What is a frog in a brick?
A frog is the bowl-shaped indentation pressed into one or both of the flat faces of a brick. This recess saves clay, reduces the weight of the brick, and allows mortar to key into it, which strengthens the mechanical bond within the brickwork.
Function of the frog
The frog serves several purposes at once:
- Material saving: less clay is needed, making the brick lighter and cheaper.
- Weight reduction: lighter bricks are easier to handle and lay.
- Better bond: the mortar that fills the recess forms a key between the bricks and improves adhesion.
Because the mortar fills the frog, a stronger mechanical connection is created between the courses.
Laying frogged bricks
A frogged brick is best laid with the frog facing upwards. In this position the recess fills completely with mortar as the next course is laid, leaving no voids that would weaken the wall or reduce its resistance to moisture. For bricks with a frog on both faces the orientation is less critical, but a fully filled joint remains the basis for strong and durable brickwork.
Related terms
- Brick
- Bricklaying
- Mortar
- Bed joint
- Brick bond
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