Why a proper sub-base matters (and why we never skip it)

What proper groundworks buy you on a landscaping job — for your patio, your wall, and the next twenty years.

This is the geek post on the site. It's about the part of landscaping that nobody talks about, nobody photographs for Instagram, and nobody wakes up excited about. It's also the part that decides whether your garden looks the same in twenty years or has been redone three times by then.

If you've had a patio fail — slabs lifting, joints cracking, the whole thing rocking like a sea-saw under a chair — the sub-base was almost certainly the reason.

What a sub-base is

A sub-base is the layer of compacted hardcore between the natural ground (soil, clay, sand) and your finished surface (paving, lawn, walls). It does three jobs:

1. Spreads load. The weight of the patio plus furniture plus people plus snow gets distributed evenly across the soil below.
2. Resists movement. Sub-bases are designed to not compress, not heave, not move with seasonal soil cycles.
3. Drains water. A properly-built sub-base lets water run through it to a drainage layer below.

Think of a sub-base as the foundation under the foundation. The patio sits on bedding mortar. The bedding mortar sits on sub-base. The sub-base sits on soil. Without the middle layer, the bedding moves with every freeze-thaw cycle, the slabs go with it, and the patio fails.

What we use

For a domestic patio in South London, our default spec:

• MOT Type 1 hardcore. A graded mix of crushed stone (typically limestone or granite), 0-40mm particle size.
• Depth: 100-150mm. For a standard patio. More for a driveway, more on soft ground.
• Laid in 50-75mm layers — never more in one go.
• Plate-compacted between each layer with a Wacker plate compactor.
• Bound at the edges with concrete haunching to stop the sub-base creeping outwards over time.

For a wall foundation:

• Concrete strip footing — a continuous concrete beam under the wall.
• Depth: 600-900mm — below the frost line and below the seasonally-wet zone in clay soils.
• Width: 2-3x the wall thickness to spread load.
• Reinforcement (rebar) on retaining walls and walls over 1m.

For a garden room:

• Either screw-pile foundations or an insulated concrete raft, depending on ground conditions.

Each is over-spec for what's strictly needed. We over-build deliberately because the cost of doing it right once is a fraction of the cost of redoing it later.

What this means for your garden

Your patio stays flat for decades. With a proper compacted sub-base and a full mortar bed, a Yorkstone patio in London will look the same in 25 years as it does on day one.

Your walls stay vertical. With proper footings and DPC, a stock-brick boundary wall stands forever.

Your lawn drains. With a properly-graded topsoil layer over the right substrate, a lawn over heavy London clay actually works.

Groundworks aren't glamorous. They are the difference between a garden that ages well and a garden that needs redoing.

What to ask your landscaper

Four questions:

1. What's your sub-base spec? A proper answer gives a depth in mm and a material name (Type 1 / MOT Type 1).

2. How thick will the sub-base be? 100mm minimum on a patio, 150mm preferred. Less is risky.

3. How will you compact it? "Plate compactor between layers" is the right answer.

4. Are you using a full bed of mortar or spot bedding? Full bed is non-negotiable on a quality job.

A landscaper who can't answer these four questions on the spot probably isn't the one to use.