Engineering brick and brick durability grades

Frequently asked questions

What actually makes a brick an engineering brick?

Two numbers together, which is what makes the class untranslatable. British classification sets a minimum compressive strength AND a maximum water absorption, and a brick has to meet both to carry the designation — Class A is the more demanding pair, Class B the less. The combination is the point: strength alone gives you a hard brick that still soaks up water and fails in frost, and low absorption alone gives you a dense brick that may not carry load. American grading splits those properties across two clauses of two different standards, so a specification there names a weathering grade and a compressive strength separately and a reader has to check both. Asking a US supplier for a Class B engineering brick will not produce anything; asking for a minimum strength and an SW grade with a stated absorption limit will.

Where is an engineering brick actually required?

Where the masonry is either heavily loaded or permanently wet, and often both. The classic British uses are below the damp-proof course, where the wall is in contact with wet ground and cannot be allowed to wick; in manholes and inspection chambers, where it is wet and occasionally under traffic; in retaining structures; and under a heavy concentrated load such as a beam bearing or a pier. The low absorption is doing the work in the wet cases and the strength in the loaded ones. It is worth knowing the reverse too: engineering brick is a poor choice for a general facing wall built in lime mortar, because a brick far denser than its mortar reverses the sacrificial relationship the wall depends on, which is the same mechanism that makes a cement repair destroy soft brickwork.

Why does a brick's suction matter as much as its strength?

Because it decides whether the mortar sets or is robbed. A brick's initial rate of absorption — how fast it draws water out of the mortar it is bedded on — governs the bond between the two. A very absorbent brick laid dry pulls water out of the mortar before it can hydrate, leaving a weak, dusty joint and a bond that fails under the first frost; that is why highly absorbent bricks are wetted before laying. At the other end, a very dense low-absorption brick such as an engineering brick takes almost nothing, so the mortar stays wet, the bricks float on the bed, and a wall built too fast in them slumps. Both failures come from the same property read from opposite ends, and it is why suction is measured and specified rather than judged by eye.