Why the build-up under your patio matters more than the slab on top — a homeowner's guide to BS 7533, MOT Type 1, and the right sub-base for Leicestershire clay soil.
Why this matters
Most failed patios in the UK don't fail because of the slab — they fail because of what's underneath. Settling, cracking, pooling water, lifting joints and weed growth all trace back to sub-base shortcuts. Spending an extra day on the sub-base is the single biggest factor in whether a patio still looks good in 10 or 20 years' time.
What BS 7533 actually says
BS 7533 is the British Standard for paved areas. It's broken into multiple parts (BS 7533-1 through BS 7533-13) covering different types of paving and traffic loads. The two most relevant parts for domestic patios are BS 7533-4 (laying precast concrete paving blocks) and BS 7533-12 (specifies design of pavements with natural stone, concrete and clay paving). The standard specifies sub-base depths, jointing materials, bedding mortar mixes, and edge restraint requirements. It is the benchmark a professional installer should be working to — and the document an insurance assessor will reference if a patio fails.
MOT Type 1 vs Type 3 — what's the difference?
MOT Type 1 is a graded, well-compacting granular material (typically crushed limestone or granite, 0–63mm). It is the default sub-base for domestic patios because it compacts to a stable, load-bearing layer. MOT Type 3 is an open-graded version (4–63mm) with no fines, designed to allow water to drain through. Type 3 is used under permeable surfaces; Type 1 under traditional patios where water will run off the surface.
Recommended depths for a domestic patio
For a typical foot-traffic patio: 100 mm compacted MOT Type 1, laid in two compacted layers, on a properly excavated and prepared formation. For a patio that will see occasional vehicle traffic (e.g. an extended driveway/patio at the side of a house): 150 mm compacted Type 1. We typically excavate to give 100 mm of sub-base, 30–50 mm of bedding mortar, plus the thickness of the slab itself (usually 20–22 mm for porcelain or 22–25 mm for natural stone).
Why Leicestershire clay needs more care
Most of Leicester and Leicestershire sits on Mercia Mudstone or boulder clay. Both are prone to seasonal heave — they swell when wet and shrink when dry, which can lift or settle paving over the course of a year. Two things help: (1) a deeper sub-base (we often spec 125 mm instead of 100 mm on the more clay-heavy sites in Oadby, Wigston and Syston); (2) a geotextile separation membrane between the clay and the sub-base, to stop the clay fines migrating up into the Type 1 and reducing its compaction.
Bedding: full mortar bed, not the five-dot method
BS 7533 is very clear: paving slabs should be laid on a continuous, full mortar bed — typically a 3:1 sharp sand and cement mix at 30–50 mm thickness. The 'five-dot' method (a blob of mortar at each corner and one in the middle) is explicitly NOT compliant and is one of the most common reasons patios crack. Hollow voids under a slab concentrate load on the contact points and the slab snaps under foot traffic.
Jointing: don't use kiln-dried sand on a patio
Kiln-dried sand is for block paving — it relies on tightly-butted joints. For patios with wider joints (3–10mm), the right product is a brush-in resin jointing compound or a wet slurry-style jointing mortar. These bond to the slab edges, lock the pattern, and don't wash out with the first hosepipe blast. We use brush-in resin jointing on most porcelain installs.
Edge restraint matters
Without a proper edge restraint, slabs can drift, joints open up, and the whole patio can spread over years of freeze-thaw. BS 7533 specifies haunched concrete edge restraint or a properly bedded edging course. We typically lay a haunched concrete kerb around the perimeter, or use a complementary edging slab bedded on mortar.
Drainage falls — the 1:80 minimum
Patios should have a minimum fall of 1:80 (1 cm drop per 80 cm of run) away from the house, to prevent water pooling and avoid bridging the damp-proof course. On Leicestershire clay we often go to 1:60 to give a bit more margin during heavy rainfall.
Getting a quote that includes the right sub-base
Our free instant patio quote includes a clear sub-base specification — depth, material, geotextile membrane (where appropriate), bedding spec and jointing product. If you're comparing quotes from other installers, ask them to put their sub-base spec in writing. A £2,000 patio with a 50 mm Type 1 sub-base will cost you more in 5 years' time than a £2,800 patio with the right build-up.
Frequently Asked Questions
How deep should a patio sub-base be?
For a foot-traffic domestic patio, 100 mm of well-compacted MOT Type 1 is the BS 7533 recommendation. For vehicular traffic, 150 mm. On heavier Leicestershire clay we sometimes increase to 125 mm and add a geotextile separation membrane.
What's wrong with the 'five-dot' method of laying slabs?
It's not compliant with BS 7533 and it creates hollow voids under the slab. Foot traffic concentrates load on the dots, and slabs often crack in the centre within a year or two. A full mortar bed at 30–50 mm is what the standard requires.
Should I use kiln-dried sand to fill my patio joints?
No — kiln-dried sand is for block paving. Patios with 3–10 mm joints need a brush-in resin compound or a wet slurry jointing mortar that bonds to the slab edges.
Related Reading & Service Pages
Read next: Choosing the Best Patio Materials for UK Weather
Relevant service: Visit our patios service page →
See real installs: Browse our project case studies →
Areas covered in this article:
Get your free instant quote & AI garden design
Itemised pricing plus a free AI design preview — delivered in under 90 seconds. No charge, no obligation, no sales call.
Get My Free Quote & Design