At a glance
Laying paving slabs correctly is primarily an exercise in base preparation. The slabs themselves are the visible part of a system, and the system stands or falls on what lies underneath them. The single most common cause of paved areas failing – sinking, rocking, cracking, pooling water – is an inadequate or poorly compacted sub-base. Getting the groundwork right takes time and involves materials that are not glamorous, but it is the difference between a paved area that looks as good after fifteen years as it did on day one and a repair job after the first hard winter. Everything else in the process – the mortar bed, the laying pattern, the pointing – flows from a base that is correctly prepared and properly compacted.
This guide covers standard concrete or natural stone slabs bedded on a mortar mix over a compacted hardcore sub-base – the correct method for pedestrian patios and paths in UK conditions. Porcelain slabs follow the same principle but require a full mortar bed rather than the five-point method described below, because their low porosity does not allow them to bond adequately via spot contacts alone. Slabs intended for driveways subject to vehicle loading need a deeper sub-base (typically 150-200mm of MOT Type 1 compacted) and a stronger mortar mix than described here. If in doubt about loading requirements or planning considerations for a larger area, check with your local authority before starting work.
What You’ll Need
Base Preparation – the Most Important Step
The correct base depth for a pedestrian patio or path in UK conditions is 100mm of compacted MOT Type 1 (crushed limestone or granite hardcore) topped with a mortar bed of 30-50mm. The total excavation depth therefore needs to be approximately 150-200mm below your finished slab surface level, plus the thickness of your slabs (typically 35-50mm for standard concrete slabs, 20-30mm for porcelain). Mark this depth clearly on pegs driven into the ground at the perimeter and use a string line to check levels as you excavate.
Once excavated, spread the MOT Type 1 and compact it in two passes with a hired plate compactor. Never skip compaction and never compact in one deep layer – 50-75mm compacted lifts give better results than a single thick application. After compaction, the surface should feel firm underfoot with no movement. A sharp tap with a lump hammer on a piece of scrap wood laid flat should produce only a dull thud and no depression. Check levels with a long spirit level or a length of straight timber – the sub-base should replicate the fall you want in the finished surface, typically 1:80 (about 12mm per metre) away from the house to prevent water running towards walls or foundations.
Set the fall before you lay a single slab. A fall of 1:80 away from any building is the UK standard for external paving. Mark this fall on your perimeter pegs using a spirit level and thin packers, run string lines across the area at the correct height, and use these lines to set every mortar bed before the slab goes on. Trying to adjust the fall by tilting individual slabs after the fact produces an uneven, unprofessional result that will never drain properly.
Step by Step – Laying the Slabs
Mix the mortar bed
Mix 4 parts sharp sand to 1 part cement (by volume) dry, then add water gradually until you reach a consistency that holds its shape when squeezed but is not sloppy. It should not slump when formed into a ball. Too wet a mix leads to slabs sinking and joints cracking; too dry and adhesion is poor. For a standard concrete slab, aim for a bed depth of 35-50mm before the slab is pressed down.
Apply the five-point mortar bed
For standard concrete and natural stone slabs, place five blobs of mortar – one at each corner and one in the centre. Each blob should be roughly 150mm in diameter and higher than the target bed depth so the slab can be tapped down to the correct level. This method allows slight adjustment as you bed the slab and ensures contact at all four corners. Do not use a continuous wet mortar bed for standard slabs as it can trap water and cause frost damage.
Place and level the slab
Lower the slab carefully onto the mortar blobs. Use tile spacers or small pieces of timber to set the joint width consistently at 10-15mm. Tap the slab down with a rubber mallet – never a steel hammer – while checking against your string line and spirit level. The slab should sit firm with no rocking and at the correct height relative to adjacent slabs and your string line. Any corner that sits proud can be tapped down; any that sits low requires the slab to come up and more mortar to be added.
Work from a board, not the slabs
Always kneel or stand on a board laid on already-placed slabs, never directly on a freshly bedded slab. Freshly bedded slabs can shift under body weight before the mortar has set, even if they feel solid. Move the board as you progress across the area. Leave the slabs undisturbed for at least 24 hours before walking on them and 48-72 hours before pointing.
Cut slabs for edges and awkward spaces
Use an angle grinder with a dry diamond blade to cut slabs to size. Score and snap works on thinner slabs but an angle grinder gives cleaner cuts on thicker stone or concrete. Always cut face-up, wear eye protection and a dust mask, and wet the cut line if possible to reduce dust. Cut edges can be dressed with the grinder to remove sharp corners.
Pointing and Finishing
Pointing fills the joints between slabs and is the finishing detail that determines how the paved area looks and performs over time. Traditional mortar pointing (the same 4:1 sharp sand and cement mix used for the bed) is the most durable option but requires care to avoid staining the slab faces. Brush-in pointing compound is easier to apply and less likely to stain but is less durable in exposed positions and should be re-applied every few years. For natural stone slabs where cement staining is a concern, a kiln-dried jointing sand with a stabilising agent is available and works well on natural products with texture.
Common Problems and How to Avoid Them
The most frequent failure in DIY paving projects is slabs that rock or sink after the first winter. This invariably traces back to an under-compacted or shallow sub-base. Frost heave – where water trapped in a poorly compacted base freezes and expands – lifts slabs out of level and breaks mortar bonds. The solution is always adequate compaction at sub-base stage, never trying to compensate with thicker mortar. A properly compacted 100mm MOT Type 1 base does not move in frost; an uncompacted one will shift within the first cold spell. The second most common cause of failure is pointing applied too soon – before the mortar bed beneath the slabs has fully cured. Walk on or point over freshly laid slabs and you introduce micro-movements that crack joints within a season. Allow the mortar bed to cure fully – a minimum of 48 hours in dry conditions, longer in cool or damp weather – before pointing.
Never lay paving in frost or near-freezing conditions. Mortar that freezes before it cures does not harden correctly – it crumbles and loses strength. In the UK, avoid paving work when temperatures are forecast to drop below 3 degrees Celsius within 24 hours of laying. If frost is unexpected, cover newly laid slabs with insulating fleece or old carpet overnight. Similarly, laying in direct summer sun on a hot surface draws water from the mortar too quickly – dampen the sub-base and work in shade where possible.
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