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Patio slab calculator: slabs, sub-base, sand and cement
Measure the patio, pick your slab and joint and see how many slabs, tonnes of sub-base, bags of sand and cement and tubs of jointing compound to order, plus the fall and how deep to dig. It follows the Marshalls and Screwfix laying guides: 100mm of compacted MOT Type 1, a full 30 to 40mm bed at 4:1, never five-spot, and a fall of 1:80 away from the house.
What you need
Paving
- Patio area
- 12.0 m²
- Indian sandstone
- 13 m²plus 5% wastage
- Slabs
- 34 × 600 × 600mm2.7 per m² with a 10mm joint, plus wastage
Sub-base and bed
- MOT Type 1 sub-base
- 2.8 tonnes100mm compacted in two 50mm layers, at 2.1 tonnes per m³ plus 10%. That is 4 bulk bags (~800 kg)Marshalls: 100mm in two layers; tradecalculator: 2.1t per compacted m³
- Geotextile membrane
- 14 m²under the sub-base, 10% for lapsMarshalls
- Sharp sand
- 1 bulk bags (~800 kg)0.4m³ for a 40mm full wet bed at 4:1 (a bulk bag is about 0.5m³)Marshalls: full bed, never five-spot
- Cement (for the 4:1 bed)
- 6 × 25kg bagsabout one bag per 2m² at 40mm; add plasticiser
- Priming slurry (SBR bond bridge)
- 2 × 20kg bagsbrushed on the back of every slab, about 9m² a bag. Coverage varies by product: check the bagMarshalls: primer required for porcelain
Jointing
- Jointing compound
- 1 × 15kg tubs14.1kg: 3.3m of joint per m² × 10mm wide × 20mm deep, plus 10%. Joint depth is the slab less 5mm, 20mm minimum, 35mm mostMarshalls Weatherpoint 365: ~7m² a tub at 450 × 450, 10 × 35mm
Groundwork
- Fall across the width
- 38 mm1:80 away from the house over 3.0m, about 12.5mm per metre. Finished level at least 150mm below the DPCMarshalls, Screwfix
- Dig depth
- 160 mm100mm sub-base + 40mm bed + 20mm slab, below the finished level at the low edge
- Spoil to remove
- 2.5 m³1.9m³ in the ground, bulked 30% once dug, about 3.2 tonnes: 1 × 6-yard skip (4.6 m³, ~6 t of soil). See the hardcore and excavation calculator
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
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Or claim your free profile link nowHow the maths works
- Paving is the patio area plus wastage. Slabs are counted from the slab plus its joint: Marshalls say 3 to 5mm for porcelain, and 10mm is usual for sandstone and concrete, 15mm for riven stone. If you enter the m² per pack the paving is rounded up to whole packs.
- The sub-base is MOT Type 1 at the depth you set, 100mm compacted in two 50mm layers for a patio (Marshalls, Screwfix), at 2.1 tonnes per compacted cubic metre plus 10%. A geotextile membrane goes under it.
- The bed is a full wet bed of sharp sand and cement, 30 to 40mm (Marshalls) or up to 50mm (Screwfix), never five-spot or dot and dab. Sharp sand is the bed volume less the cement share, at 1.6 tonnes per m³ in 0.5m³ bulk bags. Cement is the mix share at 1.4 tonnes per m³ in 25kg bags: 4:1 is the Marshalls and Screwfix mix, 6:1 the usual contractor mix, 10:1 a DIY mix for patio flags (Pavingexpert).
- Priming slurry or an SBR bond bridge goes on the back of every slab, which Marshalls require for porcelain and is good practice for sandstone, at roughly 9m² a 20kg bag. Coverage varies by product.
- Jointing compound comes from the joint geometry: metres of joint per m² (1 ÷ slab length + 1 ÷ slab width) × joint width × joint depth (slab thickness less 5mm, between 20 and 35mm) × 1.6 tonnes per m³, plus 10%, in 15kg tubs. That reproduces Marshalls' Weatherpoint 365 figure of about 7m² a tub at 450 × 450 with 10mm joints.
- The fall is 1:80 away from the house (Marshalls, Screwfix), about 12.5mm per metre, with the finished level at least 150mm below the damp-proof course. The dig is the sub-base plus bed plus slab thickness, and spoil bulks by 30% once dug.
Worked example
A 4m by 3m sandstone patio is 12m². With 5% wastage that is 13m² of paving, or 34 slabs at 600 × 600mm with 10mm joints, 2.8 tonnes of MOT Type 1 for a 100mm sub-base, 1 bulk bag of sharp sand and 6 bags of cement for a 40mm bed at 4:1, 2 bags of priming slurry and 1 tub of jointing compound. The fall across 3m is 38mm and the dig is 160mm deep.
Questions people ask
How many slabs do I need per square metre?
Divide 1 by the slab-plus-joint area. A 600 × 600mm slab with a 10mm joint covers 0.372m², so about 2.7 per m². A 450 × 450mm slab is about 4.7 per m². Add 5 to 10% for cuts, then round up to whole packs.
How deep should the sub-base be for a patio?
100mm of compacted MOT Type 1 for a patio that only takes foot traffic, laid and whacked in two 50mm layers (Marshalls). Go to 150mm on soft or clay ground. A driveway needs 150mm or more.
Should I lay slabs on a full mortar bed or spots?
A full wet bed of sharp sand and cement, 30 to 40mm thick. Marshalls say never five-spot: dabs at the corners leave voids that fill with water, and the slabs rock, stain and crack. Porcelain also needs a priming slurry on the back or it will not bond.
What fall does a patio need?
1:80 away from the house, which is 12.5mm for every metre, so a 3m wide patio drops about 38mm (Marshalls, Screwfix). The finished level should be at least 150mm below the damp-proof course. Set the fall in the sub-base and bed, not by varying the joints.
How much jointing compound do I need?
It depends on the joint length, width and depth, not just the area. Marshalls Weatherpoint 365 does about 7m² a tub at 450 × 450 with 10mm joints 35mm deep; 600 × 600 at the same joint is about 8m² a tub; porcelain with 4mm joints 15mm deep goes to 50m² or more. Joints must be at least 20mm deep.
Does a patio need planning permission?
Not usually at the back of a house. At the front, any hard surface over 5m² needs to drain to a lawn or border, or be permeable, or it needs permission. Check with your council if unsure.
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