ProjectCalcThe requirement
Approved Document C (2013 edition) covers site preparation and resistance to contaminants and moisture. For walls the requirement is that they resist the passage of moisture from the ground to the inside, are not damaged by ground moisture, and — for external walls — resist the penetration of precipitation to the structure and to the inside, are not adversely affected by interstitial condensation, and do not promote surface condensation or mould growth under reasonable occupancy conditions.
The document states plainly that "any internal or external wall will meet the requirement if a damp proof course is provided". The detail is in how.
The numbers that matter for a wall
| Element | The rule |
|---|---|
| Damp-proof course | Damp-proof course of bituminous material, polyethylene, engineering bricks or slates in cement mortar or any other material that will prevent the passage of moisture — and it must be continuous with any damp-proof membrane in the floors — 5.5(a). |
| Height above ground (external wall) | The damp-proof course should be at least 150 mm above the level of the adjoining ground, unless the design is such that part of the building protects the wall — 5.5(b). |
| Cavity (external cavity wall) | The cavity should be taken down at least 225 mm below the level of the lowest damp-proof course, or a damp-proof tray provided, with weep holes every 900 mm to assist the transfer of moisture through the external leaf — 5.5(c). |
| Damp-proof tray stop ends | Where the tray does not extend the full length of the exposed wall, stop ends and at least two weep holes should be provided — 5.5(c). |
Those three figures — 150 mm, 225 mm, 900 mm — are the ones worth knowing before a paving level, a deck height or a planting bed is set out, because the external ground level is what the 150 mm is measured from. Raising the ground against a wall after the fact is exactly the mistake the rule exists to prevent.
Ground floors and suspended floors
For a suspended timber floor next to the ground, paragraph 4.14 sets out a construction that will meet the requirement:
- a ground covering, either unreinforced concrete at least 100 mm thick to mix ST 1 in BS 8500, or concrete or inert fine aggregate at least 50 mm thick laid on at least 300 µm (1200 gauge) polyethylene sheet;
- a ventilated air space measuring at least 75 mm from the ground covering to the underside of any wall-plates, and at least 150 mm to the underside of the suspended timber floor (or insulation, if provided);
- ventilation openings of not less than 1,500 mm² per metre run of external wall or 500 mm² per square metre of floor area, whichever gives the greater, placed so that air has a free path between opposing external walls and to all parts.
A suspended concrete floor is dealt with at 4.19: it needs a damp-proof membrane where the ground below has been excavated below the lowest level of the surrounding ground and will not be effectively drained, plus a ventilated air space measuring at least 150 mm clear from the ground to the underside of the floor or insulation.
Floors and condensation
Paragraph 4.22 deals with resistance to surface condensation and mould growth: a ground floor should be designed and constructed so that the thermal transmittance does not exceed 0.7 W/m²K at any point, and the junctions between elements should follow Accredited Construction Details or equivalent guidance.
Interstitial condensation in a floor exposed from below — over an open parking space or passageway — is covered at 4.21, which sends the design to BS 5250, BS EN ISO 13788 and BR 262.
Why this matters to a quantities job
None of these numbers change how much paint, board or sand you buy — they change whether the build you are measuring will stay dry. A bathroom wall that keeps coming back damp, a floor that feels cold and smells musty, and a deck laid against a raised bed are all Part C problems rather than decorating problems, and no amount of preparation fixes them.
The editions this document cites
These are British and European standards this approved document names. The edition column is the one the document itself cites; the last column is what BSI catalogues today (22 September 2026). Where the two differ, the document is citing an edition that has since been replaced or withdrawn — that is normal for an approved document, and it is why the current edition is listed beside it rather than instead of it.
| Standard | Title as published | Edition cited by the approved document | Current edition (BSI) |
|---|---|---|---|
| BS 8500-1 | Concrete. Complementary British Standard to BS EN 206 — Method of specifying and guidance for the specifier | BS 8500-1:2006+A1:2012 — cited in Approved Document A, Section 1 | BS 8500-1:2023 — Current (30 November 2023) |
| BS 8215 | Code of practice for design and installation of damp-proof courses in masonry construction | BS 8215:1991 — cited by Approved Document C | BS 8215:1991 — Current (28 February 1991) |
| BS 8102 | Protection of below ground structures against water ingress. Code of practice | BS 8102:1990 — cited by Approved Document C as "Code of practice for protection of structures against water from the ground" | BS 8102:2022 — Current (31 March 2022). Note the title has changed as well as the edition |
| BS 5250 | Management of moisture in buildings. Code of practice | BS 5250:2002 — cited by Approved Document C as "Code of practice for the control of condensation in buildings" | BS 5250:2021 — Current |
| BS EN ISO 13788 | Hygrothermal performance of building components and building elements — Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods | BS EN ISO 13788:2002 — cited by Approved Document C | BS EN ISO 13788:2012 — Current |
| BS 8104 | Code of practice for assessing exposure of walls to wind-driven rain | BS 8104:1992 — cited by Approved Document C | BS 8104:1992 — Current, Under Review |
| BS CP 102 | Code of practice for protection of buildings against water from the ground | BS CP 102:1973 — cited by Approved Document C | CP 102:1973 — Current (catalogued without the BS prefix) |
Source: BSI catalogue records read at knowledge.bsigroup.com on 22 September 2026. Each title, designation, status and date in the last two columns was read from the BSI catalogue page for that standard.
Last verified 22 September 2026. Every requirement quoted on this page was read from the primary source listed below on that date, not from a summary. If a figure could not be confirmed in the primary source it is not on this page.
Where these figures come from
- Approved Document C: Site preparation and resistance to contaminants and moisture, 2013 edition (paragraphs 4.14, 4.19, 4.21, 4.22 and 5.4-5.5; BS 8500 and BS 8215/BS 8102 references) — GOV.UK publications, "Site preparation and resistance to contaminants and moisture: Approved Document C", checked 22 September 2026.
- BSI catalogue (“BSI Knowledge”) — product record for each standard listed above: title, current edition and status — knowledge.bsigroup.com, checked 22 September 2026.
Guidance only, not advice
This page is a plain-English summary of published regulations and standards, written to help you plan a job. It is not professional advice, it is not a substitute for the actual document, and it is not a building-control decision.
Building regulations are enforced by your local authority (or an approved inspector) and the facts of a particular job can change the answer. Confirm with your local building control before you start work, and where a job is structural, get it designed by a suitably qualified person. If this page and the source document disagree, the source document wins.