In short
- Porcelain is dense and low-maintenance, but it must be laid on a full mortar bed with a bonding primer on the back of every slab.
- Natural stones such as sandstone, limestone and slate weather and vary in colour, and each needs slightly different care.
- For slip resistance, a wet pendulum test value of 36 or more indicates low slip potential, according to HSE guidance. R ratings are a different test and are less useful outdoors.
- Patios need a fall so water runs away from the house, typically at least 1 in 80, to a border, gully or drain.
- Keep the finished paving at least 150 mm below the damp-proof course, in line with Approved Document C and manufacturers' laying guides.
Seven patio materials side by side
The table gives a broad picture. Products vary a great deal within each type, so look at the manufacturer's data sheet for the specific paving you are considering, and ask to see a laid example.
| Material | Look and feel | Grip when wet | Usual laying method | Maintenance |
|---|---|---|---|---|
| Porcelain | Uniform colour and size, many finishes including stone and wood effects | Varies by finish. Check the wet pendulum value | Full mortar bed with slurry primer, narrow grouted joints | Low. Resists staining, rarely needs sealing |
| Indian sandstone | Natural colour variation, often riven texture | Riven textures usually grip well; smooth sawn finishes can be slippery | Full mortar bed, usually primed, mortar or compound joints | Moderate. Weathers, can grow algae in shade, sometimes sealed |
| Limestone | Calm, even tones, often smooth or lightly textured | Smooth finishes can be slippery when wet | Full mortar bed, primed, compound joints | Moderate. Sensitive to acidic cleaners and spills |
| Slate | Dark, layered, crisp | Can be slippery when wet, especially in shade | Full mortar bed, primed | Moderate. Some slates flake if poorly selected |
| Concrete flags | Wide range, from plain to textured and stone-effect | Textured finishes usually grip well | Mortar bed | Low to moderate. Can fade or stain |
| Concrete block paving | Small units, many patterns and colours | Generally good | Flexible: compacted sand bed, kiln-dried sand joints, edge restraints | Low. Re-sand joints, treat weeds and moss |
| Natural stone setts | Small chunky blocks of granite or sandstone, traditional | Generally good, depending on finish | Usually a mortar bed with mortar joints | Low. Joints may need repointing over time |
If the patio also has to take a car, or you are paving a front garden, the rules and the construction change. Our sister site Kent Resin Services covers driveway surfaces, front garden paving and dropped kerbs.
Porcelain paving: dense, consistent and precise to lay
Outdoor porcelain paving, usually 20 mm thick, is made from fired clay and minerals and absorbs very little water. That is why it resists stains and frost, and why it rarely needs sealing. It also means the slabs won't grip ordinary mortar on their own.
- Bonding: Marshalls' porcelain laying advice says a paving primer or similar flexible bonding agent should be applied to the back of each slab before it goes onto the mortar bed. London Stone says a primer must be used with ultra-low porosity materials such as porcelain.
- Bed: Marshalls says porcelain should be laid on a full wet mortar bed and never on a five-spot or similar partial bed, which leaves voids that can crack or rock.
- Joints: Marshalls gives joint widths of 3 to 5 mm, and London Stone recommends a 5 mm gap filled with a good quality external grout or jointing compound. Dry or semi-dry sand and cement brushed into the joints isn't suitable.
- Cutting: porcelain is hard and needs proper diamond blades and some skill to cut neatly, which adds to labour time.
Because porcelain is flat and consistent, small errors in the bed or the falls show up clearly. It suits landscapers who lay it regularly, so ask to see porcelain patios they have finished.
Indian sandstone, limestone and slate
Natural stone gives a garden texture and colour variation that manufactured paving can't quite match. The trade-off is that every delivery varies, and the stone changes as it weathers.
- Indian sandstone is widely available in riven (split) and sawn finishes. Riven slabs vary in thickness, so a full mortar bed is needed to take up the differences. Sandstone is relatively porous, so it can pick up algae, leaf stains and iron marks, especially in shade.
- Limestone, including popular imported blue-black and grey types, gives a smoother, more formal look. It reacts with acids, so acidic patio cleaners, some fertilisers and spilt wine or citrus can etch or mark it. Check the supplier's cleaning advice.
- Slate is dense and crisp, with a layered surface. Choose a grade intended for external paving, because poorer slates can flake. Smooth slate can be slippery in shade.
- Granite, often used for setts, steps and edging, is very hard-wearing and comes in flamed or textured finishes that grip well.
Most natural stone is laid on a full mortar bed, and many manufacturers recommend priming the back of the slab, particularly with denser stones. Ask the supplier whether a sealer is recommended. Sealing can reduce staining but may change the colour, and it needs reapplying.
Concrete flags, block paving and natural stone setts
Concrete products are often overlooked, but modern textured flags and block paving can look smart and are usually less expensive than natural stone.
- Concrete flags come in plain, textured and stone-effect finishes. They are usually laid on a mortar bed. Quality varies, so look for products made to the relevant British and European standard.
- Block paving is laid as a flexible pavement. Marshalls' installation details describe a sub-base, a compacted laying course of grit sand about 30 mm thick, joints of 2 to 5 mm filled with kiln-dried jointing sand, and firm edge restraints. Falls are built into the sub-base, not the sand.
- Permeable block paving has wider joints filled with fine stone and a free-draining sub-base, so rain soaks through rather than running off. It works best on ground that drains, and on heavier soils it can be built with an outlet pipe.
- Setts, in granite or sandstone, are small, heavy blocks that suit paths, edging, circles and traditional cottage gardens. They take longer to lay than large slabs, so labour is a bigger part of the cost.
Slip resistance: pendulum values and R ratings explained
Manufacturers describe grip in two main ways, and it helps to know which test you are looking at.
| Rating | How it is measured | What to look for |
|---|---|---|
| Pendulum test value (PTV) | A swinging arm with a standard rubber slider sweeps across the surface, usually tested wet. The HSE recommends it as the most reliable field test. | HSE guidance, from the UK Slip Resistance Group, classes 0 to 24 as high slip potential, 25 to 35 as moderate and 36 or more as low. Look for a wet value of 36 or more. |
| R rating (DIN 51130) | People wearing cleated safety boots walk on an oily, tilting ramp until they slip. Results run from R9 to R13. | The HSE has reservations about this test because oil and safety boots don't reflect normal conditions. It notes that R9, and sometimes R10, surfaces will be slippery when wet. |
An R11 rating is often quoted for outdoor porcelain, but because the ramp test uses oil and safety boots, it doesn't tell you much about a wet patio in garden shoes or bare feet. If you can, ask for the wet pendulum value, ideally with the slider used for shod pedestrians. A test result is a starting point. Moss, algae, leaf litter and shade all reduce grip over time, so a north-facing patio under trees needs a grippier finish and more regular cleaning than a sunny one.
Laying methods: bedding, primers and joints
How a patio is built matters more than the material on top. Most problems, from rocking slabs to sinking corners and cracked joints, start with the base or the bed.
- Dig out and set the levelsExcavate to allow for the sub-base, the bed and the paving, with the finished level well below the damp-proof course and the falls planned.
- Lay and compact the sub-baseMarshalls says a 100 mm sub-base should be sufficient for most gardens, and London Stone gives a minimum compacted depth of 100 mm for foot traffic. Soft clay, made-up ground or vehicle loads need more.
- Lay the bedSlabs go onto a full mortar bed. Marshalls describes a wet workable mix of about 1 part cement to 4 parts sharp sand, 15 to 30 mm thick, for porcelain. Block paving uses a compacted sand laying course instead.
- Prime and placePorcelain and dense stones are primed on the back with a slurry or bonding primer, then tapped down to level and fall.
- JointJoints are filled with a cement mortar, a resin or polymer jointing compound, or an external grout, depending on the paving and joint width. Block paving uses kiln-dried sand.
Laying stops in frost. Marshalls' block paving guidance, for example, says installation should be suspended in weather that could harm the paving and that joints can't be filled in damp conditions. Ask each landscaper which bed and jointing products they plan to use with the paving you have chosen, and whether they follow the manufacturer's laying guide.
Falls, drainage and where patio water goes
A patio that looks level isn't. It should slope gently so rain runs off rather than pooling, freezing or soaking towards the house.
- London Stone recommends a fall of 1 in 80 for porcelain, which is about 1 cm for every 80 cm of patio.
- Marshalls' block paving details give a minimum fall of 1 in 80 along the paving and 1 in 40 across it.
- Riven stone and setts have a rougher surface and are often laid to a slightly steeper fall so water doesn't sit in the hollows. Ask your landscaper what fall they recommend.
- The fall should always run away from the house and towards somewhere that can take the water: a lawn or border, a gully, a channel drain or a soakaway.
Where the patio meets the house, or where it can't fall away, a slot or channel drain along the edge collects the water. On Kent clay, such as the Weald Clay around Tonbridge and Paddock Wood or the London Clay of Sheppey and Whitstable, water from a large patio can make an adjoining lawn wetter, so think about where it ends up. The drainage guide on this site covers soakaways and other options.
Keeping the patio below the damp-proof course
The damp-proof course, or DPC, is the barrier built into the wall near ground level that stops moisture rising into the house. In older brick houses it may be a visible line of slate or engineering bricks, or a row of injection holes where a chemical DPC was added later.
- Approved Document C says that in an external wall, the DPC should be at least 150 mm above the level of the adjoining ground, unless part of the building protects the wall.
- NHBC Standards similarly call for the DPC to be at least 150 mm above the finished ground or paving level.
- Marshalls' laying guides say a patio or block paving laid up to an existing structure should be a minimum of 150 mm below the DPC to avoid problems with damp.
Raising a patio to meet the back door is one of the most common mistakes in garden projects. Rain splashes up the wall above the DPC, and paving or soil piled against it can bridge it completely. Don't cover air bricks or vents either, because they ventilate suspended timber floors. If you want a level threshold at a door, it needs a specific design, usually with a drainage channel, so ask for it to be detailed. If your home is a new build still within its warranty period, check the warranty terms before changing levels around the house.
Cleaning, sealing and long-term care
Every patio needs some care. A little regular attention avoids the need for harsh cleaning later.
- Sweep regularly, especially in autumn, so leaves don't stain the surface or hold moisture.
- Clean porcelain with water and a soft brush or a cleaner recommended by the manufacturer.
- Treat algae on natural stone with a product suitable for that stone. Avoid acidic cleaners on limestone and check with the supplier before using them on sandstone.
- Pressure washers can damage joints and the surface of softer stones. Use a low setting and keep the nozzle at a distance, or avoid them on pointed joints.
- Top up kiln-dried sand in block paving joints and treat weeds early.
- Repair cracked or missing joints promptly so water doesn't get under the slabs and lift them in frost.
Sealers are optional for most materials. They can make stains easier to clean and may enhance the colour of natural stone, but they wear and need reapplying, and some change the look of the surface. Ask your landscaper or supplier whether a sealer is recommended for the specific product, and whether it affects slip resistance.
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Sources
- HSE: Assessing the slip resistance of flooring, technical information sheet GEIS2: www.hse.gov.uk/pubns/geis2.pdf
- GOV.UK: Approved Document C, site preparation and resistance to contaminants and moisture (paragraph 5.5): www.gov.uk/government/publications/site-preparation-and-resistance-to-contaminates-and-moisture-approved-document-c
- GOV.UK: Approved Document C frequently asked questions: assets.publishing.service.gov.uk/media/5a7f8f89e5274a2e87db6976/160321_Part_C_FAQ.pdf
- NHBC Standards: 5.1.19 Damp proof course: nhbc-standards.co.uk/5-substructure-ground-floors-drainage-and-basements/5-1-substructure-and-ground-bearing-floors/5-1-19-damp-proof-course/
- Marshalls: How to lay a porcelain patio: www.marshalls.co.uk/gardens-and-driveways/blog/how-to-lay-a-porcelain-patio
- Marshalls: Standard concrete block paving installation details (domestic): media.marshalls.co.uk/image/upload/v1559300893/Standard-Concrete-Block-Paving-Installation-Details-January-2019.pdf
- London Stone: How to lay porcelain slabs: www.londonstone.co.uk/blog/blog-porcelain-stone-clay-paving/how-to-lay-porcelain-slabs