This double-wall technique uses bricks on edge with a cross brick between each and produces a 9-inch thick wall with an insulating air cavity in between. Surprisingly, this technique reduces the number of bricks required by 25%, thereby reducing material used, including mortar (1:8 mix), and overall cost. Rat-trap technique is equal to the strength of a solid 9-inch wall in either Flemish or English bond.
Finishing of such walls is not generally required and exposed brick themselves are appealing and form beautiful patterns. This saves the cost of plastering, painting, and maintenance. But the sizes of the bricks vary, so only one surface of the wall can be even. The other face (generally inner) can be smoothed by patch pointing. By avoiding unnecessary plastering, we can save up to 10% of the brickwork cost.