Patio Calculator UK - Materials for Your Whole Patio Build

A Patio Build-Up, Layer By Layer

Every patio is built the same way, whatever's on top: excavate to the right depth, compact a sub-base, lay a bedding layer, place the slabs, then joint between them. Skipping or skimping a layer is where patios fail - a thin sub-base lets the ground move under load, a dabbed-not-bedded slab rocks and cracks, and unfilled joints let water into the bedding below. Work through the calculator above in build order and you get a complete list, not just a slab count.

How Deep to Excavate, and Why

Total excavation depth is sub-base depth, plus bedding depth, plus slab thickness - typically 175-200mm for a foot-traffic patio (100mm sub-base + 40mm bedding + ~35mm slab), more for a vehicle-rated sub-base. Dig the full area to that depth, not just a trench under the edges - the sub-base needs to support the whole slab, not just its perimeter. The excavated material (spoil) needs somewhere to go: our figure includes the loose bulking that happens when you dig compacted ground up, so it's a genuine "how much will fill a skip" number, not just the excavated volume.

Falls and Drainage: Where the Water Goes

A minimum fall of 1:80 away from any building is the standard UK guidance - 12.5mm of drop for every metre of run. Water should head towards a lawn, border or drain, never towards a wall. For a patio bordered by beds on multiple sides, a fall in two directions (away from the house, plus a shallow crossfall) avoids the whole surface looking visibly tilted one way.

The 150mm DPC Rule

The finished patio surface must sit at least 150mm below the damp-proof course of any adjoining wall - roughly two brick courses. This is the rule people get wrong most often, because it's invisible until damp shows up inside months later: build up too high against the house and rain splashing off the patio (or the patio itself holding moisture against the wall) can bridge the DPC. The calculator above checks this directly against your own build-up depth and DPC height, not just states the rule.

Sub-Base Depth: Patio vs Driveway

100mm of compacted MOT Type 1 is standard for a foot-traffic-only patio. A driveway needs at least 150mm to carry vehicle weight, more again on soft ground or for regular heavy vehicles. These figures match our sub-base calculator exactly - if your ground is soft, clay, or you need a different depth than the two presets here, use that calculator directly for ground-condition-adjusted depths.

Edging: When You Need It

Any side of the patio bordered by lawn, planting or loose material needs edging - block edging bedded in mortar, or the more permanent block-and-haunching combination where a concrete haunch locks the edging unit in place. Without it, the sub-base and bedding spread sideways under load over time, and lawns creep over the slab edge. A side that already butts a wall, fence base or building doesn't need edging - only measure the sides that actually need it.

Patio By Area: Quick Reference

600 x 600mm slabs, stack bond, foot-traffic sub-base, 5% waste, figures regenerated directly from the calculator above.

Use the calculator above for excavation, edging and your own laying pattern. Travis Perkins and Jewson supply full patio material lists for trade quantities; Wickes and B&Q suit smaller DIY-scale orders. For the detailed slab options and cut modelling behind these figures, see our paving slab calculator; for sub-base ground-condition adjustments, see our sub-base calculator.

Frequently Asked Questions

What does a patio build-up look like, layer by layer?

From the ground up: sub-grade (the excavated, undisturbed ground), a compacted sub-base (usually MOT Type 1), a bedding layer (sand and cement mortar, or sometimes sand alone under porcelain), the paving slabs themselves, and jointing between them. Each layer has a job: the sub-base spreads load and stops the ground moving, the bedding gives a stable, level base for the slab, and the joints lock the slabs together and keep water out of the bedding below.

How deep do I need to dig for a patio?

Total excavation depth is sub-base depth plus bedding depth plus slab thickness - typically around 175-200mm for a standard foot-traffic patio (100mm sub-base, 40mm bedding, ~35mm slab), more for a driveway-rated sub-base. Dig to that depth across the whole area, not just under the edges, and allow extra if the ground is soft or you're building up to meet a level.

What fall does a patio need, and where does the water go?

A minimum fall of 1:80 away from any building - 12.5mm of drop for every metre you move away from the house. It should direct water towards a lawn, border or a drain, never towards a wall or a neighbouring property. On a large patio, a fall in two directions (a shallow crossfall as well as away from the house) can help avoid a visibly sloped single-direction surface.

What is the 150mm DPC rule, and why does it matter?

The finished patio surface must sit at least 150mm below the damp-proof course (DPC) of any adjoining building - roughly two brick courses. Get this wrong and rainwater splashing off the patio, or the patio itself holding damp against the wall, can bridge the DPC and let damp into the house. It's one of the most common patio mistakes, precisely because it's invisible until the damage shows up inside.

Do I need a deeper sub-base for a patio than a driveway?

Yes, less deep. A patio only carries foot traffic, so 100mm of compacted MOT Type 1 is standard. A driveway carries vehicle weight and needs at least 150mm, more on soft ground or for regular heavy vehicles. Using driveway-depth sub-base under a patio isn't wrong, just unnecessary cost and spoil to remove.

Do I need edging around a patio?

If the patio is bordered by a lawn, border or loose material on any side, yes - edging (block edging or haunched concrete kerb) stops the sub-base and bedding spreading sideways under load and keeps the lawn from creeping over the slabs. Where a side already butts against a wall, fence base or building, you don't need edging on that side.