Learning how to lay porcelain paving slabs correctly is not simply a matter of placing outdoor tiles onto mortar. Porcelain is dense, has extremely low water absorption and does not form a dependable bond with an ordinary sand-and-cement bed unless the underside is fully coated with a suitable slurry primer.
This guide explains how to install 20 mm outdoor porcelain paving on a conventional UK patio build-up. It covers finished levels, drainage falls, excavation, MOT Type 1, the full mortar bed, slurry primer, cutting, joint widths, curing and jointing. It also explains when laying porcelain over an existing concrete base may be appropriate.
How to Lay Porcelain Paving Slabs: Quick Answer
- Inspect and dry-lay slabs from different packs.
- Plan the layout, finished levels and drainage route.
- Keep the paving at least 150 mm below the damp-proof course unless a correctly designed threshold detail applies.
- Excavate for the full construction depth.
- Install and compact approximately 100 mm of MOT Type 1 for a normal pedestrian patio.
- Prepare a full mortar bed approximately 30–40 mm thick.
- Brush slurry primer over the complete underside of each slab.
- Lay each slab while the mortar and primer are fresh.
- Maintain a consistent fall away from the building and joints normally around 5 mm.
- Allow the bedding mortar to cure before filling the joints.
Can You Lay Porcelain Paving Yourself?
A competent DIY installer can lay a small, straightforward porcelain patio, but porcelain is less forgiving than many natural stone slabs. The main difficulties are achieving a stable and accurately graded sub-base, cutting dense 20 mm slabs cleanly, maintaining full mortar contact and preventing slurry primer or grout from setting on the decorative face.
Professional installation is advisable where the site includes soft or made-up ground, retaining structures, drainage problems, low door thresholds, large-format slabs, steps, multiple levels or extensive cutting.
Porcelain Patio Construction: Typical Build-Up
For a standard pedestrian patio constructed over soil, a typical build-up from the ground upwards is:
- Stable sub-grade: firm ground with vegetation, topsoil, roots and soft spots removed.
- Geotextile membrane where required: used as a separation layer on weak or unstable ground, not as a substitute for proper excavation or compaction.
- MOT Type 1 sub-base: normally about 100 mm after compaction for a conventional domestic patio.
- Full mortar bed: generally about 30–40 mm thick.
- Slurry primer: applied continuously to the complete underside of each porcelain slab.
- Outdoor porcelain paving: normally 20 mm thick.
- Exterior porcelain jointing material: selected for the joint width, depth and site conditions.
The precise construction should reflect the ground conditions and expected load. Soft clay, filled ground, drainage trenches and areas carrying heavier loads may require a deeper or specifically engineered base.
Tools and Materials Needed
Materials
- 20 mm outdoor porcelain paving slabs
- MOT Type 1 granular sub-base
- Sharp sand or washed concreting sand
- Cement
- Proprietary slurry primer suitable for porcelain
- Exterior porcelain grout or compatible jointing compound
- Clean water
- Tile spacers
- Suitable edge restraints where required
- Linear drainage channel where required by the design
Tools
- Shovel, rake and wheelbarrow
- Tape measure, pegs, string lines and marker
- Long spirit level and straight edge
- Plate compactor
- Cement mixer
- Builder’s trowel
- White or non-marking rubber mallet
- Electric paddle mixer
- Wide slurry-primer brush
- Wet bridge saw or suitable angle grinder with a porcelain-rated continuous-rim diamond blade
- Rubber grout float or grout gun, depending on the jointing product
- Clean buckets, sponges and microfibre cloths
Wear suitable gloves, protective footwear, eye protection, hearing protection and respiratory protection. Wet cutting is preferable where practical because it reduces airborne silica dust and helps cool the blade.
How to Lay Porcelain Paving Slabs Step by Step
Step 1: Check the Slabs Before Laying
Inspect the paving as soon as it arrives. Confirm the product, dimensions, quantity, surface finish, batch references and overall condition before installation begins.
Porcelain designs are normally printed across a limited number of faces. Dry-lay slabs from several packs and rotate them to distribute repeated prints and tonal variation naturally. Do not install visibly damaged, incorrect or unacceptable slabs. Once a slab has been cut, primed or fixed, resolving a product issue becomes considerably more difficult.
Step 2: Plan the Layout and Cuts
Mark the patio perimeter and decide the laying direction before excavation. Consider doors, steps, inspection covers, drainage channels, walls and the position of cut slabs.
Measure the overall area against the slab module, including the intended joint width. Moving the starting line by half a slab can often avoid narrow cuts along a doorway or prominent edge. Large-format porcelain makes poor setting-out particularly visible, so establish square reference lines rather than relying on an existing wall to be perfectly straight.
Step 3: Set the Finished Level and Damp-Proof-Course Clearance
The finished paving surface should normally remain at least 150 mm below the property’s damp-proof course. Do not cover air bricks, weep holes or ventilation openings.
Low or level-access thresholds require a properly designed drainage and waterproofing detail. Simply raising a patio close to a door threshold without suitable drainage can direct water towards the building and increase damp risk.
Step 4: Establish the Drainage Fall
Porcelain absorbs very little water, so rain remains on the surface until it runs off or evaporates. The finished paving must therefore have a deliberate and continuous fall towards a suitable discharge point.
For many conventional patios, a fall of approximately 1:60 is a practical target. This equals roughly 10 mm of drop for every 600 mm of horizontal distance, or about 17 mm per metre. A 3 m patio would therefore require approximately 50 mm of total fall.
Some installations can use a shallower fall, subject to the paving texture, slab flatness, drainage design and manufacturer’s guidance. Do not assume that an apparently level patio will drain adequately. Set the fall with pegs and string lines before preparing the base.
Step 5: Excavate the Patio Area
Excavate sufficiently to accommodate the sub-base, mortar bed and 20 mm slab while maintaining the intended finished level. A typical build-up of 100 mm compacted Type 1, 30–40 mm mortar and 20 mm porcelain requires approximately 150–160 mm below the finished paving level.
This is not a universal excavation depth. Remove all topsoil, roots, organic matter and loose material. If the exposed ground pumps, deforms or allows a boot heel to sink easily, further excavation, stabilisation or specialist ground preparation may be necessary.
Step 6: Install Edge Restraints and Drainage
Install any required kerbs, edging units or rigid restraints before laying the main field of paving. Unsupported mortar at an exposed perimeter can break away and allow outer slabs to move.
Where surface water cannot discharge safely into the garden, install a suitable channel drain or other designed outlet. Drainage channels must be set below the paving surface, follow the intended fall and connect to an appropriate discharge point. Do not direct patio runoff towards the building or neighbouring property.
Step 7: Lay and Compact the MOT Type 1 Sub-Base
Spread MOT Type 1 in manageable layers. For a 100 mm compacted build-up, two compacted layers are preferable to placing the full depth loosely and attempting to compact it in one pass.
Compact each layer thoroughly with a plate compactor, making repeated passes in different directions. Pay particular attention to corners, edges and backfilled service trenches. The completed sub-base should be firm, even and constructed to approximately the same fall as the finished patio.
Step 8: Prepare the Full Mortar Bed
Porcelain paving should be laid on a continuous full mortar bed, not on isolated mortar spots, rings or dabs. Full support distributes loads, limits rocking and reduces the risk of cracked joints or hollow unsupported areas.
A commonly used site mix is approximately four parts sharp sand to one part cement. Some proprietary systems and installers specify a different ratio, so follow the paving, primer and bedding-product instructions where these apply.
Add clean water gradually. The mortar should be moist, cohesive and workable: wet enough to compact and achieve full contact, but firm enough to support the slab without slumping.
- Too dry: the mortar may crumble, resist compaction and fail to make full contact.
- Too wet: slabs may sink, slide or lose the intended fall.
Spread enough mortar for one slab or a small controlled working area. A bed thickness of approximately 30–40 mm is common, although local adjustments may be needed to achieve the finished line.
Step 9: Mix and Apply Slurry Primer
Slurry primer is essential for a conventional mortar-bed installation. Porcelain is too dense and non-porous to form a reliable direct bond with ordinary bedding mortar.
Mix a proprietary porcelain slurry primer exactly as directed by its manufacturer. Apply it with a wide brush over the entire underside of the slab, including every edge and corner. Do not apply a few stripes or patches.
Lay the slab while the primer remains wet and active. Dried primer will not provide the intended wet-on-wet bond. Protect the decorative face carefully because cured cementitious primer can be extremely difficult to remove.
Step 10: Lay Each Slab onto the Full Bed
Lower the primed slab squarely onto the fresh mortar without scraping slurry primer onto adjacent paving. Tap the slab down evenly with a clean, non-marking rubber mallet.
Check three things before moving to the next slab:
- the slab follows the required drainage fall;
- the surface is aligned with the string lines and adjacent slabs;
- the complete underside is supported by mortar.
During the first section, lift an occasional slab immediately after bedding it. The underside should show near-complete mortar transfer without substantial voids. If contact is poor, correct the mortar consistency or bedding method before continuing.
Step 11: Maintain Consistent Joint Widths
Use rigid spacers appropriate for external porcelain. Do not butt-joint the slabs.
Rectified 20 mm porcelain paving is commonly installed with joints of approximately 3–5 mm. PSU generally regards about 5 mm as a practical width for many UK patio installations because it accommodates small tolerances and allows the jointing product to be installed properly. Always check the minimum width and depth required by the selected grout or compound.
Check alignment frequently in both directions. A 1–2 mm error repeated over several rows can become conspicuous across a large patio.
Step 12: Cut Porcelain Paving Correctly
Twenty-millimetre porcelain is extremely hard. For repeated straight cuts, a suitable wet bridge saw fitted with a porcelain-rated continuous-rim diamond blade normally gives the cleanest and most controlled result.
An angle grinder may be suitable for limited cuts in experienced hands, but it creates more dust and makes long, accurate cuts harder to control. Do not use an ordinary masonry blade.
Support the slab fully, mark the cut accurately and feed the blade at a controlled rate. Forcing the cut increases vibration, heat and edge chipping. Dry-fit every cut piece before priming and bedding it.
Step 13: Clean as You Work
Remove mortar, slurry primer and cement residue immediately with clean water and a clean sponge. Change the water frequently rather than spreading a cement film over the surface.
Do not allow primer or grout residue to cure on textured porcelain. The texture that provides outdoor grip can also retain cement haze. A compatible pre-grout barrier treatment may be useful on some textured or light-coloured products, but it must be tested and used according to the manufacturer’s instructions.
Step 14: Allow the Mortar to Cure
Keep the newly laid paving free from foot traffic until the bedding mortar has developed sufficient strength. Protect the work from frost, heavy rain, excessive heat and rapid drying.
There is no reliable universal waiting time because curing depends on temperature, moisture, bed thickness and the products used. Follow the relevant product instructions and allow longer in cold or damp UK weather. Do not joint the patio merely because the slab surface feels firm.
Step 15: Fill the Porcelain Paving Joints
Before jointing, confirm that the bedding mortar has cured sufficiently and that the joints are clean, open to the required depth and free from loose mortar.
Use an exterior grout or jointing compound explicitly approved for porcelain, the selected joint width and the site conditions. Fill the joints completely rather than forming a thin cap over an empty void.
Clean the surface at the stage specified by the jointing-product manufacturer. Excessive washing can weaken some cement-based grouts, create patchy colour or draw material from the joints.
How to Lay Porcelain Paving on an Existing Concrete Base
Porcelain can sometimes be installed over sound concrete, but only when the existing slab is stable, adequately reinforced, correctly drained and free from significant movement or cracking.
The concrete surface must be clean and suitable for the selected fixing system. Depending on the project specification, slabs may be installed using an exterior-rated flexible tile adhesive or another approved bonded system. Existing structural or movement joints must not simply be bridged and concealed beneath the porcelain.
Do not pave over concrete that is breaking up, holding water, moving independently or sloping towards the building. Adding porcelain will not correct a defective substrate.
Can Porcelain Paving Be Laid on Sand, Soil or Paving Supports?
Directly on sand
No. A permanent 20 mm porcelain patio should not normally be laid loose on building sand or sharp sand alone. Sand does not create the required bonded, load-bearing installation.
Directly on soil
No. Soil changes with moisture, frost and loading. Porcelain laid directly on soil is likely to move, rock and develop failed joints.
On adjustable pedestals
Some porcelain products can be installed on proprietary pedestal systems for roof terraces, balconies or raised patios. This is a different engineered installation method. The slabs, support spacing, edge restraint, wind loading and pedestal system must all be suitable for the application.
Porcelain Paving Installation Mistakes to Avoid
- Starting without checking batches, printed faces and slab condition
- Using an existing wall as the only setting-out reference
- Placing paving too close to the damp-proof course
- Blocking air bricks or drainage openings
- Constructing the patio without a continuous drainage fall
- Directing runoff towards the house
- Laying over topsoil, loose sand or uncompacted ground
- Compacting a deep sub-base in one loose layer
- Leaving exposed patio edges without suitable restraint
- Spot-bedding or using mortar dabs
- Using a dry, crumbly or excessively wet mortar mix
- Applying primer in patches instead of full coverage
- Allowing slurry primer to dry before laying
- Leaving significant voids under large slabs
- Butt-jointing porcelain
- Using an unsuitable cutting blade
- Allowing cement residue to dry on the slab face
- Walking on or jointing the patio before the mortar has cured
- Ignoring movement joints around fixed structures or across large areas
How Much Material Is Needed?
Material quantities depend on the area, construction depth, compaction and wastage. As a planning guide:
- Porcelain slabs: order the measured area plus an allowance for cuts, breakages and future replacements. Around 10% is common for a straightforward rectangular patio; complex layouts may require more.
- MOT Type 1: calculate from the compacted volume and allow for compaction. A 20 m² patio with a 100 mm compacted sub-base represents 2 m³ of finished compacted material.
- Mortar: calculate from the patio area and average bed thickness. A 20 m² patio with a 35 mm bed represents approximately 0.7 m³ of installed mortar before allowing for waste and local level adjustment.
- Slurry primer and grout: use the manufacturer’s stated coverage and add a sensible waste allowance.
Coverage figures are estimates, not substitutes for an accurate site calculation.
Porcelain Paving Installation FAQs
Do porcelain paving slabs need slurry primer?
Yes, for a conventional mortar-bed installation. Porcelain has extremely low water absorption and does not form a dependable direct bond with ordinary bedding mortar. Apply a suitable slurry primer over the complete underside immediately before laying.
What is the best base for porcelain paving slabs?
For a normal pedestrian patio over soil, a stable sub-grade, approximately 100 mm of compacted MOT Type 1 and a continuous full mortar bed are commonly used. Ground conditions and expected loading may require a different specification.
What mortar mix should be used for porcelain paving?
A mix of approximately four parts sharp sand to one part cement is commonly used for a site-mixed full mortar bed. Follow any specific instructions supplied with the paving, primer or proprietary bedding system.
How thick should the mortar bed be?
Approximately 30–40 mm is common over a correctly prepared sub-base. The mortar must remain thick enough to provide full and continuous support after the slab has been tapped to level.
How much MOT Type 1 is needed under a porcelain patio?
A conventional pedestrian patio commonly uses approximately 100 mm after compaction. Soft ground, made-up ground or heavier loads can require a deeper or specially designed construction.
What fall should porcelain paving have?
A fall of approximately 1:60 is a practical target for many UK patios, equivalent to about 17 mm per metre. The exact fall should suit the slab surface, site levels and drainage arrangement.
Can porcelain paving be laid directly on sand?
No. Loose sand alone does not provide a durable bonded base for a conventional 20 mm porcelain patio.
Can porcelain paving be laid over concrete?
Yes, where the concrete is stable, sound, suitably drained and compatible with the selected external fixing system. Cracked, moving or badly graded concrete should be repaired or replaced rather than concealed.
What joint width should be used for outdoor porcelain?
Rectified outdoor porcelain is commonly laid with joints of approximately 3–5 mm. Around 5 mm is practical for many patios, subject to the slab and jointing-product instructions.
Can porcelain slabs be laid without gaps?
No. Butt-jointing leaves insufficient space for jointing material, laying tolerances and movement, and it increases the risk of edge damage.
What blade is best for cutting 20 mm porcelain paving?
Use a high-quality continuous-rim diamond blade specifically rated for porcelain. A suitable wet bridge saw normally provides the best control for repeated straight cuts.
How long should porcelain paving cure before jointing?
Joint only after the bedding mortar has developed sufficient strength. The time varies with temperature, moisture, bed thickness and the products used, so follow the relevant manufacturer’s instructions and allow longer in cold or damp weather.
Final Installation Checklist
- Finished level and DPC clearance checked
- Drainage route and fall established
- Ground excavated to stable material
- Sub-base compacted in layers
- Edges and drainage restrained correctly
- Full mortar bed used beneath every slab
- Slurry primer applied to 100% of each slab underside
- Slabs laid while mortar and primer remain fresh
- Alignment, fall and joint widths checked continuously
- Cement residue removed immediately
- Bedding mortar allowed to cure before jointing
- Joints filled completely with a porcelain-compatible product
A durable porcelain patio is determined primarily by the work beneath the visible surface. Stable ground, a thoroughly compacted sub-base, a full mortar bed, complete slurry-primer coverage and controlled drainage are more important than cosmetic adjustments made after laying.
Browse the PSU range of outdoor porcelain paving slabs, or read our porcelain paving FAQs for further product and installation guidance.
