How to Lay a Patio: Sub-Base, Bedding, Pointing & Drainage Falls

Summary

Patio laying is one of the most common domestic landscaping jobs, but also one of the most commonly done poorly. The difference between a patio that lasts 20 years and one that sinks, cracks, and floods a back door within 18 months is almost entirely in the preparation: sub-base depth, compaction, bedding consistency, and drainage falls. These fundamentals are not complicated, but they require proper equipment (whacker plate, spirit level with 1:80 slope gauge, or a long level with known spacers) and a systematic approach.

The regulatory context for patios catches many homeowners and some tradespeople by surprise. Since October 2008, front garden and driveway paving over 5m² with impermeable materials requires planning permission in England unless it drains to a soakaway (Householder Permitted Development rights were amended). Rear patios are unaffected by this change, but all hard surfacing must direct rainwater away from buildings and, critically, must not direct it onto the public highway.

For tradespeople, understanding the drainage requirement protects you from callbacks. A patio that was laid perfectly flat, or with a fall toward the house, will cause problems — waterlogged beds against the house foundation, pooling against the DPC, and potential water ingress. The client may not understand why this matters at quoting stage; explaining it and demonstrating the 1:80 fall is part of a professional service.

Key Facts

• Minimum fall — 1:80 (1.25%) away from any building; 12.5mm per metre; steeper on clay soils or areas receiving heavy rainfall
• Sub-base depth (general) — 100mm compacted MOT Type 1 (crushed limestone/granite) over 50mm sand blinding
• Sub-base depth (clay soils) — minimum 150mm compacted MOT Type 1; consider geotextile membrane beneath sub-base on very poor ground
• Bedding options — semi-dry mortar (1:6 cement:sharp sand, 50mm deep); wet mortar (1:5, 25mm deep); or 25mm sharp sand for permeable installations
• Minimum slab thickness — 35–40mm for standard paving; 50mm for heavy-traffic areas; 20mm porcelain needs solid full-bed mortar
• Pointing — brush-in grout (fine kiln-dried sand) for clay pavers; mortar pointing for natural stone and concrete flags; brush-in flexible compound for porcelain (prevents joint failure)
• Permeable paving — allows water to drain through joints or porous material to sub-base and soakaway beneath; avoids planning permission for front garden paving
• Jointing sand — for block paving: kiln-dried sand; for permeable paving: permeable jointing compound or coarse grit
• Edge restraints — all paved areas require edge restraint to prevent lateral spread; concrete haunching to blocks; soldier course to flags
• Distance from DPC — hard surfacing should be at least 150mm below the DPC of the adjacent wall; reduces splash-back risk and rising damp potential
• Geotextile membrane — woven geotextile between subgrade and sub-base prevents fines migration; use in clay soil and anywhere tree roots are present
• Compaction — each layer of sub-base compacted in 50–75mm lifts with vibrating plate compactor; never compact more than 75mm depth in one pass

Quick Reference Table

Detailed Guidance

Excavation and Ground Preparation

Start by calculating the finished surface level and work backwards. The total depth of excavation is the sum of: slab thickness + bedding layer + sub-base + any geotextile + margin below DPC.

For a standard concrete flag patio adjacent to a house:

• Finished level: 150mm below DPC
• 50mm flag: 50mm
• Bedding: 50mm semi-dry mortar
• Sub-base: 100mm MOT Type 1
• Total depth below finished level: 200mm
• Therefore excavate 350mm below DPC (150 + 200)

Check for services before excavating — gas, water, and electric cables run in gardens, particularly near the house. Water boards and gas companies have online mapping tools. For depths below 300mm, call 0800 96 93 35 (Dial Before You Dig equivalent service in the UK [verify current number]).

Excavate to a uniform depth. Break up and remove all vegetable matter, topsoil, and roots. On clay, the subgrade must be firm — if it's soft and deforms underfoot, remove more material and/or lay a geotextile membrane before sub-base.

Sub-Base Installation

MOT Type 1 (also called DTp Type 1, Clause 803 material) is crushed angular aggregate, well-graded from 40mm down to fines. It compacts to form a stable, load-bearing layer. Do not use soft core (crushed brick or mixed demolition waste) — it lacks long-term stability.

Lay and compact in maximum 75mm lifts. Use a vibrating plate compactor (whacker plate) — minimum plate size 300mm × 400mm for most domestic work. Make 2–3 passes over each lift, overlapping by 50%. The surface should be firm, level, and not move under foot traffic.

Check the compacted sub-base level against a string line or datum. Adjust before bedding — bumps in the sub-base create thin or thick spots in the bed that will cause differential settlement.

Setting Out Falls

The 1:80 minimum fall is the absolute minimum. For areas likely to receive heavy rainfall, a steeper 1:60 fall is better. Mark the fall direction with a string line and calculate the level difference at each end.

Example: 3m patio at 1:80 fall = 3/80 = 37.5mm level difference across 3m; highest point at house, lowest point at garden edge.

Use a spirit level with a wedge/block under one end to check fall consistently. A 1m level with a 12.5mm block under the lower end creates a 1:80 fall check. Check every course of slabs in both directions.

Bedding

Semi-dry mix (most common for flags): 1:6 Portland cement:sharp sand, consistency of damp sand that holds its shape when squeezed. Lay to 50mm depth, screed flat with a gauge screed board, then compact lightly with a hand tamper. Place slab, tap level with a rubber mallet, check falls with level, adjust as needed. The slab should be solid and stable with no hollow spots — a hollow tap means insufficient mortar contact.

Full wet mortar (1:5) for stone: natural stone often benefits from a wetter, thinner bed (25mm) to ensure full contact on uneven stone backs. Prime the back of the stone with SBR adhesive/slurry coat before laying to improve adhesion, particularly on smooth-sawn stone.

Full adhesive bed (porcelain): porcelain has low absorption and will not bond to mortar alone. Use a flexible tile adhesive (C2TE classification per BS EN 12004) as a bedding adhesive. Also back-butter the tile. Minimum 3mm adhesive on tile back, 10–15mm on prepared base. Very important for outdoor use — porcelain tiles on mortar bed without adhesive will lift in frost.

Pointing

Mortar pointing (natural stone, reconstituted stone): mix 1:3 sharp sand:cement (weaker than structural mortar — allows movement without cracking slabs). Work in sections. Brush mortars are available in premixed form (e.g., Rompox, Romex) — these are polymer-modified and more durable. Avoid pointing in wet or freezing conditions.

Brush-in dry compound (block paving): kiln-dried sand brushed into joints and vibrated in with plate compactor. Top up and brush in twice. Reapply annually as sand migrates over time.

Flexible pointing compound (porcelain): specifically formulated for porcelain with minimal water absorption. Standard mortar pointing on porcelain will crack and lift in frost because there's no movement accommodation. Use Rompox Easy, Azpects EASYJoint, or similar. These are often more expensive but essential for longevity.

Drainage and Surface Water Management

Water must flow off the patio surface and away from the house. The fall direction matters as much as the fall gradient:

• Fall should direct water toward a lawn, border, or drainage gully
• Never fall toward the house wall
• Never fall directly onto a neighbour's property or the public highway

For large patios, a linear drainage channel (e.g., ACO drain) across the patio close to the house collects water before it reaches the building. Connect to soakaway (for clay-free soils), existing surface water drain, or an infiltration crate.

On heavy clay soils, infiltration is slow — design drainage to a surface water drain or to a French drain system running along the edge of the patio.

Frequently Asked Questions

How thick should the mortar bed be for natural stone?

25–50mm for most natural stone flags. Natural stone has more variation in thickness than manufactured flags, so the mortar bed compensates. Always use a full bed (no spot bedding) — spot bedding leaves hollow areas where the stone flexes and cracks, particularly for softer stones like sandstone.

Do I need planning permission for a patio in the back garden?

No planning permission is needed for a patio or hard surfacing in the rear garden of a house — this is within Permitted Development rights. Front gardens are different: impermeable paving over 5m² requires planning permission in England (GPDO 2015, Schedule 2, Part 1, Class F.2). Using permeable paving, or draining to a soakaway, avoids the planning requirement.

What causes a patio to sink?

Almost always inadequate sub-base preparation: insufficient depth, use of soft fill, poor compaction, or laying over topsoil and organic material. Frost heave can also lift slabs where bedding is thin and water can penetrate beneath. The fix is usually to relay — patch repairs rarely level a sunken slab permanently.

Can I lay slabs on sand only?

Sand-only bedding (unbound laying) is suitable for permeable paving systems (block paving, permeable flags) where movement through the joints is expected and joints can be re-sanded. It's not suitable for natural stone or porcelain — these need a mortar or adhesive bed to be stable. Sand-only bedding under heavy natural stone is a common cause of premature failure.

Regulations & Standards

• Town and Country Planning (General Permitted Development) (England) Order 2015 — Class F.2 (front garden paving), Schedule 2, Part 1

• BS 7263-1 — precast concrete flags, kerbs, channels and edgings

• BS 7533 — pavements constructed with clay, natural stone or concrete pavers

• Building Regulations Approved Document C — site drainage adjacent to buildings

• SUDS Manual (CIRIA C753) — sustainable drainage design principles

• Planning Portal — Paving Your Front Garden — planning guidance

• Marshalls Technical Guidance — installation guidance for flags and block paving

• Natural Stone Institute — stone types and installation

• CIRIA C753 SUDS Manual — drainage design reference

• block paving — block paving sub-base, compaction, and permeable options

• drainage landscaping — French drains and surface water management

• retaining walls — retaining walls adjacent to patios

• dpc replacement — DPC levels and hard surfacing adjacency