The three categories
Bricks are classified by what they are for, not by what they are made of. Almost all clay bricks fall into one of three groups:
- Facing: Made to be seen. Consistent colour and texture, controlled dimensions, adequate durability. The great majority of what you see on a house.
- Common: Made to be covered. No control over appearance, cheaper, structurally sound. For walls that will be rendered, plastered or hidden.
- Engineering: Made to resist water and crushing. Dense, low absorption, high compressive strength. Below ground, in retaining structures, in manholes and anywhere exposure is severe.
Paying facing-brick prices for a wall nobody will see is money wasted. Using a common brick where a facing brick belongs is a decision you look at every day for thirty years.
What engineering bricks actually do differently
The two properties that define them are compressive strength and water absorption, and the second is usually the reason you need them.
Class A and Class B are the usual grades, A being the denser. A brick that absorbs little water resists frost, because frost damage happens when absorbed water freezes and expands inside the brick. It also resists sulphate attack from groundwater and stops moisture wicking upward.
That is why engineering brick belongs below the damp-proof course. A facing brick in that position will draw water up into the wall and, in a hard winter, will spall its face off.
The trade-off is that dense bricks are less forgiving. They bond less readily to mortar and they crack more suddenly when overloaded rather than deforming first.
Size changes the count more than people expect
Brick sizes vary by market, and the difference is not cosmetic — it changes how many you buy:
- Modular (190 × 57 mm): With a 10 mm joint, each occupies 200 × 67 mm of wall face. About 75 per square metre.
- Standard (203 × 57 mm): 213 × 67 mm of face. About 70 per square metre.
- Queen (203 × 70 mm): 213 × 80 mm of face. About 59 per square metre.
- King (245 × 70 mm): 255 × 80 mm of face. About 49 per square metre.
Between a modular and a king brick the count per square metre differs by more than a third. On a house that is thousands of units, and it is why the brick size belongs in the estimate before anything is priced.
Larger bricks also lay faster — fewer units per square metre means less labour — which is exactly why king-size and oversize bricks exist.
Frogs, perforations and solids
- Frogged: A depression in one face. Laid frog up and filled with mortar, which increases the mortar quantity noticeably over a solid brick.
- Perforated: Vertical holes through the brick. Lighter, fires more evenly, and the holes key with mortar. Not suitable where the brick is cut or exposed on edge.
- Solid: No voids. Required where the brick will be seen from more than one face, or cut.
Any estimate that treats every brick as a solid rectangular block will read low on mortar for frogged bricks and high on brick volume for perforated ones.
Matching existing work
On an extension or a repair, the hardest part is not specification but matching. Brick colour varies between batches from the same manufacturer, let alone between makers, and a new wall butted against an old one shows every difference.
Two practical habits: order the whole job in one delivery so it comes from one batch, and mix bricks from several packs as you lay rather than working pack by pack. Laying pack by pack produces visible bands of colour up the wall that no amount of pointing will disguise.