UK Construction Standards Reference

MOT Type 1 (SHW Clause 803)

What it covers

MOT Type 1 is a well-graded crushed stone, typically limestone or granite, from 40mm down to dust. The fines (dust) fill the gaps between the larger stones, so it locks together tightly when compacted, giving a dense, strong, load-bearing layer with limited drainage. It's the default sub-base choice for driveways, paths and patios, and what most UK merchants mean when they just say "MOT" or "hardcore" informally.

The Specification for Highway Works guidance behind Clause 803 also specifies how it should be laid: compacted in layers of no more than 100mm at a time, not dumped in and compacted all at once, or the bottom of a thicker layer won't compact properly no matter how many passes you make over the top.

What it does not cover

Clause 803 is a grading and compaction specification - it defines what the material is and how to lay it, not how to convert a job's area and depth into tonnes to order. That conversion needs a bulk density figure, which isn't part of the grading spec itself. Our calculators use 1,920 kg/m³ at compacted density for this - sourced independently, not read off the clause - and are explicit that this is the compacted figure, not the loose (as-delivered) density, since the two differ by roughly 7% and conflating them is a common source of under-ordering.

"Crusher run" is a related but distinct term worth not confusing with this: it's crushed stone graded similarly to Type 1, but not necessarily certified to the Clause 803 specification. Worth checking with your supplier whether theirs meets Type 1 spec if that matters for your job.

Which of our calculators rely on it

• Sub-Base Calculator - MOT Type 1 is the default material option, with compacted volume, loose volume to order and tonnage all built around the 1,920 kg/m³ figure.
• Paving Slab Calculator and Patio Calculator - both use the same 1,920 kg/m³ figure for their own sub-base tonnage line, matching the sub-base calculator exactly rather than using an independently-guessed number.
• Shed Base Calculator - MOT Type 1 sub-base sits beneath both the concrete slab and paving-slab base types.
• Gravel Calculator - offers MOT Type 1 as one of its selectable material densities, for the (less common, but real) case of ordering it loose rather than as a sub-base layer.

MOT Type 3 (SHW Clause 805)

What it covers

MOT Type 3 uses the same stone sizes as Type 1, but with the fines (dust) screened out. Removing the fines means the stones can't lock together as densely, which is the point: it makes the material free-draining rather than dense, so water passes through it instead of sitting on top or running off. It's used under permeable block paving and in SuDS (sustainable drainage) schemes, wherever the design intent is for water to soak away through the sub-base rather than be shed off the surface.

What it does not cover

Like Clause 803, Clause 805 is a grading specification - it defines the stone sizing and fines content that makes the material free-draining, not a materials-buying guide. It doesn't compact as densely as Type 1 by design, so a tonnage figure for Type 1 should never be reused for Type 3 on the same job without adjusting the density assumption. Our sub-base calculator treats it as a distinct material option with its own compacted density, not a variant of the Type 1 figure.

Which of our calculators rely on it

• Sub-Base Calculator - MOT Type 3 is offered as a distinct material option alongside Type 1, crusher run, recycled aggregate and hardcore, with its own compacted density rather than an adjusted Type 1 figure.

BS 7533-102:2025

What it covers

BS 7533-102:2025 is the current UK code of practice for installing modular paving - concrete blocks, clay pavers, flags, natural stone slabs, setts and porcelain slabs - for pedestrian and vehicular use up to traffic category 3, consolidated into one part, effective from 31 March 2025. It supersedes BS 7533-3:2005+A1:2009 (which covered laying precast concrete blocks and clay pavers for flexible pavements) and BS 7533-4:2006 (which covered laying precast concrete flags and natural stone slabs) outright, along with BS 7533-9:2010 and BS 7533-11:2003, and partially supersedes BS 7533-6:1999 and BS 7533-7:2010.

For bedding method, it describes a full mortar bed - mortar spread evenly under the whole slab, not just at the corners and centre - as the specification-compliant method for laying flags. A full bed avoids the voids that "dabs" (blobs of mortar at a few points) leave underneath a slab, which is what lets a slab rock and eventually crack under load, especially with modern thinner, more consistent slabs.

What it does not cover

The standard describes full bedding as the specification-compliant method; it does not document mortar dabs as an approved alternative. Dabs are a widely-used shortcut on smaller DIY jobs precisely because they're faster, not because the standard endorses them - a distinction worth being clear about rather than presenting a common shortcut as if it were an equally valid documented method.

It also does not specify the paving units themselves - concrete paving blocks are covered separately by BS EN 1338, and concrete paving flags by BS EN 1339. Both are still-current product specifications, not superseded by this installation code of practice.

Which of our calculators rely on it

• Paving Slab Calculator - bedding volume and mortar quantities are calculated for a full bed, matching the standard's specification-compliant method, not a dabs allowance.
• Patio Calculator - inherits the same full-bed bedding assumption for its own bedding mortar line.

BS 3882:2015

What it covers

BS 3882:2015 is the UK specification for topsoil - the classification and quality requirements for material intended to support plant growth.

What it does not cover

It explicitly excludes subsoil from its scope altogether. Subsoil is the mineral layer beneath topsoil: low in organic matter and nutrients, and not classified as a growing medium at all under this specification. That's not a minor technicality - it's the reason subsoil is fine as bulk fill or for levelling beneath a topsoil layer, but should never be the final layer wherever anything actually needs to grow.

Which of our calculators rely on it

• Topsoil Calculator - the subsoil-vs-topsoil distinction is used directly in the calculator's own guidance on when subsoil is (and isn't) an appropriate substitute.