Quick answer: A proper patio sub-base involves excavating to the correct depth, laying a geotextile membrane over the subgrade, then compacting MOT Type 1 hardcore in layers to a depth of 100–150mm, checked for level and fall throughout, before any mortar bed or slabs are laid on top.

 

Why the Sub-Base Matters More Than the Slabs

Whatever paving material you've chosen — Indian sandstone, porcelain, or granite — none of it performs well sitting on an inadequately prepared base. The sub-base is what actually carries the structural load and resists ground movement; the mortar bed and slabs above it are only as stable as what's underneath. Skipping or rushing sub-base preparation is the single most common cause of sunken, cracked or rocking patios reported by UK homeowners.

Step 1: Excavate the Subgrade

Start by digging out the patio area to a total depth that accommodates your sub-base, mortar bed and slab thickness combined — typically 150–200mm total for a domestic patio, though this can vary depending on ground conditions and slab thickness. Remove any topsoil, roots, and organic material entirely, since organic matter left beneath a patio will decompose over time and cause settlement. The exposed subgrade should be reasonably firm; if you hit soft, waterlogged or unstable ground, it may need additional sub-base depth or, in more serious cases, professional assessment before proceeding.

Step 2: Compact the Subgrade

Before adding any sub-base material, compact the exposed subgrade itself using a plate compactor. This firms up the natural ground and reduces the risk of it settling unevenly under the sub-base layer above once the patio is in use.

Step 3: Lay Geotextile Membrane

A geotextile fabric laid directly over the compacted subgrade, before the sub-base hardcore goes down, serves two important purposes. First, it prevents the sub-base material from gradually working its way down into the softer subgrade beneath over time, which can lead to an uneven, sunken patio years later. Second, it acts as a weed suppression layer, significantly reducing the chance of weeds pushing up through the paving from below. This step is inexpensive relative to the overall project cost and is one of the most overlooked steps in DIY installations — professional installers rarely skip it.

Step 4: Lay and Compact MOT Type 1

MOT Type 1 is a graded, crushed aggregate specified for sub-base use in the UK, designed to compact into a dense, stable, free-draining layer. Lay it in layers of no more than 100mm at a time, compacting each layer thoroughly with a plate compactor before adding the next — trying to compact the full depth in one pass typically results in a weaker, less consistent sub-base. Aim for a total compacted sub-base depth of roughly 100–150mm for a typical domestic patio, adjusting for ground conditions and expected use (a driveway or heavily trafficked area generally needs a deeper sub-base than a lightly used garden patio).

Step 5: Check Level and Fall Throughout

Don't wait until the slabs are being laid to check your levels — the sub-base itself should already be shaped to reflect your intended patio fall, typically around 1:80 directing water away from the house. Checking this at the sub-base stage, using string lines and a spirit level or laser level, makes every subsequent stage (mortar bed, slab laying) significantly easier and more accurate. Getting the fall wrong at sub-base stage is difficult and costly to correct later, so it's worth taking the time here. Our full guide on drainage and water management covers how to plan falls and manage surface water across the wider patio design.

How Sub-Base Requirements Vary by Project

A lightly used garden patio for furniture and foot traffic generally needs a shallower sub-base than a driveway intended for vehicle loading. If you're laying granite setts or cobblestones for a driveway, a deeper, more heavily compacted sub-base is typically required to handle the additional load — always check the specific guidance for your intended use and material before finalising your excavation depth.

Common Mistakes to Avoid

Leaving organic material in the subgrade. Roots and topsoil left beneath a patio decompose and cause uneven settlement over time.

Skipping geotextile membrane. This inexpensive step prevents long-term sub-base migration and weed growth, and skipping it is a false economy.

Compacting MOT Type 1 in one thick layer instead of multiple thinner ones. This produces a weaker, less consistent sub-base than proper layered compaction.

Not checking fall until the slabs are being laid. Correcting drainage issues after the sub-base is down is far more difficult than getting it right from the start.

Underestimating sub-base depth for driveways. Vehicle loading requires significantly more sub-base than pedestrian-only areas.

Calculating Materials for Your Project

Once you know your patio's dimensions and required sub-base depth, our paving slabs calculator can help estimate quantities, and our trade and bulk buying guide is worth a look if you're sourcing materials for a larger project.

Conclusion

A patio is only ever as good as what's underneath it. Taking the time to properly excavate, membrane, layer and compact your sub-base — and getting your fall right before a single slab goes down — is the foundation (quite literally) for a patio that stays flat, stable and problem-free for decades rather than years.