It has been claimed that as a result of the pandemic, homes that are more ‘adaptable’ have become increasingly popular. One of the areas that provides the greatest amount of adaptability has to be basements. The options for a basement conversion are endless. If you want to create a home gym, music room or den, a warm, dry basement can be the ideal solution.
Providing a dry environment in a basement can be challenging, but not getting it right first time can be costly and difficult to remedy. Although hidden away, structural waterproofing can be particularly sensitive to failures, largely due to poor workmanship, the inappropriate use of materials or bad design.
Key considerations when undertaking basement conversions
As a result, a range of factors need to be carefully considered when undertaking such works, to ensure a positive outcome.
What consideration needs to be given to a waterproofing designer?
A design team includes an experienced waterproofing specialist, who should be engaged at the earliest possible stage.
What are some of the key considerations in terms of ground conditions and site evaluation?
It’s important that the geology and hydrogeology, the external drainage options, and the groundwater conditions of a site are carefully considered. The type of soil and the water table also need to be evaluated, as well as any ground gas concerns.
Are there different levels of dryness to consider?
There are three grades to consider, 1, 2 and 3. Examples of a Grade 1 structure include:
- a car park, or
- a plant room without electrical equipment – where it is considered tolerable to have some seepage and damp areas.
No water penetration is acceptable for Grade 2 structures, and for damp areas that are tolerable, ventilation might be required.
No water penetration is acceptable for Grade 3 structures, and this performance level also specifies ventilation, dehumidification or air conditioning as necessary, and appropriate to the intended use. If you are looking to use the new basement room as habitable space then you will need to achieve Grade 3.
What are the waterproofing options available?
There are three waterproofing systems: Type A, B and C. However, in reality, for most basement conversions you will be limited to certain Type A and Type C systems.
- Type A, usually referred to as ‘tanked protection,’ is a structure with no integral protection against water penetration. It relies totally on a waterproofing membrane to keep water out. Type A forms of waterproofing may be applied internally or to the outside of the structure or, in some cases, sandwiched between two skins of masonry or concrete.
- Type B are structures built with a water-resistant shell, usually constructed out of reinforced concrete to an appropriate design code, which gives guidance in the grade of concrete to be used and spacing of the reinforcing steel. Special additives may also be used.
- Type C or ‘drained cavity’ systems rely on a drained cavity within the basement structure. There is a permanent reliance on the cavity to collect groundwater that enters through the fabric of the structure. The drainage system directs the water to a drain or sump, where it can be removed from the building by gravity or pumping.
What are the considerations regarding defects and remedial measures?
First and foremost, the construction teams applying, installing and building the underground space must be fully aware of the critical nature of what they are doing, the importance of accuracy and the implications of any defects and errors.
Contingency planning for any localised defects should also be included as part of the overall water-resisting design for the structure. If feasible repair is not possible, then it may be required to look at what can be done in respect of the risk posed by groundwater.
Will I need planning permission?
In most cases, planning permission is unlikely to be needed when converting an existing basement or cellar. However before progressing with a basement conversion, you should check with your local authority whether planning permission is required.
It looks right on paper but how do I ensure it is done right on site? Where can I find reliable experienced contractors?
The Property Care Association can signpost architects to members with the necessary expertise to ensure their project is a success, with expertise in this complex, highly skilled aspect of construction.
The Association oversees the industry recognised qualification, Certificated Surveyor in Structural Waterproofing (CSSW), attained by professionals in the sector.
The PCA also has a register of Waterproofing Design Specialists who have signed-up to our Continuing Professional Development (CPD) scheme and are able to provide standalone design services, who, along with a geotechnical specialist, make up the core components of a waterproofing design team.
Join our ‘Building Regulations for Basement Conversions’ webinar!
To gain a broader understanding of planning for basement conversions and develop your knowledge of when building regulations become applicable, why not join our next webinar on 22nd July? To register for the live broadcast, simply click on the button to find out more:
Get in touch with the Training Team
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