Rat trap Bond brickwork
What is rat trap bond:
Introduced by the master architect Laurie Baker in India, Rat trap bond is an alternative technique to conventional brickwork to reduce the quantity of bricks and mortar in walls. The non-conventional arrangement of bricks forms a cavity in the wall, which acts as thermal insulation. The bond also effectively makes the structural system cost-effective.
It's a type of bond where bricks are used vertically, exposing their rowlock stretcher and rowlock header to the surface; the arrangement forms a cavity between two bricks. The thickness of the wall remains the same as that of the conventional brickwork (65mm+100mm cavity+65mm=230mm). The bond is very useful to reduce the number of bricks that get used in the wall. The cavity in the wall provides thermal insulation as well.
How to do it:
Generally, the clay bricks have a size of 230mm x 115mm x 65mm. In the first course, the bricks are to be arranged vertically, as shown in the sketch. The second or alternate courses are to be staggered in order to keep the centre of the rowlock on top of the rowlock stretcher. Special attention should be given to the corner junctions in the case of a load-bearing structure.
Ideally, the bottommost and topmost layers should be done continuously with rowlock headers for better wall stability. This layer also helps to create wata/rounding with weep holes for water seepage in case rat trap bond walls are kept exposed in heavy rainfall regions. Similarly, the window sill should also have a continuous band of rowlock headers to close the cavity.
Reduction of Material:
Rat trap bond saves approximately 27% of bricks from conventional brickwork. Similarly, there has been a 50% reduction in cement mortar consumption as well. Care should be taken to avoid cement mortar falling into the cavity.
unTAG has always given preference to employing local masons to make projects cost-effective. However, very few masons are equipped with the knowledge and expertise to practice alternative construction techniques. Hence, educating local masons through hands-on training becomes crucial. As part of our studio explorations, we have made in-house miniature brick models of different layers. This model conveys the intent of a Rat trap bond very well to the site masons.
As a part of on-site training, we get the dry mockup done on site. This helps Mason understand the laying procedure for bricks. It is important to supervise the start of rat trap walls, as masons should maintain consistency in mortar thickness in case walls are to be kept exposed. The window opening, sills, and lintels should also be supervised in order to close the cavity part of walls.
Brick quality should be checked before starting work; ideally, good-quality and dimensionally stable bricks should be used as they give good stability and aesthetics to the walls. The broken edges of bricks should be placed towards the cavity side, and the fair face of bricks should be kept on the exposed side. Ideally, electrical conduits should be placed during the construction of walls. The chisling of walls for electrical conduits should be minimal in case they are not placed during the construction of walls. Special attention should be given to the mortar, as they could become potential seepage points for the exposed rat trap bond walls.
With some precaution and guidance, even regular masons can do rat trap bond walls. It's a great alternate technique that reduces the number of bricks by 27% and cement mortar by 55%. The cavity between two bricks creates a thermal buffer, which reduces the heat transfer to the spaces in a harsh summer climate to a good extent.
Team members- Nikita Patil, Yash Prabhu, Kavya Shah, Sakshi Maeen, Manthan Mulik, Gauri Satam, Tejesh Patil
Sketches- Sakshi Waman