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Stonehouse was introduced to this failed basement in a large terrace property in Notting Hill in January 2015 via Newton Waterproofing. The client had a second level basement excavated below an existing basement to the full footprint of the property, taking in both front and rear gardens.

Sadly, the basement development had been badly designed, together with poor workmanship from the concrete underpinning to the waterproofing and with inadequate drainage. The basement flooded at Christmas following completion.

The builder ceased trading and the architect and structural engineers were found culpable following a legal case.

As a specialist, Stonehouse was approached to assess the basement failure.

Stonehouse highlighted the following defects:

  • A weak, permeable structure.
  • No continuous waterproofing between the basement and lower ground floor levels even though both were below ground structures.
  • Type C drained cavity system blocked with free-lime, drainage profiles cut into the slab were blocked with debris and not accessible for maintenance.
  • A wholly inadequate single chamber pumping system overwhelmed by combined drainage from the main roof, front & rear decks, cavity drain membrane and sewage system.
  • The foul system vented to the living space as a consequence of the combined drainage.
  • No effective external deck waterproofing or drainage. Cavity drain membrane was applied to the flat soffit and, as an afterthought, holes were drilled through the deck to direct the water accumulated on the deck into the basement below.
  • Condensation to the underside of the decks due to the lack of an effective vapour barrier and insulation.

Working in collaboration with three successive architect practices we provided a robust solution which could be relied on in the most adverse weather conditions.

Stonehouse Proposal

  • The structure must be capable of withstanding any water pressure that may come to bear.
  • An internal Type A barrier membrane applied to the entire retaining structure, including the soffits, to limit water ingress.
  • The internal Type C cavity drain waterproofing system must be replaced with a maintainable system and have a dedicated pumping system with the installation of an additional sump chamber in the lower basement.
  • A continuous vapour barrier should be applied to the underside of the decks
  • All surface water from the rear roof and deck must remain external and be directed to the main front elevation drainage via a patio sump.
  • A continuous external Type A liquid waterproofing barrier membrane should be applied to the deck areas and retaining garden walls.
  • The sewage pumping system must be independent and upgraded.

Stonehouse Process

  • Structural Engineers, GBG, designed a solid, strong reinforced structure avoiding the need to replace the underpinning.
  • External decks were cleared and laid to falls away from the building. Internal floors were levelled.
  • The dry pack joint was reinforced with Waterplug and Newton 103-S.
  • Voids and points of weakness in the structure were rectified with Newton Injection resins.
  • All internal walls and floors were ground by hand to remove laitance and contaminants before spray-applying Newton 103-S to all internal surfaces. This membrane was then monitored for three months and any weak points reinforced.
  • Foamglas insulation adhered to the underside of the soffit which also provided an integral vapour barrier.
  • A full, maintainable Newton CDM System Type C cavity drain protection was installed with a dedicated Titan-Pro sump chamber and NP750 dual pumping system with Victron battery back-up for protection during any power failure.
  • Externally, all rear and front decks were prepared and waterproofed with Newton 110 WATstop as a primer, then Newton 114 Acriflex as a reinforced liquid waterproofing membrane overlaid and protected with Newton 408 DeckDrain drainage membrane.
  • Upgraded Newton sewage pumping system with two DAB pumps.

Stonehouse Guaranteed

Our clients now have a watertight basement structure on two levels, designed, waterproofed and guaranteed by Stonehouse and the prospect of fully enjoying their home in full for the first time in many years.


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