Gary Laird discusses how to manage basement conversions successfully.
08 May 2018 | Gary Laird
Structural waterproofing is a critical part of any below-ground build or conversion project and is often the difference between success and failure. The Chartered Institute of Building says 75 per cent of building failure is the result of water ingress - with failures leading to mould, corrosion or other moisture-related problems such as dry rot and even structural problems.
At works on a historically significant building in West Yorkshire that was subject to a new build, the original contractor had incorrectly installed a type A barrier protection system and it never bonded with the new concrete walls.
The entire site had to be re-excavated so the system could be installed again. This unforeseen cost could have been avoided had the project been managed correctly.
The National House-Building Council (NHBC) consistently reviews its guidance and requirements on structural waterproofing. Although NHBC standards do not have direct relevance to commercial builds, it is worth considering their recommendations as best practice. For example, chapter 5.4 of its published standards makes it clear that any waterproofing project must include early input from a waterproofing design specialist and build on the guidance of BS 8102:2009.
What you need to know
BS 8102:2009 lists the different waterproofing options and combinations. It specifies three grades of protection, based on building use.
-In grade 1 builds, such as car parking facilities or plant rooms, some seepage and damp areas are tolerated.
-In grade 2 builds, such as workshops or storage facilities, water penetration is not acceptable, but moisture vapour is tolerated.
-Grade 3 builds, such as offices, living spaces and storage areas where easily damaged goods (clothes and food) are kept, must be a completely dry environment.
Get advice early
We recommend getting a qualified certified surveyor in structural waterproofing involved in any new-build or conversion project. Not only is it the best way to minimise risk, but it will also help to lower waterproofing costs, which can represent a significant proportion of the total build costs. Early structural waterproof planning will enable your surveyor to tailor the waterproofing design to your construction type, rather than having to tailor it around other committed build considerations.
Types of waterproofing
In any below-ground space that falls into grade 3 and/or high-risk environments, at least two different types of waterproofing systems
should be used.
This is in line with the NHBC's standards. BS 8102 categorises the types of waterproofing systems as:
Type A: This provides a barrier against groundwater and water ingress by surrounding the external walls, floor and roof with a waterproof material - literally creating a barrier between water and the structure. This can be applied either internally or on the building's exterior.
Type B: Relies upon the design and materials incorporated into the external shell of the structure itself. Protection should be constructed of reinforced water-resistant concrete or structural steel.
Type C: Involves adequately disposing of water, either by gravity or through a sump and pump to a suitable outlet. This must be designed with adequate access points for servicing and maintenance, with a one-way valve to prevent backflow. Type C systems use a cavity allowing water to reach the drain, while being protected by an internal lining, with all joints and junctions correctly lapped and sealed.
Each has its own benefits and pitfalls; while type C waterproofing will require continuing maintenance, it is easier to identify and fix any problems that might arise. All share a common characteristic - retrofitting is expensive, disruptive and difficult, if not impossible in some cases.
Given the complexity of waterproofing a building, a design specialist should always be involved as early as possible.
Gary Laird is national commercial manager at damp-proofing specialist Peter Cox