Designing a chajja

It can do as much harm as good, if not well designed, says Sathya Prakash Varanashi

The idea of seeking shade from sunlight is as ancient as the human civilisation itself and this statement may elevate the respect we should give to sunshades above our windows. However, we have reduced it to a ubiquitous flat RCC slab, often used everywhere without any reason.

The word commonly known today as ‘chajja’ is the projected element just above the window, which shades the opening, stops rain from entering the room and reduces sky glare while looking out of the room.

Ideally, each window needs to have specifically designed element based on the orientation, sun directions, rain patterns, monsoon wind, sun path and such others, besides the activities inside the room.

However, it is of common practice today simply to cast a flat RCC slab as an extension of the RCC lintel beam, which can do as much harm as good, if not well designed. Between buildings with very narrow gap, sunshades will block the little light and air we can get, hence should not be provided. The buildings shade each other.

If the chajja is to mainly serve the looks and elevations, the ordinary flat projection will adversely effect. In north facing windows which get minimal rain and harsh light, chajjas can be generally avoided, but for aesthetic reasons. So, fixing a flat slab is not a solution in every context.

Imagine alternative, more attractive windows like arched windows, which is liked by many people for the character it gives to the building. In typical cases it is avoided, because not all masons can build it and equally well because it cannot have a chajja!

In historic structures, a small cornice band would appear above the void, acting as a drip mould – directing the water flowing down the surface away from window. Today, such decorative bands are out of practice. Negotiating the curve with a vaulted RCC is too costly due to centring and finishing it; Mangalore tiled chajja is ruled out in this profile and there can be no standardised chajja considering that curvature may vary between two windows. Unlike a normal window, arched ones definitely need protection from both top, called vertical shadow, and from sides, called horizontal shadow.

One option is to place the chajja also in a curved profile, with the projection anchored to the wall itself. Inverted T sections can be grouted just above the arch following the same curvature which can take flat clay hourdi tiles, stone slabs or even light weight in situ cast RCC slabs. Only the joints between the top materials need to be water proofed.

(The writer can be contacted at [email protected])

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