Tips on waterproofing, floor levels and more.


Architect Navneet Malhotra is a perpetual student. He loves to break myths and unmask the true cause for bad work by constantly sharing experience… “Path to gaining knowledge is through sharing,” is his motto.



While installing services-related equipment, ie solar water heaters, water tanks and pressure pumps, outdoor units of air conditioners, etc, on the terrace of our house, we were confused regarding the order of various protective layers for the roof slab, such as waterproofing, insulation and brick tiles. Should we anchor and secure all the equipment to the terrace first and then do waterproofing and insulation layers, or should it be the other way round? I want to ensure that we choose a solution that gives us a leakage- and maintenance-free surface for a long time. As per the contractor, all the equipment should be anchored to the concrete beams that will be specially made on top of the roof slab, and these should be created before all the other layers.
Sandeep Anand Goyal, Faridabad

The (correct) location of the waterproofing layer is the critical decision here. Failure in this area could lead to either an endless search for the source of leakage or complete uprooting and redoing of the entire terrace services. Most architects and designers have a divergent view on the location and type of waterproofing to employ at any given location. But a majority agrees that a membrane-type layer is the most successful option for building terraces where the structure is exposed to the extremes of weather, which results  in thermal expansion and contraction.

If we follow what the contractor is suggesting and anchor all the services to the terrace first, then we will either have to avoid putting the waterproofing layer under the support legs (not recommended) or try and patch the waterproofing layer around the legs. Patch-fixing of waterproofing layers is never a good idea as this will introduce unwarranted joints that are naturally prone to leakages.

Here again, the debate is endless. Some designers prefer to place the waterproofing layer above all the other layers and make it as the first line of defense against moisture. They propagate the need to control the water where it falls and prevent it from penetrating any deeper into the softer outer skin of the building. Others, like me, believe that we need to protect this invaluable layer as much as it protects us so that it remains as an impenetrable barrier for as long as possible. Placing this fragile layer near the surface of the roof will only bring it closer to the onslaught of weather and also expose it to damage by other activities.

I personally prefer to fix the waterproofing layer directly on top of a raw RCC (reinforced cement concrete) roof slab where it gets a hard and stable surface to house itself. Knowing the value of this layer and the need to protect it, it is recommended that this fragile surface should be covered with plain cement concrete as soon as possible. This 40-mm to 50-mm-thick layer of cement mortar will easily cover the entire surface and protect the under-layer from damage, while the rest of the services-related work continues on the terrace. The concrete mortar could be in the ratio 1:2:4 (1 part cement, 2 parts course sand and 4 parts aggregate of size 12 mm and less). All these ingredients are measured by volume and mixed together along with adequate water.

Once the services have been secured in their desired location, the rest of the outer layers of the terrace treatment can follow, starting from the insulation layer and followed by the outermost layer that will help protect the under-layers as well as generate a slope on the surface of the terrace. The slope was often generated by either a layer of plain cement concrete or mud-phuska and brick tiles. Mud-phuska is a mixture of earth with pieces of straw and some other binders that is spread over the terrace surface in uneven thickness such that it creates a slope of 1 cm drop in 100 cm. This loosely compacted mud-phuska is eventually covered with a layer of brick tiles with their joints filled with cement mortar. This is a popular system employed in construction in north India as mud acts as an inexpensive insulation and slope material, while the top layer of brick tiles saves it (mud) from being washed away.

We have an underground RCC water tank that has developed a crack in its base due to the roots of an adjoining tree. The tank feeds the workers’ toilet in our factory. Now, in addition to losing precious water, we now use a lot of energy in refilling this tank. Will fixing ceramic tiles or placing a plastic tank in the cavity help? We seek a quick and inexpensive solution that would last us till we can locate and create a new water tank in a fresh location as we are not keen on harming or dislocating the tree.
Manu Jain, New Delhi

I’m very happy that you are considering an alternate location for the water tank, instead of uprooting the tree. In this case, you need a cheap solution that will help hold the water, even if for a few days. The solutions involving ceramic tiles and a PVC tank are good but temporary (given the situation) as well as expensive. If the strong walls of the RCC water tank succumbed to the force applied by the roots, I doubt if ceramic tiles or a prefabricated PVC tank wall will last.

Casting another strong RCC tank inside the existing cavity will give us better results, but this will drastically reduce the holding capacity of the tank. This solution is expensive, both in terms of time and money. Fortunately, since the water is not being used for drinking purposes, we can consider applying simple waterproofing solutions.

Using any form of petroleum tar-based waterproofing layer could create a short-term barrier, but the tar may further attract the plant roots, leading to a quick collapse of its fragile structure. Joint sealers such as M-seal may also fail for lack of adequate surface area for gripping and, like the waterproofing layer, will require a dry surface for better bonding.

I would suggest you look at joint sealers along with epoxy-based surface sealers as a possible quick solution. While the joint sealers will temporarily seal cracks, the brush-applied epoxy compound (such as Araldite) will give the entire surface the desired (short–term) strength and grip. Don’t forget to use the special-purpose (4 hour) quick-setting product instead of the regular 24-hour setting one as you will need at least 2 to 3 coats of the same for desired results. Make sure that the water from this tank is not used for human consumption as contact with these chemicals may prove to be harmful.

I have extended my living room by 4 ft by taking the balcony inside. The living room is now 16 ft x 20 ft. The flooring of the original living room is mosaic tiles. The contractor has done a plain PCC over the balcony floor. The joint between the two floors is uneven and at places has a height difference of 2 mm to 3 mm. Also, the floor of balcony area itself is undulating. I want to lay PVC floor tiles on the entire surface. The vendor has asked me to level the floor. The contractor says if he scrapes off the cement floor, it will come out in large clumps. To compound the problem, some of the mosaic tiles have chipped edges and there are gaps between tiles at places. What is the easiest solution to level this floor and make it ready for installing PVC floor tiles?
Kamal Jain, Jalandhar

The civil contractor is right. There is no way to scrape off the uneven surface. An expensive, time-consuming method is to remove the top 40-mm layer of PCC (plain cement concrete) and re-lay the same in level with the existing mosaic tiles. Another solution, if the surface is not too uneven, is to use a stone-grinding machine and grind the excessive PCC off the surface.

Alternatively, here’s a process that is both simple and cheap.  Make a levelling compound by mixing plaster of paris and Fevicol-SH. Mix the two compounds well on a flat, dry and dust-free surface by using a metal plate (patti, typically employed by painters). Now spread this compound evenly over the entire PCC surface and on the existing mosaic floor (to atleast 1 ft). This is done to reduce the drastic level variation between the old and the new floor.

This application is like putting a layer of plaster of paris on an uneven wall before applying paint. Make sure you fill this mixture in all the undulations and that the layer isn’t too thick as this may lead to surface (shrinkage) cracks. Allow the surface to dry completely (for about 24 hours).

Other concerns while laying a vinyl carpet over the existing floor are gaps between tiles and chipped edges. While the gaps can be re-grouted using the above-mentioned tile grout, the loose or broken pieces will need to be removed and the empty sections filled and patched up with cement mortar (1:4) such that it is in level with the surrounding tiles. This will ensure that the undulation will not be felt through the new vinyl flooring. Also check the level of the floor under the doors. You may need to shave the door shutter slightly from the bottom to account for the thickness of the vinyl carpet.

In case the existing mosaic tile skirting is flushed with the surrounding wall plaster and you choose to retain it, I suggest you cover it with 8-mm to 10-mm-thick plywood or fibre reinforced cement board (of equivalent thickness) as the vinyl may not grip the existing mosaic surface well. But in case the skirting is protruding out of the wall, remove it completely, re-plastering and do the above-mentioned ply/cement board detail. Use termite-resistant and waterproof ply only.

I would like to install a pre-polished Corian counter in my bathroom. What is the process? Do I need to cast a thin RCC slab, or is it safer to rest the counter on an angle iron frame?
Ritu Jha, Gurgaon, Haryana

An RCC slab is still the most durable option available in case you choose to cantilever the counter with no supporting structure from the floor below or a furniture item to rest it over. But all this takes time and energy, and even ends up making the counter slab look thick and ugly. So, if you are not planning to use the counter slab as a table to climb on top of and change the light bulb (so to say), you could easily create a simple and stable structure with mild steel (MS) angles, rectangular pipes, etc, to sleekly house the desired Corian surface on top.

In theory, we need to create a strong frame that can be grouted to the rear wall. This frame will have to be first enveloped with a 0.5-inch-thick (or 12-mm-thick) waterproof plywood or fibre reinforced flat cement board that will give us a smooth level surface that will be eventually covered with the surface material, ie a 12-mm-thick Corian slab in your case. We need to ensure that our core structure is smaller by one inch to house the two materials (plywood + Corian). For example, if the depth of the counter is going to be 2 ft, then the depth of the frame will have to be 1 ft 11 in. The frame can be made out of MS angles (1.5 in x 1.5 in x 6 mm thick) or 1.5-in square hollow MS pipes.

Once all the sides of the frame are complete, you will need to add a member perpendicular to the rear wall if the overall length of counter exceeds 2 ft 6 in. This should be done such that the gap between any two members is not more than 2 ft and that no member interferes with the plumbing below. In the latter case, add an extra member on either side of the pipe below.

Fix the angle frame with dash fasteners to the rear wall for best results. Now take a 12-mm plywood sheet and create a mock counter with the back splash and front nosing/drop as desired. The vendor could take dimensions from this rough frame and deliver the raw material to be assembled at site, or carry the plywood counter itself to prepare and deliver the final product. Eventually, the wooden frame can be glued or screwed to the MS frame below. Make sure the final installed counter is covered in plastic or bubble wrap till all other work is complete.