How deep should the proposed basement be?

Technical issues

Depending on the intended use there will be minimum headroom dimensions to consider.

The structural thickness of the reinforced concrete slab will usually be in the region of 200mm to 250mm thick

The underpins or retaining walls will be formed with a “toe” this will form a structural function and will be the component to which the ground slab is attached to or sits upon.

There may well be underground drainage to accommodate beneath the ground bearing slab.

Floor finishes, screed, insulation and damp proof membranes will determine floor build up levels above the structure.

The coordination of these elements in design will determine the depth of “dig” and the final finished floor to ceiling heights.


How thick should the underpinning or new perimeter walls be ?


Traditional underpinning will generally be unreinforced mass concete, the thickness will usually be the same thickness as the wall above that it is carrying.

Reinforcing bar or key block construction will be used to join the individual pins to form a continuous wall.

Should a “special foundation” or wall be necessary then this will be formed of reinforced concrete and will require the particular consent of any adjoining owner.


How do we deal with damp proofing ?


There are two general methods of damp proofing –

  1. External membranes would be used in preference but require that the excavation is carried out as a “whole” with external access to apply membranes
  2. Internal systems would be either cementitious based products applied to a suitable sub-strate. Cavity drain systems which rely on diversion and suitable drainage methods or sometimes chemically applied systems –

Considerations would be hydrostatic pressure, volumes of ground water, gas permeable membranes and selection dependant on the ability to  access on a project by project basis.


It is always good practice when introducing a basement to consider ground water diversion methods.

Any interruption to natural ground water drainage ie., forming a new “concrete box” will have an effect of that water wanting to ingress to its natural route or be diverted to avoid ingress / pressure problems.

A simple form of perimeter drainage would be a French drain forming a new natural route maintain drainage to the affected area.

Unresolved drainage will cause problems elsewhere.


What do we do about drainage ?


Dependant upon the depth of the existing drainage systems will dictate the need for either

  1. Natural gravity alterations and installations to suit the existing mains sewerage systems.
  2. If the sewers are higher than the proposed systems then a pumped system will be designed to collect the rainwater, groundwater or effluent and pumped to the main drains.


A popular option with clean rainwater or groundwater is to “harvest” and reuse this water for gardening or similar purposes.


Please comment with any queries

Until then more next time





Leave a Reply

Your email address will not be published. Required fields are marked *