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Building Technology
 
Rat trap bond
My architect had always said that rat trap bond is what he'd use on our load bearing walls, but now has suddenly changed to say that you can't use it on load bearing walls.

We have a basement done with stone masonry 14" walls, and then will have another 3' in ashlar masonry, and then about 7' in Kerala brick at 9" thick.

I'd prefer to use rat trap bond. Will it be strong enough?

Loren
Loren Claassen
Responses
 
Rat trap bond
Thanks Prashant and thanks for introducing me to Costford.

Loren
Loren Claassen
Rat trap bond
Hi here are some important points about rat bond:
  • Strength is equal to standard 9" (229mm) brick wall, but consumes 20% less bricks
  • The air medium that is created by the bond helps maintaining a good thermal comfort inside the building
  • As the construction is appealing to the eye from both internally & externally, plastering is not necessary
  • The overall saving on cost of this wall compared to the traditional 9" wall is about 26%
  • An ideal mechanism for the congested low-income communities where land is scarce, but building a standard up-stair house is not a financially viable option
This technology has about 30% overall-saving on cost of a building of traditional 9" construction. The structure has proven its strength to go up to three floors with the support of brick columns.

But if I give my opinion you should never use it in basement. Basement walls should always be reinforced.. although you can less the thickness and use 4.5" brick wall with it to cut the price of construction.

Muzamil Riaz
Rat trap bond
Hi,

There are some drawbacks regarding the rat trap bond.

It has to be done by masons trained in rat trap bond, because there could be a wastage of mortar falling into the gap. I have tried using a timber plank which measures exactly the gap between the bricks. The other disadvantage is in using concealed wiring and plumbing. If you break one brick, then more than one brick will fall down. Apart from the 20% saving in bricks, the only additional saving will be in the pointing if you are using exposed brickwork. In plastering when you compare with a conventional Flemish Bond or English Bond, the cost remains the same.

One of the aspect that we do not consider in construction is that a half brick wall can also be used as a load bearing wall and compared with this, the rat trap bond is expensive. What happens is that we do not do any masonry design although the RCC slabs are designed as per the relevant codes of practice.

Regarding the thermal comfort, there are other important factors such as the overhang, cross ventilation etc which are more critical factors than the rat trap bond. Many years ago, we tried to do some measurements, but did not find much difference. For example an exterior white painted wall may be more effective in reflecting heat than an unpainted exposed brickwork.
Benny Kuriakose
Rat trap bond
While I agree with most of the points made by Benny, I must put forth a word of caution regarding the use of half brick walls as load bearing, required all the more because a lot of non-technical people read these postings, and they may be influenced by such generalised remarks and may go to the extent of applying them without proper professional guidance.

The use of a half brick wall as load bearing has its own structural restrictions and these should be specified as well.
Shubhru Gupta
Rat trap bond
Hi Loren,

Following are the advantages and disadvantages of 9" rat-trap bond masonry w.r.t. 9" solid brick work:

  1. Compressive strength is only slightly less or equal when you use normal burnt clay bricks. (Brick on-edge is used for rat-trap bond masonry, which is considered to be stronger than flat bricks due to the production process).
  2. The economics could be similar, as while you save on bricks, you need to use a richer mortar mix.
  3. They are approximately 10-20% more thermally insulating than solid brick work, due to the air gap.
  4. It's recommended to expose the wiring and plumbing, though there is a lot of space inside, because chiselling becomes a problem.
  5. Post construction retrofitting has to be done under the supervision of an experienced engineer/architect due to the fact that it is hollow and not solid.
  6. Rat-trap bond brick work definitely looks much more aesthetic than normal flat-brick solid brick work. But then you can also have 'solid' rat-trap-bond walls.
  7. You don't save 30% of bricks, but rather approximately 15-20%, as you will have solid brickwork at the edges, corners, joints, floor level, sill level, lintel level, roof level, corbels, arches, etc.
  8. Rat-trap-bond walls can take the load of up to G+2 buildings depending upon the quality of bricks and mortar, the structural design of the building, and the workmanship. You can always make it solid wherever in doubt.

I prefer it because of aesthetics and thermal insulation, but not for any other factors.

Chitradeep Sengupta
Rat trap bond
Chitradeep,
Thanks for the informative post!
P Das
Rat trap bond
Hi Loren,

9" rat trap bond wall can be considered as a load bearing wall, as I have myself used it at Costford, Kerala.

Yes, it is true that open wiring and plumbing is preferred. But, if you plan your services at an early stage of design, it can enrich your building. You can also save on plastering cost and materials, and on future costs of re-painting (it's maintenance-free).
Naresh Chhatwani
Rat trap bond
Prashant,

I have been using rat-trap bond under various circumstances for a number of years, so these are general pieces of advice.

Naresh, I agree that pre-planning of services is indeed a desirable advantage. But I think that I am also considering post-construction maintenance. Though I have not mentioned in my post.
Chitradeep Sengupta
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