Basements are a convenient way to add space when extending above ground isn’t possible. There are two critical areas which we address in this article: resistance to moisture and water resistance to make sure your project is a success.
All elements of basement construction are specialist work and therefore are not really activities that should be undertaken directly by a self-builder. The information outlined here is given to assist you in understanding the procedures. The general principle is to assess the risk of water reaching the structure and the nature of the structural options with respect to water resistance, and then to select a waterproofing system to provide the internal environment required.
Waterproofing of basements is covered by BS 8102 ‘Code of Practice for protection of below-ground structures against water from the ground’, which is referred to in the Technical Documents of the ROI Regulations but not specifically cited in the NI Regulations although it is likely to be commonly adopted, and it is intended for designers and construction companies.
Before starting any design or construction work, a site investigation, including a desk study, should be made to establish the ground conditions (including the type of subsoil), the level of the water table (including the provision for natural drainage), the potential for future variations in the water table level, and the location of any existing drains or other services. Assessment should also be made for the presence of contaminants and whether there is a risk from radon.
There are three environmental grades (1-3), which essentially define how dry a space must be for a given usage. Habitable space is defined as grade 3 where no water penetration is acceptable, and heating/ventilation is necessary to prevent condensation. Grade 2 also has no water penetration but omits heating, and condensation related dampness is therefore tolerable. Grade 2 is the minimum required for garages, and as no water penetration is acceptable this allows future flexibility in converting to habitable space (Grade 3).
Figure 1 outlines the principle factors and stages that need to be addressed in order to produce a robust waterproofing design for a basement. The directional arrows indicate that some matters are interrelated, and that there may be a need to repeat the process in order to also address suitability, buildability and reparability. It is worth emphasising that, at design stage, the team should anticipate defects in the system, some groundwater pressure, and how leakage could be remedied if unacceptable, something that is often given very little consideration.
The main points to developing a robust design are:
The position of the water table, the drainage characteristics of the soil and other site-specific properties and the implications of constructing a basement on the likely groundwater characteristics. The type of soil can greatly influence the quantity of water reaching the basement wall. Free-draining soils not subject to variability in water tables generally present fewer problems than clays (which tend to be impermeable), but can still become saturated in prolonged and heavy rainfall. It is important, therefore, to determine the soil type and, in particular, its drainage characteristics.