Approved Document C: Site Preparation and Resistance to Contaminants and Moisture – Building Regulations 2010 (England)
Approved Document C provides statutory guidance on meeting the requirements of the Building Regulations 2010 for building work carried out in England. This 2004 edition, which incorporates amendments from 2010 and 2013, details the technical standards for Requirement C1 (Preparation of site and resistance to contaminants) and Requirement C2 (Resistance to moisture). The document is issued by HM Government and functions as a practical guide for architects, structural engineers, and contractors to ensure the health and safety of persons in and about buildings by safeguarding against adverse ground conditions and environmental moisture.
The scope of the document encompasses land preparation, sub-soil drainage, and the construction of floors, walls, and roofs to prevent the ingress of water and hazardous substances. It provides a regulatory framework for identifying and mitigating hazards from Solid and Liquid Contaminants, such as heavy metals and hydrocarbons, and Methane and Other Gases including Radon. The guidance applies to new constructions, extensions, and material changes of use, including specific provisions for Historic Buildings where a balance must be struck between conservation and modern performance standards. Defined terms within the text include Contaminant, Groundwater, Interstitial condensation, and Vapour control layer.
Structured into seven primary sections, the document moves from general site performance to specific technical solutions for building elements. It references numerous British Standards and BRE Reports to define acceptable methods for site investigation, risk assessment, and moisture-resistant construction.
Clearance or Treatment of Unsuitable Material
Section 1 outlines the necessity of a thorough Site Investigation to determine the preparation required. This process is divided into four stages: planning, desk study, site reconnaissance, and the main investigation. The goal is to identify physical hazards, underlying geology, and hydro-geological properties that could affect the building's stability.
- Vegetable matter: Removal of turf and roots to a depth that prevents future growth is required for ground covered by the building.
- Shrinkable clays: Potential damage from ground heave must be assessed using Diagram 1 and Table 1 (Volume change potential for common clays).
- Infrastructure: Existing foundations, buried tanks, and services on previously developed sites must be identified and managed to avoid endangering occupants.
- Fill material: Made ground or fill must be assessed for compressibility and the potential for collapse upon wetting.
Resistance to Contaminants and Ground Gases
Requirement C1(2) mandates precautions against danger to health and safety caused by contaminants in the ground. The document adopts a tiered approach to Risk Assessment, moving from preliminary assessment to Detailed Quantitative Risk Assessment (DQRA) based on the Source-Pathway-Receptor relationship.
- Solid and Liquid Contaminants: Remedial measures include treatment (biological, chemical, or physical), containment via cover systems, or removal to licensed landfills.
- Methane and Carbon Dioxide: These hazardous soil gases require barrier systems and ventilation layers, particularly on or near landfill sites or in coal mining areas.
- Radon: Protective measures are required in areas identified by Public Health England and the British Geological Survey where more than 3% of homes are at risk.
- Sulphate Attack: Concrete floor slabs must be protected in areas containing sulphate-bearing strata, as detailed in BRE Special Digest SD1.
Subsoil Drainage and Flood Risk
Section 3 addresses the provision of Subsoil Drainage to avoid the passage of ground moisture to the building's interior and to prevent the transport of water-borne contaminants to foundations. While flood resistance is not a mandatory requirement under Schedule 1, the document provides guidance on constructing buildings to mitigate flood effects.
- Water Table: Drainage is required where the water table can rise to within 0.25m of the lowest floor.
- Active Drains: Subsoil drains cut during excavation must be re-laid with sealed joints or re-routed around the building.
- Backflow: In flood-prone areas, non-return valves and anti-flooding devices are recommended to prevent sewer surcharging.
Resistance to Moisture in Floors, Walls, and Roofs
Requirement C2 specifies that building elements must protect users from harmful effects caused by ground moisture, precipitation, and condensation. Technical solutions are provided for both ground-supported and suspended constructions.
- Ground Supported Floors: A Damp-proof membrane (DPM) of at least 1200 gauge (300µm) polyethylene is required, either above or below a minimum 100mm concrete slab.
- Suspended Timber Floors: A ventilated air space of at least 150mm is required between the ground covering and the floor joists to prevent decay.
- External Walls: A Damp-proof course (DPC) must be provided at least 150mm above the adjoining ground level.
- Cavity Walls: Cavities must be at least 50mm wide; where insulation is used, a residual cavity must be maintained unless the material is certified for full-fill.
- Condensation: Ground floors and external walls must not exceed a thermal transmittance (U-value) of 0.7W/m²K to resist surface condensation and mould growth.
Frequently Asked Questions
What is the minimum height for a damp-proof course (DPC)?
A DPC in an external wall should be placed at least 150mm above the level of the adjoining ground to prevent moisture ingress from the soil.
What thickness is required for a polyethylene damp-proof membrane (DPM)?
A DPM formed with a sheet of polyethylene should be at least 300 micrometers thick, which is equivalent to a 1200 gauge sheet.
How is radon protection determined for a new building?
Guidance on susceptibility and protective measures is obtained from BRE Report BR 211, which uses maps based on indicative atlases from Public Health England and the British Geological Survey.
What is the requirement for sub-floor ventilation in suspended timber floors?
The ventilated air space must measure at least 150mm from the ground covering to the underside of the suspended floor, with openings provided on two opposing external walls.
What defines a 'contaminant' under the Building Regulations 2010?
A contaminant is any substance which is or may become harmful to persons or buildings, including corrosive, explosive, flammable, radioactive, or toxic substances.
How should a floor over an open parking space be constructed?
Such floors must be designed and constructed to resist interstitial condensation in accordance with BS 5250:2002 and have a U-value that does not exceed 0.7W/m²K at any point.
What precautions are needed when building on shrinkable clay soils?
The potential damage from ground heave to services and floor slabs must be assessed, often requiring the removal of soil and vegetation as per NHBC Standards and BRE Digests.
When must subsoil drainage be provided?
It must be provided if it is needed to avoid the passage of ground moisture to the interior of the building or to prevent foundations from being damaged by water-borne contaminants.
