Building Regulations Approved Document C - Site Preparation and Resistance to Moisture and Contaminants

Summary: The Building Regulations Approved Document C covers the preparation of a building site and the treatment and clearance of unsuitable materials and contaminants that may be present on the site or in the soil.These regulations discuss the resistance of walls to weather and ground moisture with examples of damp proofing and ventilation provisions to prevent damage to the building and its materials by water. Additionally, Document C also discusses preparing a construction site that may have contaminants present in the soil or surroundings and how they should be properly removed or blocked to prevent them causing future structural issues.

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Waste disposal such as household waste has remained a problem in the UK and other countries for many years and there are now quite a few disused landfill sites dotted around the UK that are now being reclaimed for use as construction land and this and similar contaminated land situations is where Approved Document C comes in to play.

When building on land that could contain contaminants e.g. old scrap yards, it must be ensured that all harmful substances that could have seeped into the soil are removed or treated to neutralise them before construction can begin. The same goes with landfill sites - over time harmful gases such as Methane and Radon can build up so they need removing.

Approved Document C of the Building Regulations

Approved Document C of the Building Regulations - Download it here

The main aim of Approved Document C is to make sure that any construction site is prepared correctly and that it is resistant to moisture and contaminants. The document itself is broken up into two sections as follows:

  • C1 - Preparing a site and making it resistant to contaminants
  • C2 - Ensuring a site is resistant to moisture

C1 - Preparing a Site and Making it Resistant to Contaminants

The first section in this document is broken up into three parts with each covering a specific safety point concerned with the sub-soil and ground under and around a construction site:

  • C1(1): It must be ensured that the entire construction area where the building will stand is free of any item that could potentially affect its stability. This includes any topsoil, vegetation such as plants and trees and any foundations that may already exist
  • C1(2): If the area is known to feature contaminants, either in the ground or on it then these need to be dealt with to avoid any potential health and safety risk
  • C1(3) including (a) and (b): Suitable drainage should be included in the sub-soil to avoid any ground moisture entering the building interior or to avoid damage to the building and its structure, including the removal of water-borne contaminants to the buildings foundation

There is one more sub-section to part C1, sub-section (4). This helps to define what is meant by the word "contaminant". It states that a contaminant should be considered as any substance that could be harmful to the building and any person in or around it and should include toxic, flammable, radioactive and corrosive items.

In some cases, the owner or developer of a piece of land will be well aware of its previous usage due to initial research that should be carried out prior to purchase and a decent survey of the land and its surroundings that should be carried out during any planning application process, should highlight any potential contamination risks that might exist.

In the event that contamination is likely you should consult with your local building control department as they should be able to advise what steps will need to be followed in order to make the ground safe for the construction to begin.

In terms of the guidelines, these are stated in the Town and Country Planning Acts and also by your local authority, which should use the National Planning Framework for guidance. In some cases Environmental Protection and the Environment Agency may also become involved, but this will depend on the situation.

In light of this, the most likely authorities that will become involved when ground contamination is already or present or discovered (e.g. as part of a survey or soil test) and their roles are as follows:

  • District Council Environmental Health Department: They will need to be informed if contamination is found that has not previously been reported or known about and also if the contamination itself is affecting neighbouring land or it is coming from neighbouring land. They should also be notified if a previously identified contaminant differs from one that is actually found
  • Local Planning Authority: The complete redevelopment of a plot identified as being contaminated is always going to be the preferred method of development as in most cases the costs and levels of work involved in resolving issues to an already existing building etc.... will far outweigh clearing the ground, fixing any issues and starting construction from scratch. Due to this any issues with the land should be detailed in documents supplied with a planning application and if any changes to designs that may affect the resolution of contamination are needed then the planning authority will need to be informed
  • Environment Agency: They have several duties that are mainly focussed around water quality and waste management, particularly on ensuring that water sources are not affected. They may also need to be notified in respect to methods in which contaminants are either removed, contained or treated and if removed, how this is done and where the contaminated waste ends up

When a site requires investigation, this will normally happen over four stages:

  • Initial Planning: The objectives of the investigation are settled on and what information needs to be obtained. This will also include any requirements and the overall scope of the investigation
  • Desktop Investigation: This involves looking through all information available concerning the geological layout of the land, its history and also any environmental information that is available
  • Site Visit or Reconnaissance Survey: The site is visited and inspected so that all existing and potential hazards can be identified. This will also help to plan the overall main investigation
  • Main Report and Investigation: This stage normally involves physical sampling and testing of soil so that construction and design parameters of the building can be planned

Due to the fact that pretty much every site, location, geological makeup etc.... can be potentially different the above steps are subject to change and each investigation will most probably be tailored to suit the site and location it is to be carried out on.

Treatment of Contamination

When it comes to treating and resolving contaminated ground there are numerous methods of doing so such as biological and chemical treatments and also some physical. Each treatment can have a different effect but in general they will either decrease toxicity levels, concentration, mass or its ability to move.

The correct treatment to use is very much down to the type of contaminant, its location, concentration etc.... and due to this will certainly need expert advice.

In general, it will either need containing or removing - containment in the normal sense usually means surrounding an area to prevent its spread or escape but in construction this normally means so kind of covering.

Covering normally involves layering one or several layers of material over an area to reduce contamination levels, produce geotechnical improvements, break a supply etc.... In some cases the construction of the ground floor and its foundations can help go towards this. Due to the many factors that can both affect and remedy issues it is a good idea when considering coverings as a fix to look at Construction Industry Research and Information Association Report (or CIRIA as it is also know).

When removal is the chosen fix, this normally involves the excavation of contaminated material. This can be either concentrated on the affected area or excavated to a depth so that a cover can be applied and the material that has been removed is then replaced with new (note: New fill will also require checking for suitability).

So far we have edged around risks but we have gone into too much detail on specifics. These are covered in Section 2, 2.23 of Approved Document A and are summarised as follows:

  • Aggressive Substances: This covers mainly organic acids, alkalis and solvents and inorganic acids, alkalis and chemicals such as chlorides and sulphates. These items can severely affect the durability of materials such as plastic, metal and concrete
  • Combustible Fill: As you might imagine, this involves materials that can ignite and catch fire. Fill materials such as coal, plastic, domestic waste or ground that has been subjected to petrol and other flammable liquids could potentially cause underground fires that could severely effect a buildings foundation and stability
  • Expansive Slags: There are two main types that fall into this category, slag waste from steel making and blast furnaces. Over time and with the presence of water, they can expand and if present in the ground and near to buildings, can cause damage through movement
  • Floodwater Contamination: Where contaminants are present in the ground in areas susceptible to flooding, floodwater can cause things such as sewage or waste to breach buildings and properties

Methane and Gas Found in the Ground

Gases (including methane) can occur in the ground for several reasons, most commonly through waste found in locations such as landfill sites but it can also occur naturally. It should be noted however, this does not include volatile organic compounds of VOC's

In terms of a landfill, the act of certain micro-organisms breaking down organic materials produces methane gas and also some carbon dioxide. Methane can also be produced by organic rich soils such as peat.

In areas where any of the above situations occur, a site inspection and risk assessment will be required to ascertain the level of gases and to what extent they may affect any building or property.

On a domestic level there are really two areas where these guidelines come in to play:

  • Gas that can enter the property itself through the ground and potentially build to harmful levels within the building posing a significant health risk
  • Exposure that can occur outside the property in the garden and also in any outbuildings. This can include things such as garden sheds and even extensions

If the potential presence of gas is found to be far too much then this will need to be considered during the planning and design stage of the build. Depending on the extent of the gas issue, the control measures needed may be isolated to specific areas or they may need to cover the entire site.

Some of the common solutions include:

  • Removing the root cause of the gas e.g. digging out problematic soil
  • Insertion of coverings or gas proof barriers or membranes
  • Addition of ventilation system to allow gas to be safely vented to the atmosphere


Typically found in locations where traces of Uranium and Radium are found, radon is a naturally occurring radioactive gas that is both odourless and colourless making it almost impossible to detect without a professional Radon risk report.

Radon generally tends to be quite isolated to the West Country area of the UK, especially when it comes to high levels of radon. Typically, it radiates from the ground upwards and into the buildings and structures above requiring prevention work to be carried out to aid in reducing high concentrated levels that could pose a significant health risk.

Areas of the Uk that suffer from Radon issues can be found in the BRE Report BR 211 but may be superseded by the need for Radon Risk Report if it is suspected that there may be high levels present. Radon risk reports can be obtained from:

  • UK Radon -
  • BSG Georeports -
  • Public Health England -

When it comes to protecting those in the work place, information can be sought from the Health and Safety Executive -

Vegetation and Trees

One final item to deal with in this section is that of vegetation and trees. Before construction starts the area where the building is to go needs to be cleared to such a depth that any regrowth is prevented. The potential for tree roots to start growing into damage foundations could seriously affect the stability of a structure and also any service or drainage pipework.

Any existing and matures trees close to construction site will need to be assessed in terms of the damage they could cause now and also in the future after growth

C2 - Ensuring a Site is Resistant to Moisture

As you might have guessed, this section of the document focusses mainly on drainage and things such as damp proof courses etc.... The main point of this section is to ensure that a building has correct drainage and any excess surface or foul water is removed and also that moisture present in the ground cannot seep into floor slabs and up and into walls.

Drainage in Sub-Soil

This section does not apply to areas that are subject to flooding or have a history of flooding. The rules state that where the water table is subject to rising within 0.25m of the ground floor then adequate drainage needs to be put in place that either drains through gravity or through another approved means.

During construction, if an existing drain is present then one of the following routes will need to be taken:

  • Drainage will need to be re-laid with pipes using sealed joints and access areas will need to be put in place to access the drainage
  • Drainage will need to be routed around the building
  • Drainage will need to be re-routed to another drain source

Where there is the potential for groundwater or surface water to potentially affect the structural stability of a building, site drainage should certainly be a consideration.

If the building or property features a basement or cellar area and the foul water drainage also takes rainwater then there could be the potential for sewage waste and excess water to infiltrate this area. For guidance on potential solutions you will need to refer to Approved Document H. Additionally, for information on blockage and backflow prevention please refer to the CIRIA publication.

Construction inevitably involves excavation of some form and this in its nature can alter groundwater flow and the soils ability to deal with water as it once did. Where contamination is also present, drainage may be needed to ensure that these contaminants are not able to access foundations or services.


The guidelines within this section are broken up into five sub-sections in order to cover the most common floor types.

For any floor that sits next to the ground it should:

  • Not allow any moisture within the ground to pass upwards into surface floor above
  • Resist moisture damage
  • Resist the passage of any harmful gases that are found to be present such as Methane or Radon

It is also stated that floors of this type should ensure that their actual structure and thermal performance are not affected by interstitial condensation and also that they should not allow any surface condensation or growth of mould.

Where a suspended timber ground floor is present it should be ensured that:

  • The ground is covered so that any moisture and plant growth is prevented
  • There is a fully ventilated area of free air between the floor and the suspended timbers
  • Any materials that both touch the floor and the timbers are separated by a DPC (damp proof course)

As the guidelines are pretty specific on how floors of this type should be constructed, please refer to Section 4.14 and Diagram 5 onwards of Approved Document C for more information.

Where a suspended concrete ground floor is the preferred choice of floor and it consists of either precast concrete or beam and block, it will need to prevent any moisture from travelling into the floor above and its structural integrity will also need to be retained and resist the effects of any moisture.

Where a ground floor is exposed from below e.g. the floor of a room above a garage or any rooms that reside above a passage way within the property, they will need to comply with Clause 8.5 and Appendix D of the following British Standards:

  • BS 5250:2002
  • BE EN ISO 13788:2002
  • BR 262

One final point is that all flooring will need to resist condensation and any potential mould growth and ensure a thermal U-value of under 0.7W/msquK and also require all joints between all elements conform to Accredited Construction Details or guidance stated in BRE IP17/01


As with the section for flooring, this section is also broken up into several sub-sections with each covering a different, potential issue that can be caused through walls and their contact with the ground and the outside elements:

  • External and internal walls exposed to ground moisture
  • External walls exposed to rain and moisture (includes external solid walls, external cavity walls, external framed walls, cracking in walls, impervious cladding and joints between door and window frames and external walls and door thresholds)
  • Interstitial condensation in any wall
  • Risks from surface condensation or mould on any wall type

To clarify on what constitutes as a wall - this includes columns, parapets, piers and also chimneys if they are attached to the building.

In general, it should be ensured that any wall should:

  • Prevent the passage of water and moisture from the ground into the building
  • Resist the effects of moisture and water and ensure they do not cause it damage
  • Prevent water and moisture arising from precipitation (rain, sea spray etc....) from entering the structure into areas that it could cause damage to
  • Prevent rain and precipitation from entering the building
  • Ensure that it is designed and constructed so that both thermal and structural performance are not affected by interstitial condensation
  • Prevent any mould growth or surface condensation from forming or collecting

One consideration that will need to be taken into account is that in situations where a structure will be used for the storing of goods, as long as anyone entering the building will only be moving goods in and out and looking after the goods themselves and that no adverse effects to health and safety would occur, then parts 5.2(a) and (d) of Section 5 of Approved Document C may not apply.


The final section of this document deals with the roof area of a building and covers situations where a roof is exposed to the outside elements such as precipitation (rain and moisture), where they may be at risk from interstitial condensation and where there is a risk of condensation and mould growth on the internal surface of the roof.

The guidelines state that a roof must achieve the following:

  • Prevent any rain or moisture from entering the structure
  • Prevent damage to itself from moisture and rain and also not transfer moisture to any part of the building that it could damage
  • Ensure that structural and thermal performance is not effected by interstitial condensation

The above information should be used as a guide only. If you are referencing Approved Document C in relation to planning or a construction project please ensure that you are referring to the most up-to-date guidelines that can be found by downloading document C on the link at the top of this page from the Planning Portal website.

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