Tanking asphalt must always be applied in 3 coats to a total thickness of 20mm for vertical work and 30mm for horizontal work, in accordance with Code of Practices BS 8102.
Tanking should be laid directly to the substrate without a separating membrane. However, a separating membrane of glass fibre tissue, loose laid with minimum 50mm laps, may sometimes be necessary on horizontal areas to overcome excessive blowing of the mastic asphalt.
External tanking is defined as the application of an impervious membrane to the outer surface of structural walls and the upper surface of a concrete base.
The excavations should be taken out, allowing a minimum of 600mm working space outside the walls. A sound concrete base must be provided and extended to give a 150mm set-off beyond the outside line of the structural walls, to enable an angle fillet to be formed between the horizontal and vertical asphalt. Immediately after the horizontal asphalt has been laid a protective sand and cement screed, at least 50mm thick, should be laid to prevent damage to the asphalt. The horizontal loading coat of concrete, designed to withstand the maximum water pressure likely to be encountered after the building is completed, should then be applied as soon as possible.
When the structural walls have been build the vertical asphalt membrane is applied to their external faces, an angle fillet being formed at the junction of the horizontal and vertical asphalt. As soon as possible after the vertical mastic asphalt has been applied to the outside of a wall it should be protected against damage by the erection of a masonry wall or protective board. When the protective wall is of brickwork, blockwork or any form of masonry it should not abut directly against the asphalt but should be set back 40mm away from the vertical asphalt and the space flushed up course by course with mortar. (Fig 2.2) The vertical asphalt should be taken up at least 150mm above ground level.
Internal tanking is defined as the application of an impervious membrane to the inner surface of structural walls and the upper surface of a concrete base.
When excavating for the underground structure approximately 300mm should be allowed outside the line of the walls and damp earth before the application of the asphalt. The concrete base and the walls must be structurally sound, whether of concrete or brickwork, and the walls should be built up to 150mm above ground level.
The horizontal asphalt membrane is laid to the concrete base and vertical asphalt is s applied to the inner surface of the walls. An angle fillet is formed at the junction of the horizontal and vertical asphalt at all internal angles. A protective sand and cement screed, at least 50mm thick, should then be laid to prevent damage to the horizontal asphalt.
The loading coats, which should be designed to withstand the maximum water pressure likely to be encountered after the building is completed, should be constructed as soon as possible after the vertical asphalt has been applied.
When brickwork is used as a vertical structural loading coat it should be set 40mm away from the vertical asphalt to enable each course to be flushed up with mortar to prevent any voids occurring between the asphalt and the brickwork.
The presence of such voids can cause fractures to occur in the asphalt when the pressure of the water is applied.