Rat Trap Bond – Advantages, Disadvantages, Construction Method, Saving

Rat Trap Bond is known as the Chinese bond. This is a brick masonry method of wall construction in which the bricks get placed in a vertical position instead of horizontal position which creates the cavities or hollow space in the wall.

It maintains the same thickness as that of the brick laid by other methods. The vertical portion acts as the base of the wall. It was introduced by Architect Laurie Baker in 1970 in Kerala.

It creates the complex network of bricks inside the wall that creates the space in which the rats can be easily trapped so it is named Rat Trap Bond.

Application of Rat Trap Bond

It is an application for the structure where cost and energy saving is difficult. Some of the applications of the rat trap bond are:

• It can be used for low-cost construction because of the number of bricks and also the volume of mortar required to construct the wall decreases.
• It is used for buildings where the thermal comfortability is to be maintained.
• Wall having the thickness 9” (approx. 23cm) is appropriate for using a rat-trap bond.
• It can be used for low load-bearing walls up to three stories.

Materials used for construction of Rat Trap bond

Bricks

The bricks should be of standard size, the variation isn’t accepted

The brick should be perfectly rectangular in shape with straight and sharp edges and corner

The bricks should be uniform in size

The bricks should have compressive ratio as standard as the quality of the bricks should not degrade

Mortar

The mortar should be mixed in proper ratio if sand and cement as it plays a vital role in the bonding of the brick

The mortar needs to be placed uniformly between the bricks for attaining proper strength.

Trowel:

Should be of standard mason type for rat trap bond.

For applying and spreading mortar.

Measuring tape:

Should be standard and metallic having 5m and 30m length.

For measuring correct dimension.

Lining thread:

For ensuring straight wall.

Wooden template:

For avoiding mortar falling into cavity.

Construction Method of Rat trap Bond

The construction procedure of Rat trap bond are as follows:

Assume the standard size of brick as 230 X110 X75mm

1. In the conventional method, the bricks get laid in a horizontal position so that a 75 mm face is seen in the front elevation. In the case of the Rat trap bond, the bricks are laid in a vertical position so that the 110 mm face of the brick is seen in the front elevation instead of 75 mm face of the brick.
2. The first and the last layer is constructed like that of the conventional method using either Flemish or English bond.
3. A cavity is formed inside the wall as the width of the wall is kept to 230mm.
4. The masonry can be strengthened by providing vertical and horizontal reinforcement in the hollow space or the cavities.
5. The side openings and lintel are made of solid masonry for fixing frames.

Some rules for lying the bricks for rat trap bond:

Clean and locate corner:

Always clean the surface where bricks are to be laid before starting the process. If any dirt present between plinth level and brick masonry, bonding of cement will be reduced and result in possible dampness. And fix the corner of the wall.

The layout:

Two courses of bricks should place along the wall line without mortar by leaving a 1cm gap for the mortar joint. Bricks should be arranged such that only shiner fit in there and ensure the brick cross is maintained.

Mortar bed:

  After removing the dry layer, put full bed of mortar and furrow it with trowel.

Corner bricks:

Corner bricks should be carefully positioned by laying all the bricks with the frog inside the cavity wall to create a pleasing outer surface.

Mason’s line:

After finishing the positioning of corner bricks, fix the mason string at both sides of the wall, lay the top outside edge of bricks to this line, and make sure the lower edge of brick is directly over the course below.

Fix brick cross:

The first and second courses of brick should be laid between two corners without a vertical mortar joint. Then, fill the mortar after properly placing the brick cross.

Laying up corner:

Build the corner of the wall after the first and second courses of brick are laid which should be usually five courses higher than the first course.

Mortar:

Mortar should be applied only to the top of bricks already laid for horizontal joints. Plywood template should be used while applying the horizontal joints to avoid mortar falling into the wall cavity.

Use of level:

Check with mason’s level for alignment after laying course at the corner, making sure the corner is leveled and plumbed.

Laying bricks between corner:

Mortar should not be too far ahead of the actual laying of bricks to ensure good bonding. Cut off the extra mortar with a trowel and work back in fresh mortar as each brick is laid.

Mason should work only from one side because it is not good that two masons work from two sides as it might create unwanted gaps that will be difficult to close without distorting the correct bond.

Avoid dead mortar:

Using of dead mortar will weaken your cement sand ratio and may weaken the strength of wall.

Top layer:

After complete construction, close wall cavities with a top layer by lying with headers or rowlocks depending on the pre-fixed height of the finishing the wall.  

Advantages of rat trap bond

The advantages of the Rat trap bond are as follows:

1. It uses fewer bricks than other conventional methods which decrease the cost of the construction.
2. It uses less volume of the mortar as compared to that of the conventional method which reduces approximately 30% of the cost of the construction.
3. The wall constructed by Rat trap bonding can be used as the thick partition wall.
4. The wall can also be used as the load-bearing structure. It resists more load than that of the wall constructed by the conventional method.
5. The hollow space or the cavities in the rat trap bond acts as a thermal insulator which makes the interior cool during the summer and warmer during the winter.
6. Electrical conduit and plumbing works can easily be inserted inside the cavities.
7. It gives more structural safety as the reinforcement bars can be inserted inside the cavity up to the level of the foundation.
8. The rat bonding reduces the dead load of the structure which reduces the load in the supporting members such as beam and column.

Disadvantages of rat trap bond

1. The sound insulation isn’t very good in these kinds of bonding due to the formation of cavities or the hollow space.
2. Skilled and trained manpower and labor required for the construction of this kind of bonding walls
3. Special care should be given while constructing these kinds of structures.
4. Regular maintenance is required if plastering and painting haven’t done.

Saving on the construction of Rat Trap Bond

Assuming we are building a structure on the 100m³ area. Comparison between the construction by conventional method and by that of rat bonding is as follows

Conventional method

Total cost of construction of 100m³= Rs231,840.2

By rat trap method

Total cost of construction of 100m³= Rs 171,345

Saving during construction= 231,840.2-171345= Rs 60,495

% of money saved = 26.09% saved

Precaution while construction

1. Should use only good quality bricks with no cracks should be used.
2. Special care and supervision by engineer should be done while constructing
3. Skilled labor needs to be applied while constructing the wall
4. Proper mix of mortar should be maintained to hold the bond tightly
5. Faulty bricks with improper shape and size which are not straight and the ends of the brick not having sharp edges should be avoided.
6. Frog of the brick should be facing the wall side.

I hope this article remains helpful for you.

Happy Learning – Civil Concept

Contributed by,

Civil Engineer – Shreya Parajuli

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