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Let us have green and cool roofs

The focus area of the ‘Green Sense’ column has been eco-friendly and energy-efficient ideas towards sustainable futures. Vegetative garden is being discussed here as one of the means of achieving that objective, including passive cooling of buildings. 

Going by the mail response ‘Green Sense’ column received during the last few weeks, one wonders why we do not get to do what is close to our dreams. The terrace suddenly appears to be a potential unexplored, with pavilions and gardening being what many house owners dream of! If it is true, we need to realise what has been the obstacle for following one’s dreams and find corrective solutions.

Incidentally, the focus area of Green Sense has been eco-friendly and energy-efficient ideas towards sustainable futures; as such, vegetative garden is being discussed here as one of the means of achieving that objective, including passive cooling of buildings. This idea of terrace garden is not to force residents to divert their time towards vegetable cultivation. We are living in the age of busy schedules, with no spare time; as such, expecting everyone to grow vegetables as a weekly norm surely sounds far fetched.

Multiple advantages

So, when a reader responds asking how practical it is to expect people to do rooftop vegetation, the answer can be guessed without stating it. However, there could be many people who generally stay at home, who may take to it as a pleasurable hobby. Having said this, we need to discuss vegetative roofs, not only for the ecological benefit, but equally well for the multiple advantages they offer — exercise for the body, livelihood for gardeners, home-grown vegetables or a place for family relaxation.

In case it is an existing building with no hollow core slab and we have no time to do greening of the roof, what do we do? Give up on passive cooling? Not really, there are other excellent ideas such as painting the roof white. This cool roof concept comes from the proven fact that white colour reflects light and absorbs less heat, hence keeps the building interiors cooler. Nowadays, the cool roof concept is being popularised in green building circles, it being the easiest step towards reducing heat gain in buildings. There are special paints available which are made to reflect more heat; however they may cost more.

The traditional lime wash

Traditionally, many villages in hot, arid regions are known to lime wash the house walls and often the roof also every year as a festive preparation. This practice might have begun both as a strategy of annual maintenance and also passive cooling. Lime being a common material and economical, a coat of white lime with small quantities of blue to tone down white-brightness and a pinch of salt to increase surface density of particles is among the simplest, cheapest, and easiest solutions in the Indian sub-continent.

Unfortunately, this century-old wisdom is also among the ideas vanishing at the fastest rate, since no manufacturer, no company, no advertisement and no website is promoting local lime coat!

Walk up to the roof for vegetables

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.

Seek advice

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.

Let fruits and flowers fill the sky

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!

Roof-top vegetation

What could be the greenest roof above us? 

Terraces with shrubs and small trees keep the house cool

The expression ‘heat gain,’ often heard in discourses on green architecture, is actually a human invention. Let us imagine we have no solid roof above us and are outdoors. There is heat of course, but no heat gain as such. This example also holds good if there is only an overhead roof and no walls around us. It’s the act of enclosing space that attracts heat and also, a host of other environmental issues that we all need to rethink about. This is not to negate the need for buildings, since we humans need shelter more than any other animals, but to suggest that our problems could be of our own doing.

What could be the greenest roof above us? Of course, a big tree! Every person in all continents, except at snow-capped regions, knows the comfort under a large tree canopy; the shade in hot dry region; and the breeze in hot, humid areas. Without exceptions, traditional villages are a generous balance between trees and huts. The largest tree in the centre would be the public meeting forum, many other trees around serving numerous social activities ranging from tea shops to temples, while household services happen under the tree in the backyard. The green roof of the tree acts like an extension of the thatch roof of the houses.

Numerous local ideas

People living in flat RCC slab houses have attempted numerous local ideas to save the interiors from high temperatures. While some place potted plants, a few others have virtually nurtured a nursery there. In many houses we can see coconut leaf or other equivalent tree matter spread out. All these are simple examples of roof-top vegetation, which also include growing vegetables or in some rare cases like the house of architect Chitra and Vishwanath, growing rice on the terrace. The conventional images of roof gardens are also part of this thought, though now the idea of a designed and manicured garden for recreation has advanced to include other varieties of plant species. While flowering creepers and soft ground plants are easy to maintain, there now are terraces with shrubs and small trees!

In our context, roof-top vegetation assists in more ways than one: minimises heat gain into the building and reduces heat island effects in the city; facilitates birds and butterflies; controls rainwater run-off; acts as an absorbent of city sounds; creates an attractive look; and on an overall count, conserves energy.

While all this sounds good, we need to take certain precautions before doing extensive terrace vegetation. Vegetative roof is comparatively a new concept, where most of us have no prior experience. Basic acts like placing potted plants is possible right away, but even there draining the excessive water could be an issue. As such, we need to explore more about the problems and potentials associated with green roofs