Unfortunately, that question is like asking how long a piece of string is!
Firstly, and we can’t stress this enough to anyone considering digging under your property or having a basement conversion is to make sure you use experts every step of the way!!
Secondly, always ensure you employ a Waterproofing Design Consultant to undertake the waterproofing design of your project.
Gone are the days of architects undertaking waterproofing design of basements and below ground structures. British Standard 8102:2009 Code of Practice for Protection of Below Ground Structures Against Water from the Ground (BS8102:2009) recommends that every design team/waterproofing project incorporates a Waterproofing Design Specialist.
Structural Warranty Providers such as NHBC stipulate that it is a mandatory requirement that a CSSW qualified waterproofing designer is involved in the waterproofing design process. Homeowners with mortgages need to ensure they meet the criteria required by their mortgage company.
There are numerous factors which need consideration when designing a waterproofing system for a below ground structure. Below are some of the key considerations:
- Water table classification – high / variable or low
- Use of below ground structure / space
- is the project new build or retrofit?
- are there contaminants / aggressive chemicals / Ground gases present
- compatibility of waterproofing system to the structure
- construction and movement joints
- existing structures and services – suitability of waterproofing systems
For new projects it is recommended to obtain a full and thorough site investigation of the area, so all risks can be considered prior to selection of the waterproofing system:
- Carrying out soil investigations
- Investigate level of the water table before confirming the type and scope of works
- confirming ground conditions and soil types
In urban areas, the soil may contain traces of contaminates, ground gases and other hazardous materials such as asbestos. The type of construction used will also have a bearing on the choice of basement waterproofing solution.
Going below ground also requires that any service pipes or cables need to be identified. This is where land searches are necessity. These will include searches for assets owned by utility providers, service providers such as BT Openreach, rail networks and more.
Expect your basement designers to advise on what tests they would expect to see for the scheme to assist them with their design work.
How Much Does it Cost to Build a Basement?
Generally, a contractor will price basement work according to four stages or phases:
- construction of the shell and core
- waterproofing (determined by the size of the basement space as well as the number of sump pumps needed)
- fit out
- maintenance (if a sump pump (foul, ground or grey waste are used)).
There are also annual maintenance costs that relate to sump pumps and the cleaning of the perimeter drainage channels. It is important to consider this at design phase to ensure that the design complies to BS 8102:2009 and that it is accessible and maintainable.
Guidance on costs for basements for dwellings can be found through The Basement Information Centre (TBIC) www.tbic.org.uk
