Client: Commercial Interior Design Firm
Our client, a commercial interior design firm in London, tasked Timberwise with surveying the property to design a basement waterproofing system to protect the basement from water ingress. This property was a commercial building of brick construction, and whilst the building was occupied and furnished, the basement was not. The weather conditions were dry during the initial survey.
Challenges
There were a number of significant challenges when it came to designing and installing an effective basement waterproofing solution. Our team had to overcome active water ingress from seepage and heavy salt contamination on the already existing rendering inside the basement’s vaults.
We found active hydrostatic water pressure below the basement’s concrete slab at the wall-to-floor junctions of the far rear left elevation vaults.
We also had to consider how to address the water ingress from the defective seals of the concrete pavement lights above, which had contaminated the tanking and wall plaster. We would also need to fix the failing plaster and cement render tanking system that had significantly deteriorated in multiple areas throughout the basement vaults.
Ultimately, it was a challenge to design a system to conform to a BS 8102:2022 Grade 3 basement waterproofing system, which requires the assumption that hydrostatic pressure can occur due to changing water tables, localised flooding, blocked drains, and bouts of heavy rain. We needed to resolve complex perimeter drainage layouts to make sure water channels were not placed directly below dry-lining systems and that the jetting ports were still accessible for future maintenance.
We also needed to be able to ensure the client that the pump would continue to operate even during power outages without a battery backup system, as the basement was at risk of ground water ingress.
Observations
Our initial observations of the basement found that there was active water ingress from seepage and heavy salt contamination of the existing cement rendering, which was found in three left elevation vaults. Hydrostatic water pressure was actively present below the concrete slab at the wall-to-floor junctions to the far rear left elevation vaults.
We found failing sections of the existing render tanking, which was confirmed by high moisture readings taken in several areas throughout the vault. The concrete floor slab directly in front of the vaults’ entrance also had moderately high moisture meter readings.
The remaining vaults (which consisted of the front, right, and last two on the left elevation) were suffering from consistent water ingress and efflorescence salt contamination. Salt contamination from this had degraded both the cement tanking and the wall plaster across several vaults from previous basement waterproofing attempts.
High moisture within the building’s brickwork substrate was confirmed by inspections of the exposed brickwork vaults to the front elevation.
We also noted existing resin floor works had already been carried out to the right elevation of the basement.
Solutions
Preparatory Works
A lot of preparatory works had to be carried out by the client under separate contracts as it involved a lot of construction and electrical work. All electrical works and pipework had to be removed from the walls and floors in order for us to install the membrane.
The floor slab was rebated to form a perimeter drainage channel, maintaining slab thickness beneath the rebate to prevent any structural cracking. The floor slab concrete had to be reduced in all vaults and entrances to allow for the floor membrane to be installed and walls to be opened up for the base drain channel.
We created a 900mm x 900mm x 900mm aperture in the floor slab in order to install the sump chamber. We then provided a route for the discharge pipe from the sump chamber to a maintainable rainwater drainage system.
External contractors also installed two independent electrical feeds from the consumer unit, each of which was protected by a separate RCD, for the pumps, battery backup, and the alarm system which were then electrically connected.
New concrete pavement lights were installed with waterproof seals and the main internal floor slab had to be assured was completely flat and level, making it ready for the application of the floor resin which was to be completed by us.
Waterproofing Systems
Timberwise set to work installed a Type C drained protection waterproofing system that would comply with BS 8102:2022 Grade 3 basement waterproofing (habitable space).
We applied a TG500 anti-line coating to all concrete walls and floors in order to prevent free lime from leaching out and blocking the perimeter drainage channels Delta NB1 waterproof slurry was applied directly to the exposed brickwork in the vaults.
We installed an 8mm cavity drainage fire-resistant wall membrane to the designed walls up to the face of the pavement lights. The wall membrane was fixed with dual-purpose plugs featuring gasket seals, hung to allow subsequent client installation of timber battens or an independent stud partition.
We then installed a 20mm cavity drainage floor membrane, which was then sealed at the overlapped joints and sealed at the perimeter to the wall membrane. We applied two coats of a vapour floor resin to the main internal floor slab. In addition, we included the supply of a battery backup system to keep pumps operational during power outages.
Drainage and Pumping Systems
For the drainage and pumping systems, we installed a perimeter drainage channel that was set into the preformed floor rebate in order to collect and direct water to the sump chamber. We installed jetting ports and rodding eyes at the corner intersections along the perimeter channel for future flushing and maintenance.
A groundwater sump chamber was installed into the aperture, which was to be concreted in place. What was also installed was a dual pumping system that also contained a high-level water alarm which would alert the client when the system was reaching over capacity.
A flood text was completed upon installation to ensure the water drains were working effectively and leading water to the sump chamber without pooling. The area was then cleaned of debris.
For finishing works, the client was advised to protect the waterproofing system from follow-on trades and to request a final inspection performed by us prior to any application of floor or wall finishes.
The wall membrane was designed to be able to receive a timber battened or independent stud partition finishing option, depending on the client’s preference. The floor membrane was also designed to be able to receive a 70mm concrete screed or a chipboard floating floor.
We strongly advised the client not to fix through the waterproofing system without seeking advice from a system designer, as unapproved penetrations would void the 10-year guarantee the solution comes with.
We also recommended that the client contacted us after three months for a free post-installation assessment, which would also include cleaning where necessary. We also mentioned that regular maintenance and servicing of the pump system was required to ensure it continued working optimally.
Results
Timberwise successfully equalised the hydrostatic water pressure problem and fixed the water ingress issue, turning the space into a Grade 3 waterproof protected area. We implemented perimeter drainage channels and dual pumps which will collect and diverge water to the external drainage system we installed. These cavity drainage membranes act as a continuous water vapour barrier, which prevents damp and condensation.
We applied two coats of a vapour floor resin to the main floor slab, which will effectively prevent vapour transmission into the space. The entire system was designed and implemented by a CSSW-qualified surveyor and has a BBA Agreement Certification. Our efforts provided the client with long-term peace of mind and a 10 year guarantee for the basement waterproofing solution.
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