How Do You Prepare the Sub-base for Block Paving?

Quick Answer: Block paving sub-base preparation is critical to a long-lasting drive. Excavate to a depth of 250–300mm for cars and 350–400mm for HGV access, lay a compacted 150–200mm Type 1 MOT sub-base (BS EN 13242), then 30–50mm sharp sand laying course to BS 7533-3:2005+A1:2012. Falls of 1:60 minimum must be set across the surface for surface water. Edge restraints (haunched concrete or sectional kerb) are mandatory — without them, the paving fans out and fails within months.

Summary

A block paved driveway or patio is only as good as the layers underneath. Most "block paving failures" are sub-base failures: rutting, undulation, edge spread or sinking next to drainage points. None of these are tile or pavior faults — they are groundworks faults. BS 7533-3 sets out the construction method and is the de facto UK standard.

The principle is a continuous load path from the surface through to firm subgrade. Loads are spread by the sub-base (typically Type 1 MOT) and bedded by a granular laying course (sharp sand). The blocks themselves are the wearing surface and decorative element; they carry compression but not bending — that's the sub-base's job.

For domestic drives, the constraint is usually thickness vs. existing ground level: the sub-base eats 250–300mm, and there has to be a finished level that drains away from the house and matches the road/gully levels. Setting out and excavation are 30% of the work; getting the surface flat and falling the right way is the rest.

Key Facts

Quick Reference Table

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Use Sub-base Thickness Block Thickness Total Excavation Depth
Path / pedestrian only 100mm 50mm 180mm
Light driveway (car) 150mm 50–60mm 230–250mm
Standard driveway 200mm 60mm 280–300mm
Commercial drive / occasional HGV 250–300mm 80mm 360–400mm
HGV / heavy industrial 300–400mm 80mm 400–500mm
Permeable paving (Aquaflow / Permapave) 250–350mm permeable aggregate 80mm 350–450mm

Detailed Guidance

Setting out

The first task: establish the finished surface levels. Block paving falls toward drainage — usually toward the road, a gully, or a linear channel drain (e.g. ACO).

Block Paving Falls — Worked Example
House at 100.000m AOD (datum)
DPC must be 150mm above finished paving
Therefore paving max level at house = 99.850m
Drive length 8m, fall toward road 1:60
Fall over 8m = 8000/60 = 133mm
Paving level at road = 99.850 - 0.133 = 99.717m

A laser level set up at a stable point allows finished level pegs to be driven across the area. Run a string line between pegs and use it as the visual datum during excavation, sub-base placement and final surfacing.

Excavation depth calculation

For a 60mm block on 30mm sand on 150mm Type 1:

Add 20mm for compaction settlement in the sub-base = excavate 260mm below finished level. Adjust for the actual block thickness on the chosen product.

Subgrade preparation

Once excavated, the subgrade (the natural ground at the bottom of the dig) must be assessed:

Sub-base

Type 1 MOT is the workhorse sub-base material. Graded 0–63mm crushed limestone or granite — well-graded, angular, and stable when compacted. Specification by BS EN 13242.

Placement:

  1. Spread in lifts of 100–150mm maximum (deeper lifts don't compact)
  2. Lightly moisten if dust-dry
  3. Compact each lift with a vibrating plate (4 passes minimum, in alternating directions)
  4. Add the second lift if total exceeds 150mm
  5. Final surface should be smooth, with visible aggregate locking, no loose fines

Test by walking on it: a well-compacted sub-base feels solid underfoot with no movement. Pushing a finger down should leave no impression deeper than 5mm.

Edge restraints

Without edge restraint, blocks at the perimeter creep outward under each pass of a tyre, gaps open at the edge, the joint sand washes out, and the drive fails. Three standard methods:

Haunch the restraint to bring concrete to within 50mm of the surface, then cover with topsoil and turf.

Laying course

Sharp sand to BS EN 13242, free of clay and silt. Building sand (red, fine, slightly clay-bearing) is unsuitable — it does not compact stably and tracks water out of joints.

Procedure:

  1. Spread sand in a 35mm layer (compacts to ~25–30mm)
  2. Screed level with a screed bar pulled along screed rails or against the edge restraint
  3. Do not compact the sand before laying blocks — blocks press into the uncompacted sand during final vibration
  4. Lay blocks immediately — uncovered screeded sand is destroyed by rain, foot traffic and wind

Laying pattern and procedure

Common patterns:

Lay full blocks first, working outward from a chosen corner. Once a panel is complete, cut blocks to fit at edges using a block splitter (clean break) or wet saw (for curves and angled cuts). Aim to consume one whole pallet before starting cuts — full blocks are stronger than cuts.

Final vibration and sand-fill

  1. Once blocks are laid, sweep kiln-dried silica jointing sand over the surface
  2. Brush diagonally into the joints, working it down
  3. Vibrate the surface with a 90–120kg plate compactor fitted with a protective rubber mat
  4. Two or three passes in different directions
  5. Top up jointing sand as joints settle
  6. Final brush-off — excess sand removed
  7. Do not power-wash for 6 months — kills the joint stability before it sets

Drainage compliance (Approved Document H / SuDS)

Since 2008, front gardens of new properties have been subject to the Town and Country Planning (General Permitted Development) (Amendment) Order. If a front garden surface >5m² will be impermeable and discharges to the highway, planning permission is required — usually given subject to:

  1. Permeable paving installed (block + permeable sub-base + soakaway below)
  2. OR runoff diverted to a permeable area (lawn, planting) in the garden
  3. OR runoff captured to a soakaway

Standard concrete block paving on a Type 1 sub-base is impermeable. Permeable paving systems use an open-graded sub-base (typically 4/20 or 4/40 clean angular stone) with no fines, allowing water to soak through.

Common faults

Fault Cause Remedy
Rutting under wheel tracks Insufficient sub-base thickness or compaction Lift, increase to 200mm Type 1, re-compact
Joints washed out Insufficient sand vibration or jointing sand Top up with kiln-dried sand, re-vibrate
Edges spreading No edge restraint or restraint unhaunched Lift edge course; install haunched concrete restraint
Standing water Falls too shallow or in wrong direction Re-survey; lift and re-lay to correct fall
Sinking around gullies Inadequate compaction around fittings Excavate, compact in lifts, re-lay
Efflorescence (white powder) Lime leaching from concrete blocks Wait 6–12 months; will weather away

Frequently Asked Questions

Can I lay block paving on a concrete base?

Yes — but the design differs. The blocks are bedded on a 25mm mortar bed (3:1 sharp sand:cement) rather than loose sand, and the concrete base must have drainage falls and movement joints. Common in commercial public realm work. For domestic driveways, granular sub-base is usually faster and cheaper.

Do I need permission to pave my front garden?

Under permitted development, you do not need planning permission if (a) the surface is permeable, OR (b) runoff is directed to a permeable area in the garden. If the surface is impermeable and >5m² and discharges to the highway, planning permission is required. Local council websites confirm — check before quoting.

How long after laying can I drive on it?

Once the final vibration is complete and jointing sand topped up, the drive can take light vehicle loading within 24 hours. Avoid heavy vehicles (skip lorries, large vans) for 7 days while the joint sand settles. Avoid power washing for 6 months.

What's the minimum fall?

1:80 (1.25%) is the absolute minimum, but 1:60 (1.67%) is the recommended minimum across a domestic drive. Falls shallower than 1:80 result in standing water, especially as the drive settles slightly over time. Cross-falls should be at least 1:60 to a linear channel drain.

Can I use building sand for the laying course?

No. Building sand (typically red, fine, slightly clay-bearing) is unsuitable — it holds water, washes out, and never compacts stably. Always use sharp sand, grit sand or jointing-grade sand to BS EN 13242, with passing 4mm and minimal fines below 0.063mm.

Regulations & Standards