How to Plan a Patio Layout and Calculate Materials

Planning a Patio: Getting It Right First Time

A patio is one of the most popular garden projects in the UK, and one of the most common jobs for landscapers and general builders. Get the groundwork right and it'll last decades. Cut corners on the base or the falls and you'll be back within two years to relay it. Here's how to plan and calculate a patio properly.

Step 1: Choose Your Paving Material

The material you choose affects everything — from the sub-base depth to the jointing method and the final cost. Here's how the main options compare:

Paving Material Comparison

| Material | Typical Cost (per m²) | Pros | Cons | |---------|----------------------|------|------| | Concrete flags (600 × 600) | £15–£30 | Affordable, easy to cut, widely available | Can look basic, prone to staining | | Block paving | £25–£50 | Durable, flexible, good for driveways | Labour-intensive, needs edge restraint | | Porcelain paving | £40–£80 | Modern look, low porosity, stain-resistant | Hard to cut (needs diamond blade), needs priming for adhesion | | Natural stone (sandstone) | £30–£60 | Beautiful, each piece unique | Variable thickness, needs careful bedding | | Natural stone (granite) | £50–£100 | Extremely durable | Heavy, expensive, hard to cut | | Natural stone (slate) | £35–£65 | Contemporary look | Can delaminate, slippery when wet |

Key Considerations

• Porcelain needs a specialist adhesive (slurry primer) on the back of each slab because it's non-porous — see the British Association of Landscape Industries for paving installation guidance — mortar won't bond to it otherwise
• Natural stone varies in thickness by 5–10mm per slab, so you need a thicker mortar bed to level it
• Block paving is laid on sand, not mortar, and needs a rigid edge restraint to stop it spreading

Step 2: Calculate the Paving Quantity

Measure the length and width of your patio area. For irregular shapes, break it down into rectangles and calculate each section separately.

Area (m²) = Length (m) × Width (m)

Wastage Allowance

| Layout | Wastage to Add | |-------|---------------| | Straight (stretcher bond) | 5% | | Offset/staggered | 5–7% | | Herringbone | 10% | | Diagonal | 10–12% | | Random pattern (mixed sizes) | 5% |

For a 20m² patio in a diagonal pattern:

20 × 1.10 = 22m² of paving to order

Rather than working this out manually, plug your dimensions into the Patio Calculator to get exact slab counts for your chosen size and pattern.

Step 3: Sub-Base

The sub-base is the foundation of your patio. Skip it and the paving will sink, crack, and let weeds through within a couple of years.

Standard Sub-Base Build-Up

| Layer | Material | Thickness | |------|---------|-----------| | Sub-base | MOT Type 1 (compacted) | 100–150mm | | Mortar bed | 4:1 sharp sand to cement | 30–40mm | | Paving slab | Your chosen material | 20–50mm |

For block paving, replace the mortar bed with sharp sand (30–50mm) and add a layer of geotextile membrane between the sub-base and sand.

How Much Sub-Base Do You Need?

Volume (m³) = Area (m²) × Depth (m)

MOT Type 1 compacts down by roughly 20–25%, so order accordingly:

For a 20m² patio with 150mm sub-base: 20 × 0.15 × 1.25 = 3.75 tonnes (at 2.1 tonnes per m³, roughly 1.8m³ loose)

Use the Gravel Calculator to get the tonnage for your sub-base and any decorative aggregate borders.

Step 4: Drainage Falls

This is where many DIY patios go wrong. Water must drain away from the building — standing water against the house causes damp, and pooling on the patio makes it slippery and unusable after rain.

The Standard Fall

Minimum fall: 1:60 (roughly 17mm per metre)

This means for every metre of patio length running away from the house, the surface drops by approximately 17mm.

For a 4m deep patio:

4 × 17 = 68mm total fall from house to garden edge

How to Set Out the Fall

1. Set your datum point — the finished paving level at the house (must be at least 150mm below the DPC)
2. Measure the patio depth
3. Calculate the total fall
4. Set a string line at the far edge, dropped by the total fall
5. Use intermediate pegs to check the fall is consistent

Where Does the Water Go?

The water running off your patio needs somewhere to go:

• Onto a lawn or planted bed — the simplest option if your garden slopes away
• Into a soakaway — required if the garden is flat or slopes back towards the house
• Into an existing drainage gully — if there's one nearby

Important: Under the Flood and Water Management Act 2010, you cannot drain a patio directly into the public sewer without permission. If you're paving over 5m² at the front of a property, you either need to use permeable paving or ensure drainage goes to a soakaway/garden, not the road drain.

Use the Drainage Calculator to work out soakaway sizes and drainage runs.

Step 5: Edge Restraint and Haunching

For Slab Patios

Slabs at the edges need haunching — a triangular bead of concrete (roughly 6:1 ballast to cement) along the outer edge of the mortar bed. This stops the edge slabs sliding outwards over time.

For Block Paving

You must install a rigid edge restraint before laying. Options include:

• Concrete edging kerbs (best for heavy use)
• Aluminium or steel paving edges (neater finish, lighter duty)
• Existing walls or structures

Without edge restraint, block paving will creep outward and the joints will open up.

Step 6: Jointing

| Paving Type | Jointing Method | |------------|----------------| | Concrete flags | Wet mortar (4:1) or dry brush mix | | Natural stone | Wet mortar pointed or resin jointing compound | | Porcelain | Resin-based jointing compound (Flowpoint, UltraScape, etc.) | | Block paving | Kiln-dried sand or polymeric sand |

Avoid cement-based jointing on porcelain — it stains the surface and is almost impossible to clean off. Always use a resin-based compound.

Laying Patterns

Popular Patterns for Slabs

• Stretcher bond — slabs in rows, each offset by half. Simple, minimal waste
• Stack bond — grid pattern, all joints aligned. Modern, clean look
• Random ashlar — mixed sizes in a repeating pattern. Traditional, works well with natural stone

Popular Patterns for Block Paving

• Herringbone (45° or 90°) — strongest pattern, best for driveways. 10% wastage
• Stretcher bond — simple, quick to lay. 5% wastage
• Basket weave — decorative, but weaker interlock than herringbone

Common Room Sizes: Pre-Calculated Quantities

| Patio Size | Area | Slabs (600 × 600, 5% waste) | Sub-Base 150mm (tonnes) | Mortar Sand (kg) | |-----------|------|-----|-----------|------------| | 3m × 3m | 9m² | ~28 | 1.7t | 360 | | 4m × 3m | 12m² | ~37 | 2.3t | 480 | | 5m × 4m | 20m² | ~59 | 3.8t | 800 | | 6m × 4m | 24m² | ~70 | 4.5t | 960 | | 8m × 5m | 40m² | ~117 | 7.5t | 1,600 |

Cost Comparison: Typical Installed Prices

| Material | Supply Only (per m²) | Installed (per m²) | |---------|---------------------|-------------------| | Concrete flags | £15–£30 | £60–£90 | | Block paving | £25–£50 | £80–£120 | | Porcelain | £40–£80 | £100–£160 | | Natural sandstone | £30–£60 | £90–£140 |

These include sub-base, bedding, and jointing but exclude skip hire, excavation of existing surfaces, or drainage works.

Final Tips

• Check for underground services before you dig — gas, water, electric, and drainage all live in the first 600mm. Use the LSBUD (Linesearch Before U Dig) service to check for buried utilities
• Don't butt paving tight to the house wall — leave a 10mm expansion gap filled with mastic or use a flexible strip
• Compact the sub-base in layers — 75mm at a time with a plate compactor, not all at once
• Mix slabs from multiple packs to avoid colour banding — especially with natural stone
• Seal porcelain edges after cutting to prevent moisture ingress and frost damage