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Lay on a full 40-50mm bed of mortar (sharp sand and cement about 5:1) over 100mm of compacted MOT Type 1 sub-base, with a fall of about 1:60 away from the house and the finished level at least 150mm below the damp proof course.
What The Patio Calculator Works Out
- Patio area: length x width in square metres. For an L-shape, split it into two rectangles and run the calculator twice.
- Slabs needed: the area plus 5% for cuts and the odd breakage, divided by one slab's area, rounded up. A 4 x 3m patio in 450mm slabs needs 63, in 600mm slabs 35. Larger slabs cover faster and look calmer but weigh more; mixed-size patio packs are sold by the m2 and come with a laying pattern.
- Mortar bed: the area x a 50mm bed depth, as cubic metres and as 25kg bags of sand and cement combined (about 2,000kg per m3 dry). Never lay on dots of mortar: they leave voids that crack slabs and echo underfoot. The example patio's bed is about 0.6 m3, roughly a tonne of material.
- The details that survive the first winter: a fall of 1:60 to 1:80 away from the house, the finished surface at least 150mm below the damp proof course, joints pointed or brush-filled, and porcelain primed on the back with slurry so the mortar grips.
- Custom slab size and waste (advanced): type any slab size in millimetres (900 x 450, 800 x 400 and 1200 x 600 porcelain are common) to override the dropdown, and adjust the waste for complex shapes or diagonal patterns (10% to 15%).
- Sub-base (advanced): the area x your Type 1 depth x 2.1 tonnes per m3 compacted, with the bulk bag equivalent. 100mm of MOT Type 1 is the normal minimum for a patio, 150mm on soft ground.
- Bed as sand and cement (advanced): splits the bed into 25kg bags of cement and tonnes of sharp sand at 5:1, which is cheaper than pre-bagged mortar mix for anything bigger than a few square metres. Adjust the bed depth from 40 to 60mm to suit uneven ground.
- Jointing compound (advanced): estimates the linear metres of joint from the slab size, then the kilograms of brush-in compound needed at your joint width and a 25mm depth, in 15kg tubs. Narrow 5mm joints on porcelain use far less than 10 to 15mm joints on riven stone.
- Slab cost and labour (advanced): the slabs bought x your price per m2 (budget concrete GBP 20 to GBP 35 per m2, porcelain and sandstone GBP 35 to GBP 70), plus the area x a landscaper's rate. Fully installed patios in the UK commonly run GBP 60 to GBP 120 per m2 including the base.
Further reading: GOV.UK: paving front gardens.