Prior to commencing with the construction of a dwelling/dwelling houses it is imperative to ensure that the ground on which you are to build is suitable. Organic material and excessive moisture/water should be removed in so far as is possible and should there be any remnants they should be adequately addressed at the design stage. The following guidance is suitable for application with respect to non-complex dwellings using ordinary design and construction methods.
Site Drainage
If the site in question is subject to flooding, you should reconsider building on this site. However, should you wish to proceed you should employ the services of a suitable qualified environmental engineer to determine how best to proceed.
The following guidance is based on the assumption that the site in question is not subject to flooding. Drainage of the site should be undertaken where:
The water table can rise to within 250mm of the lowest floor level of the dwelling, or;
Where the ground water on the site would likely adversely affect the stability of the structure.
Where the above is a concern the site should be drained by gravity or other suitable means of safeguarding the building should be implemented.
Some good indicators of whether the water table on your site is high are:
The site in question is surrounded by areas of higher ground.
The ground remains damp even in dry weather.
Vegetation associated with damp ground are present on the site.
Dwellings should be constructed in such a way that surface water on the site should not be able to cause damage to the building and should not be able to enter the dwelling(s).
Subsoil Drainage
During excavation if an active subsoil drain is cut the following steps should be adhered to:
Where the subsoil drain is to pass through the building it should be re-laid in pipes having sealed joints with access points to be provided outside the building.
It should be diverted to an alternative outfall.
It should be diverted so as to run around the building as opposed to under it.
Part C of the Building Regulations relates to Site Preparation and the Resistance to Moisture and Technical Guidance Document C provides guidance on subsoil drains, see extracts below. Diagram 2 of Technical Guidance Document C demonstrates how a new subsoil drain should be provided, if required.
Diagram HC1 - Subsoil drain cut during excavation- Extract from TGD C
Diagram HC2 - Protection against ground moisture - Extract from TGD C
Organic Material
Any vegetation or turf on the site should be removed from the ground which the dwelling(s) are to be constructed. The removal from site should be to a sufficient depth that will remove all roots etc. and prevent further growth.
A site containing material that is easily compressible or tree roots poses a threat to the stability of the building and this should be considered at the design stage. Building services should be designed to either resist or accommodated and should be sufficiently strong or flexible to reflect the specific design choice. Tree roots should not be able to penetrate joints.
Further Guidance on Ground Water
Further guidance for the protection of buildings from ground water damage/ingress is contained within the following documents:
CP 102: 1973 – Code of practice for protection of buildings against water from the ground.
BS 8102: 2009 – Code of practice for protection of buildings against water from the ground – this document replaces elements of CP 102: 1973.
BS 8215: 1991 – Code of practice for the design and installation of damp proof courses in masonry construction.
I.S. EN 752: 2017 Drain and sewer systems outside buildings - sewer system management.
