Flat clay tiles make a hourdi roof easy to maintain, and the aesthetics are stunning.
What is the most critical part of a house – foundation, wall, roof, openings or staircase? The obvious answer is the roof, hence maybe the metaphorical meanings when we say ‘roof over the head’. It is of course, less visible than the walls, hence often gets lesser attention, especially where front elevation looks are more demanding.
While walls enclose the room providing protection, it is the roof that provides the true shelter. Unlike any other element of domestic architecture, here we face multiple issues to grapple with. Immediate thought could be about the structural issues, as to how to support the roof itself. Traditional architecture worked around local materials to solve this issue, while modernity has opened up innumerable options.
People across the societies have felt the need to innovate on roofs, either because there never has been a totally satisfactory design or roofs have had the scope to play with them. While the flat and sloping roofs have dominated at large, all other options have continuously knocked at the door.
Contemporary architecture using advanced software to generate a profile, computer-controlled manufacturing of components and on-site technology to assemble them all have created some amazing roof forms across the world, with architects like Frank Gehry and late Zaha Hadid leading the pack of innovators. But do they mean in our small town contexts we need to be deprived of them? No.
The Learning Centre located at Salem proves the point. With the passion of promoter Sanjay and involvement of builder Arun, their team has managed to build a magical school building. It’s it not the just the roof which is special here, but the totality of design and building.
As such and with the concept of minimising cement and concrete, the design revolved around mud and clay.
Roofing with clay Mangalore tiles has been around for over the centuries, but it needs to be only sloping due to the interlocking grooves. By replacing them with flat clay tiles, one could create a curved profile. However, placement or direction of the clay tiles is important to ensure smooth curvature and to ensure that rainwater does not clog anywhere.
Typical computer formulae will not help in resolving the S-curved roof which adorns the school building, but only an experienced structural engineer like Ravindranath could resolve it. The structure also has domes, vaults, varied kinds of filler roofs, stone slab roof and such others. As such, the behavioural patterns of roofs need to be studied and equilibrium planned for.
Of course, special attention is called for while assembling the roof for labour safety, joint filling and waterproofing the joints. The fabrication also has to be done to precision, lest the blocks may not be seated properly.
Curved clay flat hourdi roofs have advantages other than merely the novelty. They perform better for speedier construction, reduce steel with their lighter weight, ensure different aesthetics as against RCC ceilings and are easier for maintenance.
Be it the roof, wall or the whole building, architecture should not be driven by the idea of fancy, however attractive these new designs may appear to be. All the elements of design and components of construction should together aim to provide a holistic perception and experience.
Mangalore tiles could be used to build walls, roofs, pavers and more.
Majority of Indians were and still are used to having a space in the house where they could be eating at noon, children playing in the evening and cousins staying as guests sleeping there at night. The space we enter into on the first floor, often called a family hall, is even now considered for such varied roles.
Termed as ‘multi-functional spaces’ in architectural language, they have been among the basic principles of saving resources by minimising the need for multiple rooms for multiple requirements. While this has been commonly known and is continuing, what is forgotten is the idea of materials being multifunctional.
Buildings had to depend upon the few local resources for all construction needs, so people learnt how to use them with minor modifications. This also led to high theories like ‘single material approach’ much advocated by thinking architects such as Shankar Kanade and his brother Navanath. We may connect the visual powers of Taj Mahal, White House or Red Fort for their single material.
When Govindarajan, retired from IFFCO, desired to have a small farmhouse in his land, he could not but notice how people around the village built. He chanced to see a wall built with Mangalore tiles and wondered what’s the roof tile doing in the wall.
Discarded low-quality pieces and tiles from demolished houses not good for re-use get the common preferences here. Besides, one could see them on compound walls, edging a pathway in the garden, topping a parapet wall or even as pavers in broken condition. These tiles are very good at compression, hence we can load lots of weight on them!
Easy to build
A wall with Mangalore tiles is easy, cheap and fast to build with. With the rock hard tile transferring most of the load, the role of mortar joint is reduced to a levelling course. Cement mortar does not stick well to the surface, hence stabilised mud mortar is both appropriate and economical.
Routinely, these walls were inside and outside plastered. If built with stabilised mud and left exposed to sun and rain, they can perform very well on multiple fronts. They keep the house much cooler, thanks to an undulating and micro-shading surface which does not absorb much of solar heat. With a coat of lime, the walls come alive with a rare texture.
Local practices of today are being ignored in the face of regional or global ideas, but let us remember when these ideas were attempted in the past, they were not called as local practices.
They were the mainstream practices of the day, with no exposure to the global of the day. Apparently, these neglected and seemingly insignificant ideas were sustainable, while much of our celebrated newer and engineered construction ideas do not seem to provide a trouble-free stay for even two decades.
Without localisation and without realising the multifunctional potentials of each material and each technique, we cannot achieve a sustainable future.
Using stone could reduce resource consumption in the construction sector.
Human civilisation is an ever optimistic one, with a ‘never say die’ attitude. It is this undiluted trust in us that makes many people possibly believe that the current crisis of climate change can be controlled across time. They may quote many instances from the indomitable human spiritto illustrate how someday we will reach our goal of reversing the climate crisis.
However, we appear to be relying upon ideas rooted in the same technology or innovativeness, which in the first place created the crisis, to solve the crisis. This is not to demean new research, but to suggest that we also need to relook at the past to look for possible solutions. After all, days of the past did not lead to excessive consumption or climate crisis. Hence, they may offer some sober and steady ideas, however few they may be.
Using stone slab as a roofing material can be an illustrative case. Stone roofs have been popular across India. Slabs were cut into pieces two to three ft. wide and up to 12 ft. in length, depending upon the strength of the material. Stone or brick pillars were erected, upon which stone or wooden beams rested, and they were topped with stone slabs, joints finished with water proofing.
The very mention of this system would make experts in modern construction consider it as primitive. It cannot be used for making modern multi-storey office complexes nor could it be economical given the time constraint we face in projects. The present rate of urbanisation would deplete the local supply in no time.
Many such arguments can follow to delist the option for ever, unfortunately forgetting that stone roofs can be a solution in a few appropriate contexts. Today we need hundreds of such solutions to collectively reduce the massive concretisation of the construction industry. With cement and steel dominating all major items of works in every building, from foundation to water proofing, from floors to wall finish, it is imperative that we introduce a few items where we can minimise such manufactured materials.
What is strange is the way we forget our present practices and lifestyle the moment a new option is on the horizon. Why cannot a known solution co-exist with a new, innovated one? Why discard proven design and construction possibilities, when they continue to have logical and practical applications?
This is not an argument just to support stone structures, but to support thousands of such small, appropriate and sustainable ideas that could reduce resource consumption.
Ask any one with experience in construction about formwork and shuttering, they are likely to say how it is time consuming, expensive and often a headache. More so in urban centres where it is difficult to get skilled people who can erect the formwork to precision. To cast a flat concrete roof, we need to create the flat surface in advance, complete with supports and capacity to take the load of the roof. If it is complicated with levels, curves, varied thicknesses, different geometries and such others, that’s then a prescription for errors.
Incidentally, our traditional buildings escaped this burden – most often they lifted the wooden beams and masonry materials one by one to assemble at roof level. Flat or sloping roof profiles made no difference to this simple technology. Of course, they would have erected temporary supports for some kind of forms like domes, but most common buildings were built without formwork.
Would it not be wonderful to build roofs without or with minimum formwork? After all, what we finally need is the final roof and not this temporary roof support. This thought led our modern engineers to discover methods that would use components made ready on floor to be lifted up. The initial ideas explored pre-cast beams to replace wood and then place smaller segments of roof elements made in site, which led to the arch panel roof which uses no formwork at all. Everything is assembled on top.
Considering they are multiple elements, ensuring a water-proof surface has been a challenge in this technique. To that end, some prior experience is required to do the arch panel roofs. What if the roofs are homogenous? We can avoid many irritants like cracking joints, water proofing the roof and such others. The scientists at Indian Institute of Science, mainly Dr. Jagadish and Dr. Reddy, focused on the traditionally popular domes and vaults, to know how to re-introduce them as a cost cutting measure.
The main inspiration came from roof forms in historical buildings, primarily revolving around domes and vaults, i.e. curved roof forms. Architect Hassan Fathy had demonstrated an ancient technique to build them without any formwork and slowly the idea percolated to India also. These ideas are generally cheaper, appear more unique and make the job much simpler. Accordingly, we see them at the Development Alternatives building in Delhi and the building centre in Auroville. Slowly many architects practising alternative design ideas took these ideas and tried spreading it.
While the pre-condition for the research was to reduce cement joints, it also led to newer architectural forms and challenges. Coupled with further alternative materials like stabilised mud block, the roofs without shuttering met the expectations of many clients.
These alternative roofs create larger volumes inside, in case of hot regions, increasing the indoor thermal comfort. Mostly they are made of natural materials, thus reducing the embodied energy of the structure as a whole, becoming green.
The human mind never ceases to think and explore, even when we know that all the innovations may not be beneficial, for the simple reason that one valid ideation among a hundred failed ones is still worth the effort. On a comparative note, the building industry does not indulge in continuous and steady research; hence a whole lot of established concepts continue to be employed. As such, occasionally when a great new idea springs up, we should not lose out on the opportunity to explore it or even employ it.
The last century has seen innovative roofing ideas such as hyperbolic structures, catenary curved vaults, tensile membranes, domes without formwork, prefabricated shelters, geodesic domes, ferro-cement roof panels, space frames, and pre-engineered structures. Mostly known to architects and engineers, these high-ended ideas demand a thorough knowledge of roof behaviour, technological expertise to manufacture the components and skilled labour force to execute them at site. However, these conditions do not mean they should be excluded from routine public buildings and normal house constructions.
Funicular shells also belong to this set of alternative solutions. Many architects have used this idea extensively in their projects. Shell roofs are most economical in short spans of 1.5 to 3 metres, so the roof area is divided into grids of judicious spans, preferably square, even though rectangular shapes are also workable. These grids are formed by RCC beams, virtually creating a framework of side supports with the central part to be filled by the shell roof. The central filler part is a two-way curved shallow dome-like profile generated by the required geometry. Upon the form work, bricks are laid in a chosen pattern, which becomes a self-supporting system, held by the beams in the edge, with minimal steel and concrete.
In case of large halls, the main columns can be in the designed spacing, but the space in between needs to be subdivided by cross beams to take the shells, while in areas like bedrooms, each room can get one roof. The edges of this shell roof shall be thick to balance the thrust effect of the roof weight, becoming thinner as it reaches the top centre. The upper part of the shell can be filled with brick bats or lean mortar to get the level floor finish as required. The weight of the flooring material pushes the shell down, stiffening it in the process and making it stable.
Achieving perfect geometry is very important to ensure arch action within the curved shell, transferring the loads to the side supports. Considering the detailed calculations required for roof form, making fibre glass formworks in smaller parts becomes even more economical than individual formworks, besides the possibility of them in different combinations to get roof forms.
Once these basics are followed, a wide variety of patterns can be worked out for the roof. What initially appears as complex, emerges as a do-able idea.
How is the Taj Mahal roofed? It is a question that need not be asked because all of us know it is roofed by a dome. The commoners may not know of the technical details about the bulbous shape atop, but the technical experts can analyse its form and behaviour from different perspectives. Domes have dominated the historic skyline for centuries and continue to have a prominent role even today.
However, most buildings that we build today cannot take a dome for reasons like upper floors are ruled out, they effect the possible terrace, the rise of the dome may create unwanted interior volume, expert skills are required to do a stable dome or that they are possible mainly on circular plans. Does anyone wish to have a dome shape, without any of the above limitations? The answer lies in shell roofs.
Technically, shell roofs are not the same as the domes, but share a few common factors like arch action for transferring the building loads. Among the viewers, of course, they evoke similar feelings as domes due to the curved form. The shell roofs can be in thin concrete or masonry, curved in form, spanning short distances. Unlike the domes, they are not hemispherical in form and do not rise high, as such enable useable floors above them.
In India, many well-known institutions like Structural Engineering Research Centre, Auroville or Anangapur Building Centre have explored shell roofs for application towards cost-effective, fast track constructions with an alternative aesthetics. The former ones worked with varied materials including concrete roofs, while the last one worked mainly with natural masonry materials. The building centre worked with what is technically termed as funicular shells based on the catenary curve principle, which suggests the curved shell roof would transfer loads to the edges in an axial or perpendicular manner, creating least of deflection and turning moments.
For a flat roof
Shell roofs normally enable a normal flat roof, create beautiful patterns in ceilings, speed up the work, save much of concrete and reduce steel consumption by more than half. While the funicular roof best suits square profiles, it can be constructed atop rectangular plans also.
Of course, the profile needs to be evolved carefully and the formwork done to the specific curves. For all the advantages of funicular shell roofs, it demands expert advice and masonry skills. Once trained, the team may venture into even complex roofing solutions.
A vegetable garden can be organised on any flat roof.
When we visit our relatives in a village, the way they walk into the backyard, pluck vegetables and cook a delicious meal is enviable to every one of us. We all know that the vegetables we buy from a shop in a city are not fresh, however good the air conditioning in the truck or the shop be. It is not that urban living does not permit time for gardening, but the logistics such as land, manure, pest control and water management act as a deterrent.
But with increased awareness about the organic movement, many house owners have now started growing their own vegetables, though in small quantities.
A typical terrace vegetable garden can be organised on any flat roof with pots. While cement pots are easily available, they tend to absorb heat and make even the mud fill inside slightly hot.
As such, earthen pots are better suited. Between a wider and deeper pot, the deeper one should be chosen with drain hole at the bottom.
Any horticulturist or nursery expert can advice on vegetables that grow well in pots and the kind of preferred sunlight. Fixing light weight shade nets on part of terrace extends the life of plants and increases the yield.
Vegetables of shorter time cycle normally have shallow roots and better enjoyed because of early yields.
The main advantage of growing vegetables on pots is the ease of maintaining them all. Shifting for more sunlight, changing mud mix, doing replanting, changing pots and such other tasks can be handled by any one.
The pots can also be placed on balconies that receive direct sunlight. There are vegetables that grow on creepers, which may find the terrace floor too hot during summers. In such cases, place some dry palm leaves where the creeper is expected to grow into.
The overall weight on roof slab needs to be considered, hence it is safer to line up pots with walking space in between which does not lead to critical point loads on roof.
More green options
There are many other options to have more green within the building — growing wall climbers such as Ivy, hanging plants and drooping creepers, grassy lawn on sun-lit terrace, creepers on pavilion or pergola roofs turning them into leafy roofs, herbal gardens and such others.
While all these ideas appear good and commonplace, it is always safer to take expert advice at least in the beginning. Dampness in walls and roof could be a predictable problem if the building is not safeguarded against water seepage.
Potted plants require periodic mixing of mud within the pot and total replacement occasionally. Most people living in cities need advice on plant material, sowing period, crop months and plucking time since tghey are totally disconnected from nature.
Terrace gardening is a way to connect to nature again.
Besides the open yard, the only other option to grow greens is on the terrace, which incidentally is not a new idea, if we include the Hanging Gardens of Babylon in the green roof list.
We build on the ground, so theoretically the ground gets transferred to the top of the building. If the ground could have been green, should not the new ground up above also have the options of going green? Of course, yes. Besides the open yard, the only other option to grow greens is on the terrace, which incidentally is not a new idea, if we include the Hanging Gardens of Babylon in the green roof list!
It is the risks involved with roof-top cultivation and lack of remedial solutions for the common problems that makes us leave the roof largely untouched. It is curious that the major driving force today for re-discovering green roofs is not the urge for growing flowers or vegetables, but ecological concerns.
Roof-top plants reduce both the heat gain into the building and heat reflection into the atmosphere, cooling both the building and the city.
Thanks to the evaporative processes, the relative humidity gets a positive boost, though one would need large areas of green roofs to achieve a noticeable difference. If we could begin in a small way today, someday that large area could be achieved.
Three key approaches
There are three basic approaches to greening the roof — plants directly on soil medium; assemblage of potted plants; and softscapes like ground creepers, vines or lawns. Let us first look at the general garden.
Direct planting reaps the maximum benefits, but requires the maximum attention too. The roof needs to be perfectly water proof and preferably designed to take the extra weight of planting. Even if the roof is guaranteed against cracks and leaks, it is advisable to fix a layer of impervious plastic-based lining on the roof top, before filling the surface. It also ensures smoother flow of extra water at the bottom, following watering of plants or long hours of rain.
There have been cases where people tried gardening on existing roofs, with inadequate roof slopes. In such cases, good slopes need to be created during the plastic lining, before the soil fill. If it is an old house, adding a water-proofed concrete layer could be considered before doing extensive gardening.
The patch where we go green and the leftover terrace have to be edged with appropriate materials such as brick on edge. Often, this edge gets ignored and the whole terrace gets wet due to water seepage from the sides!
Lightweight coconut pith compost, peat moss and such manure soil mix are among the better choices. When we do direct planting, it is easier to keep the soil moist, unlike the case of potted plants. Incidentally, the soil mix should not absorb too much water and retain it too, for this increases the load on the roof.
Plants with only fibrous roots are possible as the top soil mix will be just 4 to 6 inches deep. Of course, there are options in shrubs, herbs, flowering plants, crotons, vegetables, orchids, rose, small palms and such others. For those willing to spare their energies, this is a good enough range!
Thatched roofs have strong visual character, making them desirable in resorts, retreats and farms.
The making of a thatched roof.
Indian construction scene faced a strange dilemma during the colonial period when the traditional methods got sidelined as kutchha , temporary and undesirable. Incidentally, most of what our elders built was kutchha using local materials. Barring palaces and temples, may be we were a kutchha country! Anyway, the divide between temporary and permanent continues, e.g., even now we need not take local authority permission for kutchha constructions.
Thatched roof, which has been the single largest roof type in India before the advent of new ideas, is amongst the hard hit in the story of kutchha . The widespread prevalence of thatch could be attributed to its perfect fit for rain-fed, warm and humid climate. Easy availability, simple skills required and minimal heat gain made it a popular choice. By default, it is very local, cheap, replaceable, and hence amenable to self-help construction.
In every drive through the countryside of south India and often even in city contexts, we come across thatched roofs. Mature grass reeds are selected, surface cleaned, seasoned with slight wetting which also make them pliable, sun dried to get the desired water content and then tied together as small mats. Such mats are then tied to the bamboo or wooden purlin below. Additional layers of grass are laid, tying them occasionally to the lower layer. Coconut or arecanut leafs are also used likewise, as an alternative to thatch. Often, the woven coconut leaf is the underside for thatch.
In many modern versions, there are structures with G.I. sheet roofing as the underside; then topped with thatch. This clears the fear of fire and rodents. Grass reeds can be tied together like broom sticks, topped on each other to get a very dense roof layer, which lasts many years. A horizontal projection at the wall top eliminates rats and snake movements. The major complaint against thatch has been its fire safety. Chemically-treated thatch has been attempted, which increases the fire resistance. However, it is better to leave thatch as it is, and have a fire-graded underside which localises the fire, in case it occurs.
Thatched roofs have reduced, yet continue to be among the best options, given certain criteria. Their strong visual character makes them desirable in resorts, retreats and farms. Being temporary, the choice of continuing or replacing them is open after 6 to 10 years, the average life span of a well-done roof, with nearly 30 degrees slope angle.
The highlight of thatch in these days of climate change could be that they are, by far, the most sustainable roof form.
In many villages, thatched roofs are still being made, but if not supported, no person with skills for making thatch roofs would be around in the generations to come. We may lose the most eco-friendly roof to the annals of history.
May be the act of designing is all about exploring and discovering the hidden potential. Perforated filler roofs fit perfectly into this theory.
The design profession is strange. Often it does not take a project context as a mere fact, but loves to fantasise it. Designers play around the building form and attempt different combinations only to explore the options available, where some good options actually never get built!
Instead of using materials in their basic and simplest form, people try exploring the myriad ways of using them to get a variety of effects from the very same stuff. May be the act of designing is all about exploring and discovering the hidden potential.
Perforated roofs are a perfect fit to the above story line. As the sun moves across the sky, the sun’s rays move across the room, as if the roof acts like a cloak.
This idea started with architects placing coloured waste glass bottles within the roofs, randomly or in a design, creating a picturesque interior. Despite the attraction, issues such as water leakage, breaking of bottles, and shifting during concreting halted the popularity of this idea.
Jaali roofs on steel frames are an extended version and comparable beauty to watch, but appear weak in areas of high security threats.
Replacing steel frame by RCC and embedding the jaali blocks into the slab gives us the normal secure concrete roof, but with the playful small void, each acting like a small pinhole camera.
Adhere well with concrete
The half-cut hollow clay blocks are by far the easiest materials to embed in the roof, though other locally available options could also be employed. Reinforcement rods are placed as per structural design, which should be based on the block sizes. Upon the normal shuttering, hollow blocks are placed with holes up facing skywards with reinforcement rods in between to get the chosen design format.
Structurally, this system acts like a filler slab; hence the blocks adhere very well with concrete, avoiding water seepage possibilities. These blocks can be patterned on any geometry, as long as the steel rods are in position as desired. The normal concreting is preferred more than the ready mix, to ensure the holes are kept clean.
Placing a piece of glass on top ensures rain protection.
Also as skylights
The perforated roofs also act like skylights, though with much lesser direct light.
In hot dry regions where light comes with high glare and sky brightness, it is climatically difficult to have large clear glass skylights, hence this design idea with small holes becomes a good fit that filters in soft light.
Incidentally, in hot regions such as Rajasthan, even the large window openings are replaced by jaalis of small voids!
The challenge is to increase the daylight factor, without increasing glare and sharp shadows. However, there would be increased indoor heat gain; hence roof bottom ventilation is a necessary provision.
The perforated RCC roof does not become lighter than the normal filler roofs, but visually appears lighter.
Besides the eco and aesthetic benefits, the major attraction for perforated roofs could be that it’s a do-able alternative idea, which demands no extra construction skills.