With space at an ever increasing premium it is no surprise that basements are becoming more popular with homeowners and developers alike. According to the Halifax*, basement planning applications between 2012 and 2017 soared by 183%. Basements add useful space to any home where land to extend is limited, and their uses are practically endless.
But with one of the wettest climates in Europe, how do we ensure our basements stay dry?
What is a basement?
It’s not such a silly question as it sounds! Sloping sites can make understanding what is considered a basement more difficult. In fact, only 2% of homes use completely submerged basements, whereas 7% make the most of sloping sites with partially submerged rooms. In these situations, we tend to refer to them as semi-basements.
We would consider a room to be a basement where any floor becomes below the ground level on one or more levels. So, if your lower level is built into a hillside, meaning part of it is below the ground level of the above floor, this would be a basement.
Ready to create a basement? Hire a waterproofing specialist
The UK climate means that water ingress is a real risk for any underground structure. This is a specialist area and that’s why LABC Warranty requires you to hire a waterproofing specialist as part of your design team. We want you to limit the risk of damp penetration and make sure you and any future owners can enjoy the home for years to come.
We would consider a credible waterproofing specialist to be a Certificated Surveyor in Structural Waterproofing (CSSW) or equivalent. The design specialist should be responsible for designing an integrated waterproofing solution to eliminate the risk of water ingress.
We know from experience that basement leaks can be difficult and time-consuming to tackle, often with significant disruption for the property owners and occupiers. So while the relevant British Standard code of practice BS8102 (protection of below ground structures against water from the ground) requires that any waterproofing system should be accessible for repair, it’s better that basements are created to such a good standard that repairs are not needed.
So how do you go about getting your basement waterproofing right first time?
Basement waterproofing: the points you need to consider
As well as the basement design, you will need to collate information about the site to understand if the design is appropriate. What might work well in one location could be completely inadequate in another.
The information we ask for will include:
- A Site Investigation Report to show the soil composition and level of any water encountered
- Risk Assessment to highlight the prevailing water table regime and potential for future waterlogging (burst mains etc.)
- Section drawings showing the basement finished floor level and external ground level. Floor plans showing where the waterproofing will be situated. Detail drawings showing what materials will be installed and how (like the detail above)
- Third Party Certification to demonstrate that the products are suitable for the proposed use
- A design Report to explain the reasoning behind the proposals and justify how it complies with BS8102:2009
- Appropriate professional qualifications: as we’ve already said, the designer must be CSSW qualified or equivalent and will take on design liability
- Professional indemnity insurance: the designer should also have suitable insurance cover for basement waterproofing design
Basement waterproofing: levels and grades of protection
BS8102 grades waterproofing protection in relation to the basement’s intended use. It also considers different types of protection against risks associated with the water table.
For LABC Warranty purposes, we require a minimum Grade 2 performance in all cases with Grade 3 required for any occupied space. We only accept Grade 1 for the construction of underground car parking.
The table below describes the three “systems” for waterproofing a basement, with commentary on each depending on the local water table classification. The system you use must take into account the site conditions as described in the site investigation report.
BS8102 goes on to point out that defects and the presence of groundwater, at some point, must be assumed in all cases, which is why it makes the case for access to repair. In this context it’s important to think about the limitations of your system:
- The structure will provide the primary resistance: your chosen waterproofing design system will decide how much
- Drainage is essential if you choose an external tanking approach
- Cavity drainage systems will not withstand hydrostatic pressure alone: you need to include a means of primary resistance into your design
- If your structure is lined or decorated, it will need another (additional) line of defence
As you can see, there are lots of variables to consider. Make sure your design is suitable and robust for the site it will occupy. Basements can make fabulous living spaces and can add a special and eye-catching space to any home. Why compromise on its integrity?
Further information and comprehensive guidance on the design and construction of basements can be found in Chapter 6 of our Technical Manual.
Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.