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The rat-trap bond was invented and popularised the famous architect Laurie Baker. This system uses bricks-on-edge with a cavity behind every facer brick. Each course of brickwork is 115mm high, comprising of two parallel facers followed by brick placed at 90o to lock the bond. In the next course, the vertical mortar joints are broken by spanning the cavity at its centre with the locking brick. T and L junctions are made solid by filling the cavity with a brick-on-edge. Horizontal courses of brick-on-edge may be taken below the damp proof course (DPC), or at lintel and roof levels to provide horizontal tying. For every 230mm height of brickwork, there is a saving of one layer of mortar.
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It goes without saying that every three bricks laid saves one brick in this bond. This bond shows better performance against flexure as against the conventional brick wall. This is due to the fact that the through brick in this bond is laid on edge and hence action is against a 115mm depth instead of a 75mm depth. Thus, its overall performance is comparable to conventional bond work considering the cavity in the wall. For the system of external rendering the normal practice adopted is to keep the internal face of the wall smooth which can take a coat of 12mm plaster. The external header face is plastered so that the surface is finished in line with the stretcher face, which is left exposed. Thus the inner surface is protected from damp.
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(The brick work in rat-trap bond is a cost-effective option and is being used in several affordable housing projects by government and public agencies)
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