Strength of wall is not compromised, it remains same as traditional masonry wall. Cavity induced in wall provides better thermal insulation, resulting in cooler interiors during summer and warmer interiors during winter.
Architect Laurie Baker introduced it in Kerala in the 1970s and used it extensively for its lower construction cost, reduced material requirement and better thermal efficiency than conventional masonry wall, without compromising the strength of the wall.
Selection of Bricks
The criteria that are set for the selection of bricks is of utmost importance as less number of bricks are used in the construction of rat trap masonry.
The size of the bricks used must be of a standard size and variation in size is not accepted. The acceptable sizes of brick in Indian scenarios are – Length 220-250 mm, Width 100-115mm and Height 65- 75mm.
The edges and corners of the bricks must be straight and sharp and perfectly rectangular in size.
Having a uniform size of bricks is important as the masonry is the modular type and to achieve good strength and finish.
Table 1: The material strength requirement Rat Trap Bond
Type of Construction
Recommended Compressive Strength of Bricks Best Practice Minimum Allowable
Recommended Mortar Ratio
Load bearing, Double storied
40 – 50 kg/cm2
1:5
Load bearing, Single storied
35 – 40 kg/ cm2
1:4
Infill masonry in frame structure, no restriction on number of floors
Min 35 kg/ cm2
Not less than 1:4
Construction of Rat Trap Bond
The bricks are placed in a vertical position so that 110 mm face is seen from front elevation, instead of the 75mm face (considering brick of standard size 230 X 110 X 75 mm).
As the width of the wall is kept as 230mm, a cavity is created inside the wall.
However, the first and the last layer of the masonry is constructed as the convention sold masonry.
In the sill, lintel and sides of openings are made of solid masonry (without cavity) for fixing of frames.
To strengthen the masonry, vertical and horizontal reinforcement bars are provided in the cavities.
Electrical conduits and plumbing pipes, with prior planning, can be put inside the cavity for better aesthetics.
Fig 2: Construction of Rat Trap Bond
Image Courtesy: ArchitectureLive!
Advantages of Rat Trap Bond
The cavities in the masonry act as thermal insulators. Thus, the interiors remain cooler in summer and warmer in winter.
Rat Trap masonry uses fewer bricks and mortar reducing the cost of masonry up to 30% when compared with conventional brick masonry.
The number of bricks used in the construction of rat trap masonry is 470, whereas, in conventional masonry, it is 550.
Walls constructed using rat trap masonry can be used as load-bearing as well as a thick partition wall.
Rat-trap bond when kept exposed, creates aesthetically pleasing wall surface and the cost of plastering and painting may also be avoided.
As this type of masonry has 30% of cavities, the dead load of the structure is reduced which in turn reduces the structure supporting members such as column and footing.
In case of more structural safety, reinforcement bars can be inserted through the cavity until the foundation.
Many buildings that were constructed decades ago have proved that this type of walling technology is durable and the maintenance costs are low.
Disadvantages of Rat Trap Bond
Due to the formation of cavities in the masonry, the building does not provide good sound insulations.
Skilled labor is required to construct this type of masonry.
Frequent cleaning of external surface required if not plastered.
Special care and attention to be given while designing and constructing rat trap bond masonry.
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