Monthly Archives: July 2011

Concrete roof rules

The basic skill to cast it has percolated down to remote villages too, however over designed or wrongly mixed it all could be.

Look around today. Everywhere everyone is in a hurry to cast RCC concrete roofs. However, if this line is taken as a compliment for RCC roof, sorry it is not. It is a statement of concern, about a versatile material being indiscriminately used, including in areas such as sitouts where concrete roof is just not necessary.

Let us accept the fact that reinforced cement concrete roof, commonly called RCC or concrete roof, has been a great discovery. It has solved innumerable problems including providing security and opened up design options such as shell roofs which were never dreamt before. Though only qualified structural engineers know exactly how to optimally design these roofs, the basic knowledge and skill levels required to cast a roof has percolated down even to the remotest of villages, however overdesigned or wrongly mixed it all could be. No wonder, small bus shelters, not so permanent milk booths, park pavilions, storage sheds or village houses, which in the past were roofed with sheets or tiles, today get RCC roof. Projects tendered by the government compound this overdoing, by insisting upon concrete roof everywhere.

Steel and cement are among the production industries that consume humongous resources; hence have very high embodied energy. Of course, today they top the list of materials for being most popular and widely used, as such are seemingly irreplaceable.

Additional burden too

Most of us forget the additional burden of the temporary support called centering or shuttering, which after some use becomes a total waste. Among all the needs of water for mixing and curing, it is the roof which singularly consumes the largest quantity per square feet. Segregating the steel reinforcement and concrete from the discarded old buildings or irreparable cracked roofs is hugely time- and resource-consuming. Even if we use cutting machines and succeed in this segregation, it is impossible to reuse old concrete but for some land fill.

Unlike any other traditional roof, a concrete roof once cast can never be returned to its original ingredients, and will continue to burden the earth for a long time to come. Till recently, concrete roofs were labour intensive, hence provided the much-needed labour opportunities.

Now-a-days, manual casting is being replaced by Ready Mix Concrete in large cities, further reducing on people benefits. Considering concrete is far from being eco-friendly, the way it has relegated all earlier roof options to the back seat is both amazing and alarming.

Having said this, let us realise the solution does not lie in discarding concrete, but in using it judiciously. Also, there are numerous methods such as arch panel roof, filler slabs and ferrocement technology which basically employ steel and cement, but create alternative options for construction.

In these days of rising price of RCC roofs, it is time to explore such alternatives, for both ecological and economical reasons.

Right designs

Mangalore tile roofs are eco-friendly and amply prove their durability and ease of installation. 

Mangalore tile roofs have often suffered for no mistakes of their own. Of course they are fragile, demanding careful handling; the bottom surface attracts salt deposit and there could be more cobwebs! However, the decreasing popularity is also a result of our generation forgetting how to build with tiles. Innumerable mistakes have been committed by us in design and construction, especially in areas such as roof angles, support members, corner junctions or managing rainwater. It’s but natural that people have lost faith in the system itself. Additionally, alternative roofing ideas with ample flexibility, security and availability have marginalised the tiled shelters.

Labour intensive

Being labour intensive and based on local resources, the fact that tiled roofs are eco-friendly goes without debate. Millions of such roofs all over India amply prove their durability and ease of doing. The frequent statements about “how cool traditional homes used to be” is a testimony to their contribution to comfort.

Traditionally, all tiled roofs rested on timber sections called joists, rafters and purlins, when it was easy to lift the tiles to get in. Nowadays, mild steel sections have replaced them in most areas, due to shortage of carpenters and cost considerations. Welding an additional bar under the tile converts the support into a typical grill, just like at windows, ensuring safety and security. The common complaint is that the bottom surface collects salt deposits and develops white patches. Painting this face with tile colours reduces the visibility of salt patches. If additional budget is possible, clay ceiling tiles can be installed under the main tile, providing aesthetic appeal and increased thermal comfort.

The right slope

Unknowingly, many designers provide a low angle for the roof, which decreases the overlap between tiles and rainwater slips into the building. A minimum of 25 degrees slope is necessary for any tiled roof, though in the past, much steeper 33 or 45 degree slopes were provided. The junction of roof with wall surface needs to be well done with part of the roof going into the wall and finished with curved water-proof beading. A sloping roof with tiles demands basic geometry in the plan form, to ensure proper matching of the slope angles, high points called ridges, low points called valleys, edges called eaves, ends called hips or gables and such others. As such, even before we start the construction, calculations and drawings become necessary to ascertain that the roof sits properly! Carpenters who routinely do such roofs have it all in their mind.

If tile has to top a RCC roof (such double roof lacks design logic), placing the tiles on mortar strips is a better detailing than pasting all tiles over a thick layer of mortar. Once pasted, locating water seepage and replacing the cracked tile is very difficult. Both tile and mortar being good conductors of heat, such roofs do not provide passive cooling.

Mangalore tiles might have been criticised, but building with sloping roofs has not reduced much, suggesting a continuing validity for the idea.

The beauty of red tiled roofs

Despite several hurdles in their manufacture, Mangalore tile roofs are amongst the most eco-friendly.

If we were asked to sketch a house when we were young, we typically would have drawn one up with sloping roofs. If it’s in South India, one may safely assume, the roof is imagined to be in Mangalore tiles. The choreography of red-tiled roofs partly covered by the lush green canopy of trees in Kerala and Karnataka or the villages in flat lands of Tamil Nadu and Andhra creating a new skyline are etched in the memories of every traveller.

This essay could have as well begun asking where were Mangalore tiles first made and without winking the eyes, the answer would have been ‘Mangalore’, a coastal town in Karnataka! Even before the missionaries of Bassel Mission from Switzerland landed in coastal Karnataka during the 18th century, rounded roof tiles, now called as country tiles, were popular all over the south. Produced in most villages, these were hand made, had no interlocking facility, and were more prone for cracks.

New concept

The Missionaries now settled at Mangalore introduced the concept of clay tiles which are flat, grooved, interlocking, and also ensured they are mechanically pressed for greater strength and durability. By the early 19th century there were numerous factories along the seaside stretching from Kundapur to Kollam, producing these tiles.

Tiled roofs are part of building with clay, possibly amongst the most sustainable construction systems. They create an induced passive cooling, wherein the tiles and the air underneath get heated up and the hot air is allowed to escape outside in the gaps of the tiled roofs. This ensures cooler air from the floor level rises up. The secret behind the thermal comforts of traditional tiled homes lie in this natural air draft, besides the soft light they let in through the tile gaps. There are minimum windows to ensure least heat gain and glare-free living. Of course, higher the roof height, the greater the benefits.

Incidentally, much of the tiled roof we see in large cities may not be actual tiled roofs! There is a trend of topping the concrete roof with tiles, often popularised by house builders. People believe this would eliminate the possibility of theft by removing tiles and stop all water leakage but still ensures that ethnic Mangalore tile look from outside. It’s comparable to providing one roof type over another roof type! We are employing two independent roofs together; hence the costs are high. The tiles are pasted upon thick mortar, hence heat transfer increases and replacing a broken tile is very difficult.

Irrespective of how the tiles are used – as original roof or cladding – Mangalore tiles are among the cultural preferences of many communities. Despite the onslaught of newer technologies, dwindling manual labour and depleting soil resources, we find tile roofs among the most eco-friendly ideas. It is good to see their continued popularity and revival by architects even in cities like Bangalore. As such, it is important now to focus on how to design with tiles.

Right design, big benefits

The roof has to be specifically designed to take and transfer the load to the walls or beams. 

Experiment: Time to go in for new roof designs

Where does the efficiency of roof design lie? In green architecture, it is rooted in how far we work with nature or go against it and then use technical solutions to make up for it. Incidentally, majority of the people cannot afford the costs of mitigating ecological problems created by our designs and silently suffer the house which is a heat trap or pay for maintenance costs of a multi-storey apartment block or clean a school that goes wet with every rain. Very few can pay for air-conditioning, but if the majority seek this solution, the city would suffer!

Roofs play a major role in the efficiency we are discussing about — be it in cracks, leakages, heat gain, activity space, structural costs or design aesthetics.

The roof has to be specifically designed to take and transfer the load to the walls or beams. As such, the stability of a structure is much dependent on its design. Simply stated, there are two ways by which the weight affects the constructed member: the factor that may bend the member, known as tension; and the factor that may crush the member, known as compression.

Wood, steel, reeds, thick stone section and such others which act like a beam are good at transferring tension loads. Arches, vaults and domes, along with other related masonry details like corbelling, take good care of compressive loads.

All roof options resolve how these two forces of tension and compression are balanced.

While professionals freshly calculate the design values for each roof case by case, where they are not involved, as is the majority, people simply do the roof by precedence. Most professional follow some default practice which they keep repeating, where the building owners have no clue at all.

Design options

Would we believe if we were told about a dozen types of roofs in Bangalore? The following list is amazing but true. Sloping thatch roof; sloping Mangalore tile roof (with or without ceiling tile); Hourdi block roofs (in slope & in curve); flat roof with stone slab; normal RCC roof (flat, sloping, curved); filler slabs in flat, slope or curve (wpc tiles, Mangalore tiles, cement blocks, clay hollow blocks etc as optional materials as fillers); arch panel roof; Madras terracing; wood plank roofs; vaults in RCC or masonry; masonry domes; corbelled roofs and sheet roofing (in varied materials). Including the variations, the list extends beyond two dozens! Each comes with its set of climatic comfort and carbon footprint; issues of execution and attractiveness; cost and benefit; problems and potentials; appropriateness and design style; and such other criteria. Incidentally, most of us do not explore all these options before deciding the final form of the building.

Options like thatch roofs could be totally out of race for an urban school, yet searching for the right fit would not only popularise some of the lesser known ideas, but also generate newer ideas.

Flat, sloped or curved?

Despite the options available, most people cast a flat RCC roof, often with unnecessarily heavy design.

If we look at Leh, Ladakh, not even one building would appear with sloping roofs, but in total contrast, all the structures in the Thiruvanthapuram Fort area have only sloping roofs. The island of Santorini is full of small domes over every house, while the floating villages in South-East Asia have thatched, hipped roofs. After all, the wall is a simple surface standing upon the ground that could be erected anywhere, with pillars that could be just done by keeping a pole vertically straight. It is the making of the roof that throws up challenges, to ensure the gravity force of the earth does not gobble it down.

Every region has discovered an appropriate system, suited for the local geography, geology and climate. Humid and rainy areas would have slopes, while snowfall areas would have a steep slope. Clay tiles, thatch, slates and such materials are found on these roofs. Hot and dry regions with ample supply of mud, like Rajasthan, built flat roofs. If good mud is not available, people tried out stone slabs, reeds and alternatives to gain flat surface.

Curved roofs are often mistaken as a new concept, they are actually very old. The Treasury of Atreus at Mycenae, Greece, has a corbelled dome roof built at least 3,250 years ago and still standing tall! From the igloo homes of the polar regions to the Mediterranean islands, early architecture explored roofs other than flat surfaces. In India, temple gopuras showcase a unique capacity in building construction. Of course, all the ancient curved roofs were geometrical following the rules of mathematics, invariably built with masonry using burnt bricks or stone.

Emerging trends

Modern technology enables us to build any type of roof anywhere! The spans have increased, curves have become flexible without rigid geometry, construction budgets have become permissive and many newer roof forms have been discovered.

The design variety has increased, but the design logic has suffered. While some are prohibitively costly, some others increase heat gain into the building. Fancy ideas and false roof looks have also found their place.

Despite this increased range of options available now for roof construction, most often we see people routinely casting a flat RCC roof, often with unnecessarily heavy design. If we combine creativity with design logic without compromising on cost or climate, more appropriate roof architecture would evolve.