Rat Trap Bond – Construction, Advantages, and Disadvantages

Rat trap bond, also known as Chinese brick bond, is a unique modular masonry technique that offers numerous benefits compared to traditional brick masonry.

This bond, introduced by architect Laurie Baker in Kerala during the 1970s, has gained popularity for its cost-effectiveness, reduced material requirement, and improved thermal efficiency.

In this article, we will explore the material criteria, construction process, as well as the advantages and disadvantages of rat trap bond masonry.

Selection of Bricks

The selection of bricks plays a crucial role in rat trap bond construction due to the reduced number of bricks used in this technique. The following criteria should be considered when choosing bricks for rat trap bond masonry:

1. Standard size: Bricks should adhere to a standard size, with acceptable dimensions in Indian scenarios being a length of 220-250 mm, width of 100-115 mm, and height of 65-75 mm.
2. Straight edges and corners: The bricks must have straight and sharp edges, maintaining a perfectly rectangular shape.
3. Uniform size: Using bricks of uniform size is essential to achieve both strength and a polished finish in the modular masonry structure.

Construction of Rat Trap Bond

The construction process of rat trap bond involves specific steps to create the desired cavity in the masonry wall. Here are the key elements of rat trap bond construction:

1. Vertical brick placement: Bricks are positioned vertically, exposing the 110 mm face from the front elevation, instead of the conventional 75 mm face (considering a standard brick size of 230 x 110 x 75 mm).
2. Cavity formation: By maintaining a wall width of 230 mm, a cavity is created inside the wall, offering additional benefits.
3. Solid masonry at specific points: The first and last layers of the masonry are constructed conventionally without cavities. Similarly, the sill, lintel, and sides of openings are made of solid masonry to facilitate the fixing of frames.
4. Reinforcement bars: To enhance the masonry’s strength, vertical and horizontal reinforcement bars are placed within the cavities.
5. Concealed services: With proper planning, electrical conduits and plumbing pipes can be concealed within the cavity, enhancing the aesthetics of the structure.

Advantages of Rat Trap Bond

Rat trap bond masonry offers several advantages over conventional brick masonry. Let’s explore the key benefits:

1. Thermal insulation: The cavities in rat trap bond masonry act as effective thermal insulators, keeping the interiors cooler during summer and warmer during winter.
2. Cost-effective: Rat trap masonry reduces the cost of construction by up to 30% compared to conventional brick masonry. It requires fewer bricks and less mortar, resulting in significant savings.
3. Reduced brick requirement: Rat trap bond masonry uses approximately 470 bricks per square meter, whereas conventional masonry requires around 550 bricks per square meter.
4. Versatile applications: Walls constructed using rat trap bond can serve as load-bearing walls as well as thick partition walls, offering flexibility in design and construction.
5. Aesthetic appeal: Exposed rat trap bond walls create an aesthetically pleasing surface, eliminating the need for plastering and painting and reducing additional costs.
6. Reduced dead load: With approximately 30% cavities, rat trap bond masonry reduces the dead load of the structure. This, in turn, reduces the requirements for supporting members like columns and footings.
7. Enhanced structural safety: Additional reinforcement bars can be inserted through the cavities to provide further structural stability, extending from the walls to the foundation.
8. Durability and low maintenance: Rat trap bond masonry has demonstrated its durability over the years, with many decades-old buildings showcasing its longevity and low maintenance requirements.

Disadvantages of Rat Trap Bond

While rat trap bond masonry offers numerous advantages, there are a few drawbacks to consider:

1. Limited sound insulation: The formation of cavities in the masonry reduces its ability to provide sound insulation. Additional measures may be required to address this concern.
2. Skilled labor requirement: Constructing rat trap bond masonry requires skilled labor with a good understanding of the specific techniques involved.
3. External surface cleaning: If the walls are left unplastered, regular cleaning of the external surface is necessary to maintain their appearance.
4. Special attention required: Designing and constructing rat trap bond masonry demands careful planning and attention to detail to ensure structural integrity and desired outcomes.

Rat trap bond masonry offers a cost-effective, thermally efficient, and aesthetically pleasing alternative to conventional brick masonry. With reduced material requirements, enhanced thermal insulation, and lower construction costs, this technique has gained recognition in the field of sustainable architecture.

However, it is essential to address the limitations associated with sound insulation, labor requirements, and careful construction planning. By understanding the material criteria, construction process, as well as the advantages and disadvantages, architects and builders can make informed decisions when considering rat trap bond masonry for their projects.