SHW Clause 803 — MOT Type 1 sub-base

Cite both, and in this order

The Specification for Highway Works, Series 800, Clause 803 (11/21 amendment) requires that the mixture meets BS EN 13285 and Table 8/1, with the grading requirements summarised in Tables 8/6a or 8/6b.

BS EN 13285 alone is the wrong citation. It is the European standard describing how unbound mixtures are specified — categories, declared values, tolerances. It contains no material called "Type 1". The numbers live in the SHW table.

This matters because "conforms to BS EN 13285" is a claim that, on its own, tells you almost nothing. Clause 803 is what makes it Type 1.

That continuous spread from 63 mm down to dust is the whole point. The fines fill the voids between the larger pieces, which is what lets Type 1 interlock into a firm load-spreading platform under a compactor.

It is also why Type 1 is the wrong material for drainage. The fines that make it compact are the fines that stop water passing. A French drain filled with Type 1 is a French drain that does nothing — see open-graded stone for the material that does.

Table 8/6b — "Type 1 F" (0/20) — is restricted to trench reinstatements and widenings under a metre. It is not a general substitute and should not be offered as one.

SHW permits up to 225 mm compacted in a single layer. That figure circulates widely on domestic advice sites, and it is dangerous there — it applies to highway plant with a specified number of passes, not to a plate compactor hired from a tool shop.

A domestic wacker will not compact through more than about 75–100 mm of loose material. A 150 mm driveway base is two lifts. A base compacted only at the top looks perfect and is soft in the middle, and the first frost finds it.

We deliberately do not offer the 225 mm figure on our calculators, and the pages say why. It is a case where quoting the standard accurately but without its context makes the advice worse.

Which of our figures come from it

• The grading envelope above — verbatim from Table 8/6a, used on our MOT Type 1 page.
• The Type 1 F restriction — from Table 8/6b, and why we do not offer it as an option.
• The 225 mm maximum lift — from SHW, cited specifically so we can explain why it does not apply to domestic work.
• Not the density. Clause 803 specifies grading, not bulk density. Our 2,100 kg/m³ compacted and 1,680 kg/m³ loose are merchant planning figures, and both are on the sources page flagged as trade convention rather than specification.