Introduction to Tanking
Tanking should be provided wherever a floor level is below ground level. In a case where the floor is wholly below ground level, as is the case in a basement, all walls should be tanked. The principle of tanking is to provide a continuous waterproof barrier or membrane; this is created using either mastic asphalt or proprietary adhesive DPM. Depending on the nature of the site, membrane can be placed either internally or externally.
The reason tanking is necessary is because basement and semi-basement constructions are prone to water penetration caused by hydrostatic pressure building up outside the retaining walls. As well as this, contaminants in the groundwater can damage reinforced concrete, in-situ and mass concrete constructions.
The tanking membrane can be sandwiched between 2 layers.
External tanking occurs in a situation where a protective wall on the excavation side protects the membrane with the structural located internally as shown.
Diagram B34 - Basement tanking
Diagram B35 - Typical detail - external tanking
Screed & Insulation
Where screed is laid on top of insulation, screed should have a minimum depth of 65mm. It is also recommended that light reinforcement be used.
In order to reduce the risk of clogging by migration of fines in the drain, it is recommended to wrap the perforated drain in a geotextile filter.
In internal tanking, the structural wall and floor slab retains the excavation with the membrane inside with the internal skin providing protection inside.
External tanking is preferred to internal tanking as it provides protection to the main structure from attack by sulphates in the surrounding soil and groundwater.
Diagram B36 - Typical internal tanking detail
Number of Coats/Thickness
Mastic asphalt should be laid in 3 coats to a total thickness of 30mm on horizontal surfaces sloping to 30 degrees to the horizontal. On vertical surfaces and slopes over 30 degrees to the horizontal, mastic asphalt should be applied in 3 coats with a total thickness greater than 20mm. The layer of mastic asphalt should extend 150mm past ground level.
In cases where angle fillets are required, they should be at least 50mm wide and applied in 2 coats at the junction of 2 planes forming an internal angle. In order to ensure the membrane is effective in waterproofing, tanking should be applied to clean dry surfaces free of sharp protrusions which may puncture the membrane.
Basement walls should be backfilled in layers of graded material of less than 150mm depth and compacted.
In order to prevent deformation of the PVC/uPVC pipe it is necessary to ensure that the granular material is well compacted either side of the pipe.
Basement design, construction and tanking is a specialised area and an engineer should be employed to oversee. The engineer must be qualified by examination, in private practice and possess professional indemnity insurance.
Laying & Application of Tanking
Ensure no damage to membranes occurs at all times. Build the protective wall as soon as possible after completion of damp proof membrane. Keep the area overlaid clear of materials used by other trades.
Externally Applied Membrane
At the junction of the membrane from the floor to the wall, provide and maintain effective temporary protection. Avoid driving mechanical fixings through membrane.
Membrane Support & Heating Material
Support to be provided completely to both sides of tanking membranes in a sandwich construction. In an internal tanking situation, ensure the inner protective wall is constructed so it completely supports the membrane.
Temperatures of Heated Material
Bonding bitumens not to be heated to temperatures greater than 260°C. Asphalt not to be heated to temperatures greater than 230°C for prolonged periods. Measure temperatures using a thermometer in the heating cauldrons or in the mastic asphalt immediately after it has been removed from a mixer.
Use of Granular Fill Material Under Concrete Floors
It is important that all granular fill material is fit for the purpose it is intended. The recommendations are as follows:
The hardcore bed should be at least 150 mm thick. Hardcore should conform with I. S. EN 13242: 2013 and meet the specification as outlined in Annex E of the accompanying guidance document to this standard, SR 21: 2014 + A1: 2017.
The layer of hardcore should be well compacted, clean and free from matter liable to cause damage to the concrete. Specific guidance is given in Clause 3.4.2 of SR 21: 2014 + A1: 2017 on limiting the presence of a reactive form of pyrite which may give rise to swelling or sulphate attack on concrete.
A blinding layer should be provided in accordance with the specification given in Annex E of SR 21: 2014 + A1: 2017, for fines material.
The blinding layer should be of adequate depth to fill surface voids thus creating an even surface and avoiding sharp projections, which may damage radon or damp-proof membranes.
The purpose of blinding is to sufficiently fill surface voids and provide firm and even support for support chairs for reinforcing steel in the case of reinforced slabs. It is also necessary to provide a good surface for working.
Avoid sharp points in the blinding which may puncture or damage the damp-proof membrane.
Damp-Proof Membrane Detailing Requirements
Listed below are the requirements to be followed on site for damp-proof membranes:
♦ DPM in floor and DPC in surrounding walls should either be continuous or sealed together.
♦ Lapping of at least 150mm should be provided at all joints. Joints should also be sealed.
♦ Punctures should be repaired with the same type of polythene and lapped at least 150mm all around the puncture.
Further reading/Information available: BS 8102: 2009 Code of practice for protection of structures against water from the ground.
One should consider the following when buying or using DPM:
1200 gauge material should be used when laying DPM below concrete. All joints should be sealed as prescribed in CP 102 and set out previously.
“Heavy duty” or “C1200” should not be used as they are not 1200 gauge.
It is strongly recommended that virgin material is used. This should comply with the requirements outlined in I.S. EN 13967 Flexible sheets for waterproofing – Plastic and rubber damp proof sheets including plastic and rubber basement tanking sheets – Definitions and characteristics.
Avoid sharp points which may damage the membrane.
Care should be exercised so as not to damage the DPM at the junction.
1. Ensure the DPM is in place before walls are built above DPC level. This is to ensure that a proper connection between DPM and DPC can be achieved. DPM must be in place before walls are built above DPC level so that a proper positive connection between DPM and DPC can be achieved.
Diagram B37 - DPM placement prior to walls being built
2. The DPM should be lapped under the DPC for the full thickness of the inner leaf.
Diagram B38 - DPM lapping under DPC for thickness of inner leaf
3. It is important to ensure that a proper seal is achieved around all services.
Diagram B39 - Proper sealing around services
4. Where required it is important to ensure that the adequate laps are maintained.
Diagram B40 - Minimum lapping requirements
It is important to ensure that proper care is taken when installing damp proof membranes to ensure the DPM is not damaged. Damage to the DPM will result in a reduction in its effectiveness.