Paving sub base depth: correct thickness for patios and drives

Get the compacted sub base depth right and the paving above it lasts decades. Get it wrong and you’re relaying slabs within two or three years.

For a standard pedestrian patio or garden path, aim for a compacted sub base depth suitable for light foot traffic. A domestic driveway carrying car weight requires a thicker compacted sub base, with heavier vehicle areas needing an even deeper base. These figures describe the compacted aggregate layer only, not the total hole you dig. PavingExpert’s guidance sets out these same bands and is worth bookmarking if you want the trade-standard reference. The material doing the work here is DTp1 or MOT Type 1, a graded crushed stone, not loose gravel.

Patios and paths: 75–100mm compacted
Domestic driveways: 100–150mm compacted
Heavy or commercial use: 150–225mm compacted
Poor or clay soil: add 50mm or more to any of the above

Skip a lift, use the wrong stone, or ignore a soft subgrade, and you get differential settlement, where one section of paving sinks faster than its neighbour and the joints start to gape.

Key Takeaways

Compacted sub base depth ranges from 75mm for pedestrian paths to 225mm for heavy use, and correct compaction in lifts matters as much as the depth itself.

Point	Details
Patio and path depth	Use 75–100mm compacted DTp1 or MOT Type 1 for pedestrian areas on firm ground.
Driveway depth	Use 100–150mm compacted, rising to 200–300mm on clay soils.
Compact in lifts	Build the base in 50–75mm loose lifts, never one thick layer, to avoid hidden settlement.
Match material to purpose	Use graded DTp1/Type 1 for structural load, Type 3 free-draining stone only for permeable systems.
Order from a trusted supplier	Poweraggregates supplies DTp1, MOT Type 1, and Type 3 aggregate direct from its own quarries with delivery and technical advice included.

Why sub base depth matters and what actually fails
Quick reference: recommended sub base depths by project
Which sub base materials work and which ruin the job
How to calculate excavation depth and order the right quantity
Compaction best practice: lifts, kit and flatness checks
When site conditions mean you need extra depth
Permeable paving and Type 3: a different set of rules
Common DIY mistakes that cause sub base failure
Buying materials: what to ask your supplier
How Poweraggregates supplies the right base for your project
Sources
FAQ

Why sub base depth matters and what actually fails

A sub base isn’t just a levelling layer. It spreads point loads, like a car tyre or a heavy pot, across a wider area of the ground beneath, so the soil never has to carry more pressure than it can bear. Think of it as a stiff mattress between your paving and the earth. Too thin, and every load transmits almost directly into the subgrade below.

When that happens, the classic failure is differential settlement: not the whole patio sinking evenly, which is rarely a problem, but one corner or one slab dropping while the rest stays put. You’ll see it as a slab that rocks underfoot, a joint that’s opened up unevenly, or a puddle that never used to form. Engineering guidance on subgrades and subbases puts the minimum for most unstabilised granular sub bases at 100mm, precisely because thinner layers can’t distribute load reliably over variable ground.

Here’s the part most DIY guides skip: depth alone doesn’t save you if the compaction is poor. A 150mm layer dumped in one go and lightly tamped can perform worse than a well-compacted 100mm layer built in stages. Trade sources are consistent on this point:

Compaction quality drives long-term performance as much as raw depth does.
Loose, uncompacted material beneath a compacted surface layer will settle later, sometimes months after the paving goes down.
Signs of an undersized or poorly compacted base include sinking slabs, cracked bedding mortar, and joints that widen unevenly rather than uniformly.

Get the depth and the compaction right together, and you’ve solved most of what causes patios to fail early.

Quick reference: recommended sub base depths by project

Use this table to pick a starting depth, then adjust for your soil using the guidance further down.

Before you commit to a number, walk the site with a spade. Dig a small test hole in a couple of spots and note what you find: firm sandy ground supports the lower end of each range, while anything soft, clayey, or holding water after rain pushes you towards the higher figure. If your test hole fills with water and stays wet, that’s your cue to look at drainage before you look at depth. Stone & Surfaces’ trade guide backs roughly 100mm for patios on good ground and at least 150mm for driveways, which lines up with the table above.

Which sub base materials work and which ruin the job

The single biggest material mistake DIYers make is reaching for whatever gravel looks cheapest at the yard. Not all aggregate is built to carry load, and the difference matters more than the depth you dig.

DTp1 / MOT Type 1 / crusher-run is a graded, angular crushed stone that includes a mix of particle sizes down to fine dust. When compacted, those smaller particles lock into the gaps between the larger stones, forming a dense, interlocking matrix that barely moves under load. This is the standard structural sub base for patios, paths, and driveways, and it’s what every depth figure in this guide assumes you’re using.

Type 3 aggregate is a different animal: an open-graded, largely fines-free stone designed to let water pass through rather than lock together tightly. Marshalls’ guidance on sub base selection draws a clear line here: Type 1 for load-bearing domestic work, Type 3 for permeable paving where drainage is the priority. Using Type 3 under a standard patio where you need structural stiffness is the wrong call, just as using Type 1 under a permeable system defeats the drainage purpose entirely.

Avoid these for a structural sub base, no exceptions:

Rounded pea gravel or single-sized 20mm gravel. It lacks fines, so the stones roll against each other rather than locking, and it will never compact into a stable base no matter how long you run a plate over it.
Random rubble, broken concrete, or builder’s leftovers. These create unpredictable hard and soft spots through the base, guaranteeing uneven settlement later.
Topsoil or anything with organic content. It compresses and decomposes over time, taking your paving down with it.

Pro Tip: Before you order, ask your supplier for the material’s compacted density and confirm the MOT grade and maximum particle size on the delivery note. A genuine DTp1 spec sheet takes thirty seconds to check and saves you from discovering the wrong stone once it’s already tipped on your drive.

How to calculate excavation depth and order the right quantity

Once you know your target compacted sub base depth, the rest is arithmetic. The formula is:

Total excavation depth = compacted sub base depth + bedding layer + paver or block thickness

There’s a wrinkle worth understanding: sub base material compacts down. Loose aggregate typically settles by roughly 20 to 25% once compacted, so you need to order more loose material than the finished compacted depth suggests. A worked example from a patio base depth guide shows this clearly.

Worked example for a standard patio:

Set your compacted base target: 100mm.
Add bedding layer: typically 25mm of sharp sand or a bedding mortar.
Add paver thickness: say 22mm for a standard concrete flag.
Sum the total excavation: 100 + 25 + 22 = 147mm, so dig to roughly 150mm.
Calculate loose material needed: because compaction reduces volume by around 20–25%, order sub base as loose volume, not compacted volume, meaning your 100mm compacted layer needs closer to 125–130mm of loose material before you compact it down.

To turn that into an order, work through area first. Multiply the patio’s length by width to get square metres, multiply by your compacted depth in metres to get cubic metres, then apply the loose conversion factor to get the volume you actually need delivered. Most DTp1 weighs approximately 1.8 to 2.1 tonnes per cubic metre once compacted, so a 20 square metre patio at 100mm compacted depth needs around 2 cubic metres of compacted material, translating to roughly 3.6 to 4 tonnes ordered loose.

Add 5 to 10% on top for waste, spillage, and the inevitable underestimate. When you call your supplier, state the compacted depth you’re targeting, not the loose depth, and confirm whether the quoted price is per tonne or per cubic metre, since the two aren’t interchangeable once density varies between materials.

Compaction best practice: lifts, kit and flatness checks

Depth on paper means nothing if you drop it all in one go. Compact in layers, or “lifts,” of no more than 50 to 75mm loose material at a time, which settles to roughly 35 to 50mm once compacted. Go thicker than that and a plate compactor can’t transmit enough energy to densify the bottom of the layer, leaving loose material hidden under a compacted crust that will settle later.

For most DIY patios and paths, a rented plate compactor in the 3,000 to 5,000 lbf range does the job. For driveways or anything over roughly 40 square metres, a powered roller or a heavier trench compactor speeds things up and gives more consistent results across a larger area.

Follow this sequence for every project:

Lay the first lift of loose sub base, spread evenly with a rake.
Compact it thoroughly, making two or three passes with the plate compactor in different directions.
Check the surface for flatness using a straight edge, aiming for no more than 3 to 4mm variation per metre.
Repeat with the next lift until you reach your total compacted depth target.
Final check before laying bedding: the surface should feel solid underfoot with no give when you walk across it.

If your boot leaves an impression or the compactor bounces rather than pressing down, that layer isn’t done. Run it again before moving on.

When site conditions mean you need extra depth

A tape measure tells you the target depth. A spade tells you whether that target is enough for your ground.

Start with a simple soil check: dig a 300mm test hole and squeeze a handful of soil. Sand falls apart and drains fast, silt feels smooth but holds together loosely, clay rolls into a sticky ball and stays wet for days after rain, and peat looks dark, fibrous, and spongy. Sandy, free-draining ground supports the lower end of the depth ranges above. Clay and peat almost always need more.

Clay soil: add at least 50mm to your standard depth, and consider going to 200–300mm under a driveway, since clay expands and contracts significantly with moisture.
High water table or poor drainage: increase depth and consider Type 3 free-draining construction, or address drainage separately before building up.
Peat or made-up ground: don’t build on it directly. Excavate the peat out and replace with engineered fill, and lay a geotextile membrane to stop the sub base migrating into soft ground below.
Standing water in your test hole: stop and sort drainage first. No amount of extra depth compensates for water sitting under your paving.
Slopes: add solid edge haunching in concrete to stop sub base migrating sideways under load, particularly on driveways.

If your spade sinks in without resistance or the hole fills with water overnight, treat that as a decision point, not a detail to work around.

Permeable paving and Type 3: a different set of rules

Permeable systems flip the usual logic. Instead of building a dense, interlocking base that keeps water out, you’re building one that lets water through and, often, stores it temporarily before it soaks away.

Type 3 aggregate does this by omitting the fine particles that make Type 1 lock together tightly, leaving open voids for water to pass through the structure rather than pooling on the surface. Depth guidance for permeable systems isn’t driven primarily by vehicle load; it’s driven by how much water the sub base needs to hold and how fast the ground beneath can absorb it.

Depths of 150mm or more are common, but the real design driver is infiltration rate and storage volume, not just weight-bearing.
A poorly draining subgrade underneath a permeable surface may need a soakaway or attenuation structure built into the design.
Sustainable drainage (SuDS) rules vary by local authority, and larger permeable installations, particularly for driveways with planning implications, often warrant a chat with a specialist or a check against local guidance before you dig.

Drainage stone for these builds needs to match the free-draining specification, not the load-bearing one your standard patio uses.

Common DIY mistakes that cause sub base failure

Most sub base failures trace back to a handful of repeated errors, and nearly all of them are avoidable once you know to look for them.

Using rounded, single-sized gravel instead of graded DTp1. It never locks together, so it shifts under load indefinitely.
Compacting in one thick lift instead of several thin ones. The base looks compacted but hides loose material underneath that settles later.
Ignoring drainage until water starts pooling. By then you’re excavating an already-laid patio.
Skipping edge restraints. Without haunching or edging, sub base migrates outward and the whole structure loses stiffness at the perimeter.
Not checking the damp-proof course. Finished paving levels should sit at least 150mm below the house DPC to avoid bridging it and causing damp.

Watch for red flags as you build: a soft patch that gives under the plate compactor, or water that ponds and won’t drain after rain. Both mean stop, dig that section out, and address the cause, whether that’s contaminated fill or a missing geotextile layer, rather than paving over the problem.

Buying materials: what to ask your supplier

Getting the depth calculation right only pays off if the material that turns up matches the specification you planned around. Before you place an order, run through this checklist with your supplier:

Exact material spec: confirm DTp1 / MOT Type 1 for structural work or Type 3 for permeable systems, not just “hardcore” or “crusher-run” without a grade attached.
Maximum particle size and delivered density, so your tonnage calculation actually matches what turns up.
Price basis: per tonne or per cubic metre, since the two aren’t directly comparable without knowing density.
Delivery vehicle and site access: tight driveways or narrow side entrances may need a smaller vehicle or a different tipping arrangement.

When you place the order, state your target compacted depth clearly, ask for a material data sheet if one’s available, and build in a 5 to 10% waste allowance. If you’re doing the job yourself, book compactor hire at the same time so the material isn’t sitting exposed to rain for days before you’re ready.

A note from decades of watching sub bases fail and succeed

Supplying aggregates for as long as Poweraggregates has means seeing the same mistakes repeat across driveways and patios: the base rushed, the wrong stone chosen, one lift instead of three. Most patios are genuinely within a competent DIYer’s reach. Driveways carrying regular vehicle weight, sloped sites, or ground that won’t stop holding water after rain are where calling in a contractor earns its cost. If you’re unsure which side of that line your project sits on, get in touch with the Poweraggregates team for advice before you dig, not after.

How Poweraggregates supplies the right base for your project

Poweraggregates supplies DTp1, MOT Type 1, and Type 3 free-draining aggregate direct from its own quarries, in bulk loads for larger driveways or bagged quantities for smaller patio jobs, with delivery arranged to suit your site access.

Getting the material right matters more than getting a good price on the wrong stone. Poweraggregates’ team will talk through your compacted depth target, your soil type, and whether you need Type 1 or Type 3 before you place an order, so you’re not guessing at grades from a bag label. For patios and paths, browse the decorative stone range for options that combine a solid structural base with a finish that suits the garden. For driveways or larger jobs needing free-draining construction, the drainage stone range covers Type 3 specifications for permeable builds. Once you’ve settled on your excavation depth and material spec, head to the shop to order bagged quantities for smaller projects, or contact the team directly for a bulk delivery quote tailored to your site.

Sources

For deeper technical detail, PavingExpert’s sub base FAQ covers depth tables and layer technique in full. Marshalls’ sub base guide is the clearest breakdown of Type 1 versus Type 3 selection. Engineering-level detail on load distribution sits in the cement.org subgrades and subbases guidance.

FAQ

How deep should a patio sub base be?

For a standard pedestrian patio, aim for a compacted DTp1 or MOT Type 1 sub base, with increased thickness on softer or clay ground to improve stability.

Can you use 20mm gravel as a sub base?

No. Single-sized, rounded 20mm gravel lacks the fine particles needed to lock into a stable matrix, so it will never compact into a reliable structural base.

How thick should the base be under pavers?

Total excavation depth equals your compacted sub base (75–150mm depending on use) plus a bedding layer of roughly 25mm and your paver thickness, typically 20–25mm for standard flags.

How deep does a driveway sub base need to be?

Domestic driveways typically require a suitably thick compacted MOT Type 1 sub base, with increased depth recommended on clay soils or where heavier vehicle loads are expected.

What’s the difference between Type 1 and Type 3 sub base?

Type 1 (DTp1) is graded with fines and compacts into a dense, load-bearing matrix suited to patios and driveways, while Type 3 omits fines to let water drain through for permeable paving systems.