In June 2013 it was announced that £500 million would be cut from the Metropolitan Police budget, 65 of the least used police front counters across the capital were proposed for closure. Marylebone Police Station had its fate sealed and the old police station closed in June 2013.

In 2015, Westminster City Council’s planning department granted The Portman Estate planning permission to demolish the existing building at 1-9 Seymour Street and to redevelop the site. The scheduled demolition and clearing of the site was to make way for a new seven storey mixed use building, which was to include a two level basement, residential block, restaurant, retail space, commercial office block and school.

Some of the key challenges to this project were:

  • Ensuring a fully functional waterproofed basement which would accommodate the building and its future use.
  • Responsibility for full façade package including onerous performance criteria.
  • Multiple façade treatments with complex interfacing. Totally bespoke projecting glass and steel bays.
  • Bespoke vitreous enamel window liners.
  • Complex inner city environment with significant delivery challenges.


1-9 Seymour Street is a 90,000 sq ft technically challenging project with a multiple façade.

Whilst the multiple façade is a key feature, it is also a disadvantage. A detailed design and a sound approach would be applicable to this project from the onset. The main contractor Galliford Try undertook the complex underpinning and formed the two levels of basement.

There is always a level of risk to damaging other buildings in the vicinity when dealing with demolition, re-build and basement construction, this risk was designed out in collaboration with renowned Architects Eric Parry.  During this phase Galliford Try successful reached all of their goals without causing any damage to surrounding structures.

This project required a sympathetic knowledgeable basement specialist. Guardian Preservation with its reputation of working with homeowners, surveyors, contractors, architects, landlords and management companies was a natural choice. Guardian Preservation have a long established relationship with Delta Membrane Systems Limited and the two businesses have regularly shared experience and knowledge.

Both Guardian Preservation and Delta Membrane Systems Ltd were included at the design phase so were able to add consideration and give expert advice during the crucial planning stage.  The basement dig was planned with the use of piling.  On this project contiguous piling was deployed in conjunction with an inner RC lining wall.

Tim Herbert, CSSW qualified Structural Surveyor and partner at Guardian Preservationcommenced his involvement in the project by giving careful consideration to products available, how they would be best suited to the project, that all BS8102:2009 standards and considerations were met, that the site was kept in a safe manner (health and safety) and that all Building Regulations were adhered to.

Tim’s approach to the project was to initially make contact with Delta Membrane System Limited’s David J Symes, Technical Director; along with David’s advice no assumptions were made.  As with any basement construction consideration to groundwater, soil type, precipitation, topography and ground conditions/site analysis needed be addressed.  Tim and David’s approach was to carry out full and throughout checks and to make a decision on what types of waterproofing would be best suited to this project, as no two projects are identical.

Waterproofing a basement ensures that the integrity of the structure is preserved whilst ensuring the usefulness of the basement.  All basements should be constructed to cope with groundwater levels up to the full retained height of the basement.

Tim and David, during consultation took into consideration the British Standard BS8102:2009.  BS8102:2009 is the code of practice for protection of below ground structures against water from the ground.

System Components

BS8102: 2009 sets out the scope and limitations for below ground structures, areas covered include:

  • Waterproofing barrier materials,
  • Waterproofing to construction joints within the RC frame construction,
  • Ensuring buildings are structurally integral with watertight construction,
  • Drained cavity construction,
  • Ground water assessment,
  • Ground water drainage; and
  • Associated risk assessments.

An initial and important consideration in the overall waterproofing strategy was to ensure that all construction joints were appropriately detailed and waterproofed using Hydrophylic waterstops – such as Koster Quellband, which expands in contact with water to seal the construction joint.

Tim sought to incorporate materials that would best meet the objectives set out in Section 10 of BS8102:2009.  Through knowledge and experience Tim sought David’s advice to incorporate 2 forms of waterproofing.  With over 2500sq.M to cover Tim decided to incorporate Delta Membrane Systems Limited’s NB1 in-depth migratory crystalline slurry (a Type A System, a water barrier), and Delta MS500 and Delta MS20 (a Type C System, cavity drain membrane system).

Köster NB1 is a crystallizing mineral waterproofing slurry system which has a capillary plugging agent ideal for sealing against pressurized water (> 13 bar) and is watertight (>130m water head).  Köster NB1 is a Type A waterproof system as set out in BS8102:2009.  NB1 is suitable for new builds and in restoration properties. Köster NB 1 Grey is characterized by its excellent resistance to pressure and abrasion as well as excellent resistance to chemicals.

Tim opted to apply the Köster NB1 with a peristaltic pump providing a uniform spraying application, this method saves considerable man hours and is a great way to ensure complete even coverage.

Having applied a Type A waterproofing system Tim opted to add a secondary Type C system offering a robust waterproofing design.

By adding a secondary Type C system the basement would be completely watertight, if any water ingress was to occur in future the Type C system would be able to deal with this so as not to compromise the structure.

The Delta MS500 is a premier cavity drain membrane and was used for the waterproofing of the walls. The Delta MS500 was fixed using Delta Qwik-seal plugs, fixed at 600mm centres with added benefits of Delta double-sided butyl tape to seal joints and overlaps.  Delta butyl Corner strip tape was added to seal wall/floor joints.

The Delta MS20 is a champion cavity drain membrane used for waterproofing the floors.  This is a heavy gauge version and incorporates 20 mm studs which offer extra drainage capacity.  Delta MS20 was used as a “cavity former” for this type of new construction. The Delta MS20 is neatly rolled out across the basement floor; the sheets are overlapped and sealed in accordance with the manufacturer’s instructions.

Corner strip tape was added to seal wall/floor joints.

Case Study Results

The basement structure has been constructed to cope with any future water ingress and will withstand the test of time.

Galliford Try were thrilled with the outcome.

The project was achieved on time and within budget.

The finished project will be a truly stunning building that adds to the growing portfolio for all involved and will add to London’s ever evolving skyline and landscape.

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