Case 1
- Introduction
- Causes of Defects
- Good Practices
- Standards
- Maintenance and Diagnostics
- Remedial
- Similar Cases
- References
Good Practices
Design
Design should ensure that for a bigger floor area, there should be sufficient outlets of suitable size to cater for the volume of rainwater. The outlets should not be too far from each other because a fall over a long distance may lead to inaccuracies in the screeding.
Design for adequate falls to allow water to drain off effectively. This would help reduce the problem and subsequent water seepage. Refer to 6367:1983 Code of Practice for drainage of roofs. See figure 2 for typical drainage layout.
It is preferable to have a uniform fall over the entire flat roof. Avoid providing steeper perimeters near the perimeter of the roof as this will cause water to collect at the junction of the roof slab and the parapet wall [Draft code of practice for Waterproofing of Reinforced Concrete Buildings].
Falls between outlets can be provided by the addition of tapered firrings to the purlins between outlets or by introducing a fall in the purlins themselves (See figure 3).
Construction
Ensure supervision while the workers are laying the screed to falls. It would help prevent possible ponding in the future. For calculation of run-off from all surfaces within on catchment area, whether they are vertical, horizontal or sloping, assumption should be made that all surfaces are impermeable or saturated.
Waterproofing membrane should cover all areas with a minimum 150mm upturn on the parapet walls at the sides. This is to prevent leakage of ponding water into the slab below. However, if the screed laying is properly done, this would nor pose a problem.
Materials
Use tapered outlets for the drain pipe as these are more efficient than those with a uniform diameter (See figure 4). A liquid-applied membrane is most advantages on congested roofs. It is also capable of bridging hairline cracks on concrete surfaces. It is useful as flashing material around drains, protrusions, curbs and parapets.
Performed membranes are good if there are no awkward corners to which waterproofing membrane should be laid. These preformed membranes are useful to identifying sources of leakages. Performed membranes are used where the condition of the substrate does not allow the materials to be fully bonded.