Waterproofing systems
Mastic asphalt tanking
Most construction textbooks show asphalt built up in 2 coats, with an angle fillet at the wall/floor junction and a 50mm thick cement infill placed between vertical asphalt and an inner brick or block wall. Horizontal floor asphalt in 3 coats is protected and loaded by a cement/sand screed, sometimes augmented by a concrete slab.
Such waterproofing, which is costly, durable and incurs severe space penalties, is not often carried out for existing buildings. But it is certainly an option, and where space and budgets permit, is extremely reliable. However, if the building were to be subject to significant movement from settlement or subsidence it could fail, and once cracked or damaged, there would be no second line of defence in a traditional asphalt tanking system, unless a perimeter drainage system was built in. Failed asphalt tanking is also difficult to repair as it is concealed behind loading walls and under floor toppings.
The BRE supports asphalt tanking as a basement waterproofing option, but points out the 'significant space penalty, in terms of area and usually height as well' (from BCA Design Guide - Basement waterproofing).
Figure 7: Internally applied mastic asphalt tanking. © Courtesy MAC Ltd
- Reinforced concrete structural floor slab.
- 25 × 25mm chase cast in concrete.
- Ground level.
- Loading and protecting brickwork.
- 40mm space flushed up with mortar (course by course).
- 20mm 3-coat mastic asphalt.
- Reinforced concrete structural wall and floor.
- Concrete loading slab.
- 50mm protective sand and cement screed.
- 30mm 3-coat mastic asphalt.