WHY THERE IS DAMPNESS IN ROOF SLAB UNDER THE TERRACE FLOOR OPEN TO SKY IF LAID WITH MARBLE/TILES/TERRAZO INSTEAD OF BRICK TILES.
By Er. Ved Parkash Takyar
 
The above is one of the problems of dampness and leakage which later on becomes unsolvable in buildings. In these days there is craze to have the terrace floor of marble/ kotah stone or the precast terrazzo tiles instead of brick tiles. But one should know that there is a lot of variation in temperature in northern India i.e. from 0 to 47 C and every material has its own character to gain heat and cold. The brick tiles being porous and moreover laid on mud and mud-plaster duly grouted with cement sand mortar do not gain too much heat in summer and thermo-action is negligible, therefore cracks do not occur in the joint of brick tiles. Whereas the marble/ kotah stone or terrazzo tiles which are generally laid over a bed of cement mortar after laying brick-ballast concrete on R.C.C. roof, gain heat too much and due to thermo-action, hair cracks occurs in the joints of stone slabs and tiles even in the first summer. When there is a continuous rain, the water penetrates in the roof through the hair cracks in the joints and dampness will appear under the roof slab. This dampness is also seen in the roof slab under the place of desert cooler, air-conditioner and where flower pots arc placed on the terrace due to penetration of water in roof through the hair cracks. Once the water seeps through the cracks in the joints of marble/ kotah stone etc., dampness will appear under the R.C.C. slab as it remain in wet condition below the top layer of floor and some times also on account of condensation by atmospheric action i.e. when the outer atmosphere is cool and inner side is warm. The dampness in the roof slab will not only give a bad look and foul adour inside the room but will also rust the M. S. Bars in the concrete and therefore, reduces the life of RCC slab.
If someone is determined to lay the flooring with marble/ kota stones and terrazzo tiles etc. then he should follow the following suggestions & specifications:
To control thermo-action, at least 2" to 3" mud and mud-plaster must be laid and compacted on the duly treated R.C.C. slab in proper level and slope towards the rain water pipes which works as a thermo insulation on the slab. Where there is apprehension of dampness and leakage, some professionals and masons give undue importance to rich concrete of (1:2:4) or (1:1~2:3) ratio for water proofing on the R.C.C. Slab under the bed of stone slabs and tiles in terrace floors and floors of bathrooms etc. The rich concrete without proper re-enforcement is prone to cracks due to thermo-action. Some professionals and masons suggest the laying of "Kuba" i.e. brick-bats grouted with sand mortar on the R.C.C. Slab which is also not suitable in this region where temperature exceeds 45C. Therefore, the character and behaviour of material needs to be taken into account during construction.
Above 2" to 3" thick mud duly compacted, l 1/2" to 2" thick base concrete (1:3:6/1:4:8) of brick-ballast "3/4" to 1" gauge should be laid with the admixture of water proof chemical and compacted on the rammed mud in proper level and slope. Before the brick ballast concrete hardens, the top surface should be brushed with steel brooms brush to have key joints between the concrete and mortar bed. It is desirable that before the laying of stone slab/ tiles, the brick concrete surface is coated with cement paint prepared with water proof chemical. For doing so 1/2" to 3/4" thick cement sand mortar (1:3) (prepared with water proof chemical) bed under the stone slab and tiles is sufficient. But most of the stone laying contractors and masons often lay lean dry concrete under such floors to avoid the mixing of 1.1/2" to 2" screed with cement sand mortar, compacting with durmat and brushing the surface to have key joints. On lean concrete, the mortar bed is laid l 1/2" to 2" thick by them without considering any treatment of the dampness which will certainly occur in the roof slab during continuous rain and dripping of regular water on the surface of floor. This practice also leads to rise in cost of work. Moreover, there this practice cannot match the results of the above stated methods and specifications and that is why dampness occurs in roof slab under the terrace floor i.e. floor open to sky including the verandah on the east, south and west direction. The hair-cracks in joint of stone/tiles of the floor will definitely occur due to thermo-action., but the above treatment will not allow the water to pass/ penetrate beyond the joints.
The end stone/tiles along the parapet should also be embedded in the parapet slightly in slope towards the courtyard as shown in the drawing. In most cases, the concrete 'gola' is not constructed along with the laying of RCC slab. Because the character and behaviour of brick masonry and stone flooring differ with each other, a crack between the wall and stone flooring is unavoidable. Through this crack, the water will penetrate during rains and washing of floors which will result to some dampness in walls. It is suggested that before laying the stone flooring, a vertical damp proof cement plaster with the wall should be done from the slab up to the stone flooring as shown in the drawing.
Generally, the stone/tiles laying contractors and masons mix the cement sand mortar with water minimum for half day work, when the ordinary Portland cement mortar begins to set after 30 minutes and completely set after 600 minutes. When the setting of cement mortar starts, the unused cement sand mortar will start to loose strength gradually, so therefore, this practice not to use cement mortar within 30 minutes after adding water and laying very thick layer of cement sand mortar under the bed of stone/tiles flooring needs to be checked and stopped by the Architects, Engineers, Builders and Contractors.
If the above methods and precautions are adopted and expansion and construction joints are suitably provided in the roof slab and along the parapet during construction, then there will be no chance of dampness in roof slabs under the terrace floor other than the brick tiles also.
It is not out of mention here that even the stone flooring of toilets & kitchen washbasin give dampness in walls due to leakage through the joints of junction because their back surface is not made water/damp proof. Similar is the case of stone cladding of outer walls during the rains. It happens because stone has no cohesive property being of plain surface and gaining more heat being homogeneous. In outer walls it some time disintegrate during summer and causes fatal accidents.
The bigger size stones other than stone patties should be laid with proper size of brass dowels & clamps. In no case terrazzo /concrete flooring should be laid cast in situ on the terrace floor, (open to sky).
by Er. Ved Parkash Takyar
( Building Consortium ) 19 - H, B. R. S. Nagar, Ludhiana.
 
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