Type 1 calculator for sub-bases

Working out a Type 1 sub-base

Type 1 is bought by the tonne but specified by its finished, compacted depth. So the calculator takes the area, multiplies it by the compacted depth for the job, adds an allowance for levels and spillage, then turns that volume into weight using 2.2 tonnes for each compacted m³. For bulk bags it divides that weight by 800kg or 850kg and rounds up, because nobody sells three quarters of a bag.

Pick path, patio or driveway and it fills in the depth: 75mm, 100mm or 150mm. You can type any other depth over the top. The allowance starts at 10% and the density at 2.2, and both can be changed to your supplier's figures.

Worked example: a back-garden patio 5m by 4m, on 100mm of compacted Type 1, with the default 10% allowance and 800kg bags.

• Area: 5 × 4 = 20m².
• Compacted volume: 20 × 0.100 = 2.00m³. Plus 10% = 2.20m³.
• Weight: 2.20 × 2.2 = 4.84 tonnes.
• 800kg bags: 4,840 ÷ 800 = 6.05, so 7 bags. In 850kg bags it is 5.69, so 6 bags.
• Loose: 5 tonnes tipped. Bullimores, a Northamptonshire supplier, gives its minimum loose order as 5 tonnes in one place on its page and 4 tonnes in another, so a patio this size is about where a loose load is worth pricing.

How deep should Type 1 be?

Allow 75mm compacted for a footpath, 100mm for a patio and 150mm for a driveway that takes cars. Those are the depths Bullimores gives on its Type 1 page, and the ones this calculator fills in. It adds that heavier loads may need a deeper sub-base, assessed by a structural engineer.

Paving makers say the same for patios and drives. Marshalls' porcelain paving installation guide says 100mm should be sufficient for most domestic applications, and its driveway guide gives 150mm. Both add that the design must follow the ground conditions and the loads expected.

Not every supplier is that generous. Thomas Bros suggests 50mm for patios and footpaths, 100mm for driveways and 150mm or more for roads and heavier use. If you price to the thinner figures and the ground is soft, the slabs or blocks are the first thing to show it.

A garden path shows how quickly the tonnes add up even on a small job: 12m long, 1.2m wide, at 75mm.

• Area: 12 × 1.2 = 14.4m².
• Volume: 14.4 × 0.075 = 1.08m³, plus 10% = 1.188m³.
• Weight: 1.188 × 2.2 = 2.61 tonnes, which is 4 bags of 800kg or 3 tonnes loose.

Why calculators differ, and compaction

Because they use different weights per cubic metre, from 1.55 to 2.2 tonnes. That is the biggest single difference between the Type 1 calculators that rank for this search, and most of them do not tell you which figure they use.

We read the figure out of each calculator's own page. Breedon's aggregates calculator multiplies MOT Type 1 by 2.2. Bullimores' calculator uses 2.1. Holcim's aggregate calculator uses 1.7 for limestone Type 1, 1.6 for hardstone and 1.55 for its recycled Type 1, and says to allow a 5% margin on top.

The gap is mostly loose against compacted. A bag of Type 1 from M&P Moran is sold as roughly half a cubic metre weighing 800 to 850kg. That puts loose Type 1 at 1.6 to 1.7 tonnes per m³ as it comes off the lorry. Once a plate compactor has knocked the air out, the same stone fills less space, so a cubic metre of finished sub-base weighs more. Measured to the finished depth, the higher figure is the one that matches what you have to buy.

Run the 5m by 4m patio again at Holcim's 1.7 and it comes to 3.74 tonnes instead of 4.84. That is 1.1 tonnes short, more than a full bag, and you find out with the plate compactor running and the stone gone.

How much to add for compaction depends on the density you use. At a compacted figure like 2.2, the shrinkage on compaction is already in the tonnes, so the extra only needs to cover the levels coming out high, spillage and the edges. At a loose figure like 1.6 or 1.7, you would need around a third more on top to end up at the same finished depth.

Supplier advice sits between 5% and 10%. Thomas Bros recommends ordering 5 to 10% extra to account for compaction and material variation. Holcim says its tool already allows a minimum and suggests a 5% margin. The calculator defaults to 10%, the top of that range, because the base is the one layer you cannot top up after the paving is down. Got a weight per m³ from the quarry or merchant? Type it in and keep the allowance small.

Type 1, Type 2, Type 3 and recycled

Type 1 is a crushed stone graded from about 31.5mm or 40mm down to dust, made to the Highways specification, clause 803. Holcim's Type 1 data sheet lists it as a 0/31.5mm mixture with no more than 9% fines passing the 0.063mm sieve. The fine material locks the bigger stones together when it is compacted, which is why it carries a car.

The Specification for Highway Works, Series 800 sets out the difference. The copy we could open is the May 2004 edition, since superseded, and it says this. Type 1 is made from crushed rock, slag, concrete, recycled aggregate or burnt shale, with no more than 10% natural sand. Type 2 may also be made from natural sands and gravels, so it has more rounded stone in it. Type 3 is open graded, with the fines taken out so water drains through.

Recycled Type 1, usually crushed concrete and masonry, is fine for most garden and driveway work. Marshalls says recycled material can be considered if it is well graded and compacts to a close-textured finish, but anything with organic matter in it should not be used. Holcim's calculator rates its recycled Type 1 about 9% lighter than its limestone Type 1, so you may get slightly more volume per tonne.

Laying and compacting the sub-base

Spread it in thin layers, and run the plate compactor over every layer until it stops moving before you add more. Marshalls sets the limit at 75mm a layer, or twice the largest stone size. So a 150mm drive base is at least two passes with a plate compactor, and a 100mm patio base is two passes of 50mm.

Work out the dig before you order. The excavation is the sub-base plus the bedding layer plus the paving. Where the paving meets the house, Marshalls' guides keep the finished surface a minimum of 150mm under the damp proof course, to keep out rising damp. On a patio, Marshalls also builds the falls into the sub-base, with a minimum of 1 in 80 lengthways and 1 in 40 crossways.

Where you are unsure of the ground under it, Marshalls recommends a geotextile membrane before the stone goes down. It keeps the stone and the soil underneath apart.

What this calculator leaves out

It is a quantity for ordering, not a pavement design. It does not test the ground or allow for soft spots, made ground, tree roots or a high water table.

Laying a patio on top? The paving calculator counts the slabs, the mortar bed and the jointing. For a gravel finish on the sub-base, the gravel calculator works out the top layer.

• No digging out. The spoil you remove and the skip or grab hire are not included.
• No bedding layer. Sharp sand under block paving and mortar under slabs are separate.
• No edgings, membrane or drainage. Price them on their own lines.
• Weights are estimates. Quarry, rock type and moisture all change them. Your supplier's figure beats ours.

Common questions