80 minimum.

Skipping the SBR primer on porcelain. The slabs start rocking within the first year of use. Remediation is full lift-and-relay at several thousand pounds. A bucket of Instarmac Pro-Prime at £25-35 prevents this. Every porcelain manufacturer's installation sheet mandates the primer step; many landscapers skip it because it slows the job and they don't believe it matters.

Ordering cheap Indian sandstone with no declared PTV or slip rating. The slab looks identical to £30/m² calibrated stock in the yard, at half the price. In rain it's lethal. Buy only from retailers who declare PTV or R-rating on the product sheet and reject stock that arrives with thickness varying more than ±2mm across the pallet.

Using kiln-dried sand as jointing between slabs. Kiln-dried sand works only on block paving with flexible sharp-sand bedding. On a rigid mortar-bedded slab patio it washes out and weeds fill the joint. Use Geo-Fix All Weather or two-part resin for slab joints.

Sealing Indian sandstone before efflorescence has cleared. Applying a sealer in the first six weeks after laying traps salts inside the stone, causing white bloom and peeling sealer. Wait 8-12 weeks minimum, ideally 12 months. Use impregnating (not topical film-forming) sealers only.

Under-estimating sub-base quantity. Ordering 3 tonnes of MOT Type 1 when the patio needs 6 is a common error. Sub-base tonnage is area × depth × 1.9 (compacted density in t/m³). A 20m² patio at 150mm depth needs 5.7 tonnes, round up to 6 to cover losses and over-digging.

Using an angle grinder to cut porcelain. Chips every cut edge so badly that joints never grout cleanly. Hire a wet saw with a porcelain-rated continuous-rim diamond blade. Budget one day's hire at £80-120 for a typical 20m² patio's cuts.