Designing roofs that do not leak

What are the precautions to be taken to prevent rainwater seepage in buildings?

BUILDING LEAKAGES: Walls damaged due to water leakage and weathering.

Heavy rain has taken a toll in most rugged houses leaving behind leaking roofs.

Exterior surfaces

The most exposed surface of the building is the top surface or the roof slab, mostly a reinforced concrete surface. Any concrete or brick material is porous. When it rains heavily, water tends to penetrate the roof slab, drip inside the house or run down the wall causing the paint to peel off, and short-circuiting electric wires.

Any surface exposed to high rainfall needs to be designed keeping in mind these important points to avoid seepage:

A good slope is required to drain off all the rain water. Only when the water stagnates, it seeps into the roof slabs and walls. The rain water pipe too can be made wider (6" dia) instead of the conventional 4" dia.

The tiling material has to be of a non-porous surface. All the joints have to be properly sealed and the edges properly rounded

A polymer coating (which is cement based) has to be used for all junctions between walls and floors and applied in two coats in a criss-cross direction.

Avoid levels in terrace because varying levels will lead to more number of joints and hence more chances of leakage.

Plaster cracks can also lead to dampness in walls. These cracks can be avoided if the plastering thickness is maintained around 20 mm and Pozzalona Portland cement is used for the same.

The ideal mix

Anti-corrosive epoxy coated steel rods are also an expensive option. This option may be suitable for long-term projects but for small residential buildings, it may be not be viable. Graded concrete mixes (in the ratio 1:2:4 or 1:1½:3) can be proportioned with 20 mm and 12 mm blue metal instead of just using 20 mm graded blue metal. Also for sloped roofs the concreting mix needs to be in ideal proportionsto avoid honey-comb formation (it is the small holes that are formed during concreting and can be seen after removing the shuttering). Extra care has to be taken for sloped roofs and may be avoided preferably for coastal cities like Chennai. Proper care has to be taken to avoid re-rolled steel bars and a tensile strength test may be performed before starting work.

Architectural detailing

Careful architectural detailing is also important to avoid seepages and cracks. Any punctures in the roof slab in the form of fan hooks and ceiling lights are vulnerable locations. Proper cover blocks and good specification hooks may help avoid leakages through the ceiling. Roof slab cut outs and box type sun shades (which may be avoided as well) essentially need to have an aluminium spout for draining the stagnant water. Some times even the AC conduits may cause sweating, leading to dampness in the wall. A sunshade also needs to have a drip groove so that it arrests the water trickling into the rooms.

Grooves in the elevations need to be properly water-proofed and plaster bands at parapet level need to have coping materials like natural stone to avoid water seepage

For low lying areas in the building which may have an external opening directly, it would be wise to provide a floor sump or collection sump where in all the seepage water flows into one area which can easily be pumped out.

Also the walls and roof can be subject to the weather tests like rain and sunshine before plastering.

The author is the Chief Architect of Murali Architects, Chennai