Monthly Archives: August 2011

Jack arch roofing

This is traditionally styled where the arches are formed in the roof itself supported by the centering. 

Between the flooring and the roofing, what is more important? Undoubtedly, it is the roofing. Yet, curiously, we find thousands of patterns and products for the flooring backed up by huge advertisement campaigns, while there is hardly a whisper about the roof, but for the builder informing about the day of roof casting! It could be linked to the fact that most often we are looking down, hence tend to ignore looking up to the roof.

If we start looking up, we would realise that arched roofs are among the more attractive ones, presenting a designer or false ceiling appearance. The more traditional ones are called jack arches where the arches are formed in the roof itself supported by the centering, while the recent researched type is arch panel roof, where the arches are made on ground to be lifted up and placed.

Across the walls, wooden or steel cross beams are placed at not more than 3 ft. spacing with the short gap in between formed into a shallow arch form, to work as the temporary roof base. Good quality local bricks, preferably the thinner type, are placed on this base in a curved arch profile. The joints are finished with rich mortar. If the upper floor is to be used, it could be levelled or water proofed to be left as the final roof. In case thin hollow clay blocks, also called as water proof course tiles (WPC tiles) are used, additional passive cooling can also be achieved, besides ensuring the roof within the normal thickness. No structural steel reinforcement is required within the arches, for they naturally transfer loads along their curvature. Only nominal 6 mm rods are placed within the joints to hold the blocks together.

Length, an advantage

Among the advantages of jack arch roofing is unlimited length. Once the width of the room is limited, say within 12 to 15 t., which can be connected by wood beam or steel girder, the length of the room can be adjusted. There is no longitudinal member at all, hence this possibility. If all the material components such as cross beams and tiles are ready, the making of the roof is fast with no extra time demanded for placing reinforcement steel or extensive curing.

Masons have to take good care to achieve proper curve in each of the arches, so keeping a template piece is advised. During casting, steel centering could be used, but the top arch surface has to be in mud or lean mortar to achieve the curve and also to ensure their easy removal after casting the roof. Since the individual arches are resting on cross beams, centering support is needed only for the cross beams. Electric conduits can be run as normal, with hole punctured in the tiles to fit roof boxes and provide the standard wall elbows.

Why this state?

Even though jack arch roofs are economical and eco-friendly, their application has reduced. The skill levels for handling the roof have also reduced. If we desire, they can be revived.

Many solutions for the roof

The new roofing technology is more advanced, but at some places the old practices are appropriate, and we need to blend the two.

Let us recollect our visits to villages and small towns, check out the kind of roofs we might have seen and compare them with the practice today. The idea of a monolith single material concrete roof we now know, is of recent origin. Traditionally, many building materials were put together to create the roof which then made sense because a range of local resources could be mobilised to get the best of roofing solutions. Clay plus wood for Mangalore tiles roof, steel plus bricks for Madras terrace, wood plus cudappah slab for stone roof, wooden beams plus planks for intermediate ceiling and such others that exist even today in our century-old houses and buildings amply prove their advantage, affordability and versatility.

Simplifying the roof

If we need to roof a room 18′ long and we find it difficult, what’s the common sense solution? We may place a wooden cross beam between the supports at every 3′, so effectively we need to roof six spans of 3′ each, which is much simpler. If only small stone slabs are available as roofing material, this could be reduced to 2′ each, by placing more of the cross beams. Of course the width of the room has to be limited to the available length of cross beams, say within 12′, which is fine with majority of residential activities. The space in between the beams or the narrowed span has to be now filled, using any of locally available appropriate materials.

In case the width is more, the simple cross beam has to be made stronger by triangular truss-like formations, provided with a central pillar support or wooden members replaced by steel sections. For buildings demanding larger indoor spaces, more advanced solutions need to be applied.

Technically, there is nothing wrong in this approach. However, advancing time discovered newer modes of roofing, based primarily on reinforced concrete and structural steel, as necessitated by the specifications of different projects.

Standards and codes

During the colonial and post-independence era, the building construction industry got formalised with standards and codes written, where certain practices were retained while the others were discouraged. Formal education, idea of textbooks and degree-based professionalism further damaged the image of local practices. Just like in medicine or folk wisdom, many effective solutions got sidelined, and today hundreds of time-tested design ideas get neither support nor promotion by the building industry.

If this narration sounds familiar, where the march of time leads to the new replacing the old, yes, it is a familiar story. If only the ‘new alone’ could have been our saviour, if all the modern ideas were to be ecologically sustainable and if the technical solutions of today are affordable by all – may be, we would not have sought any other option but modernity and high technology. The new roofing technology is more advanced, but at places the old practices are appropriate; as such we need to find ways of blending the past and present.

Exciting options with filler roofs

Filler roofs are designed using the same principles as RCC, except for replacing part of the concrete with some alternative material. 

Last week belonged to queries about filler roofs, following the preceding essay introducing the idea. No wonder there were so many questions about it, considering it is an exciting solution, though less heard of. While the regular RCC is widespread even in remote villages, it is surprising that filler roofs are hardly known around and have minimal visibility.

By the early 1980s, many roofs were cast with filler material in Kerala and later in Bangalore too. With nearly 30 years behind them, their durability has been repeatedly proven. Incidentally, when a commoner wonders if the roof is strong enough, what actually he enquires about is whether the roof will structurally perform well with load transfer, tension, compression, deflection and such technical matters. Since filler roofs are designed using the same principles as normal RCC, except for replacing part of the concrete by some alternative material, worries about strength can go unquestioned. It is the designing for tension load, hence the spacing of steel reinforcement that would differ as per the sizes of filler material.

No new problem

Behaviourally, the slab acts like any other slab, and as such does not pose any new problem not seen in regular RCC roof — be it with possibilities of water seepage or cracks! In either case, normal RCC or filler, such issues need to be attended to, should they appear unfortunately. Quality of construction at site is a pre-condition which can affect both the mainstream and the alternative practices, and as such should not be used only to degrade the alternative!

Availability of the hollow clay filler blocks could be an issue, to be explored by the potential user, but any local clay roof tile supplier could be of help. Alternatively, a variety of locally available materials could be inserted as filler, including stabilised sun dried mud bricks. When we place glass bottles or perforated jali blocks, we get skylight effect.

Cement blocks have already been experimented with, which then could be plastered and concealed. There are many filler roofs done with mud pots or bowls as filler material, where we get the looks of an artistic ceiling, at a lower cost.

While concreting, the filler blocks may move due to labour movement, which demands careful on site handling. Depending upon the room size, there could be savings in the steel cost; however the cost of block and increased labour would ensure our savings are spent! In large span roofs, where it is desired to avoid the beams, one may design a thicker slab, then introduce filler concept to save on concrete.

When the whole roof gets cast as clay filler slab, it’s possible that some family member may not like the red look at the bottom of the roof. Such people have the choice of getting the ceiling painted white, giving a designer false ceiling appearance, thanks to the fluted profile of the block!

Reduce the concrete in filler slabs

Filler slabs do not demand greater technical skills or supervision than the normal slabs, and hence can be employed widely.

Roof

We are talking common sense. When we need to buy something, if it costs high, either we shop for a different item or reduce the quantity of purchase. In the case of roof, concrete is not replaceable on most occasions. So, the least we can do is to reduce the RCC quantity, which can be done by partly replacing concrete by any other economical, beneficial or attractive material. Since the roof thickness gets filled so, technically such roofs are termed as filler slabs.

Incidentally, even though all RCC roofs are a single monolith cast of concrete along with steel, the area between the steel reinforcements play no role in load transfer. Yet, we cast the roof in full because it cannot be cast with innumerable voids in between, considering the form work or the temporary supports that are required for casting the roof.

Baker’s style

One solution popularised by Lauri Baker, the pioneer architect of cost effective architecture in India, was to place two numbers of low grade Mangalore roof tiles in between the steel rods. Once the centering is done, the filler material is placed and then the steel rods tied as per the required spacing. Since then, architects and engineers have placed everything possible under the sun, including bottles, computer key boards, cement blocks, mud pots and such others to achieve filler slabs. The criteria in selecting the material has largely been the cost and its looks.

Multiple advantages

Since the heat gain from the roof is the highest, if the filler slab could be done using hollow materials, we get multiple advantages – passive solar cooling where the voids reduce the heat transfer, lesser roof weight thanks to the hollowness, different looks when seen from the room below, reduced sound transmission between the floors thanks to the voids in between and reduction of steel consumption considering greater distance between them. Accordingly, nowadays a special block called hollow clay roof block, also called as maruthi block or filler block, has gained popularity. These are manufactured by clay brick and tile manufacturers, mainly in Kerala and Karnataka.

Once the roof centering is complete, it is levelled by a layer of stabilised mud or very weak cement mortar. First the roof blocks are placed end to end as per the structural engineers design, placing them closely and compactly. The minor gaps in between are sealed with the same lean mortar to reduce curing water flowing down after the roof casting. These blocks come with specific spaces to run the steel rods, in both the directions as required by room spans. A layer of thin steel rods is placed on top, electric conduits are tied and the roof is concreted as usual.

Following the stipulated 21 days of curing and de-shuttering, the construction proceeds just like any other roof. Filler slabs do not demand greater technical skills or supervision than the normal slabs, hence can be employed widely