Frog and perforations in a brick

Frequently asked questions

Frog up or frog down — is there a right answer?

Up, for anything structural, and the reason is the bed joint. Laid frog UP, the recess fills with mortar as the next brick is bedded, so the joint is solid through its whole depth and the two bricks are keyed to each other. Laid frog DOWN, the recess is a void unless the bricklayer deliberately fills it — and most do not, because it is slower and uses noticeably more mortar — so the bed joint is reduced to the ring of mortar around the edge of the frog. That reduces the bearing area, reduces compressive strength, and leaves a cavity in the wall that can hold water and transmit sound. Frog down is faster and uses less material, which is exactly why it happens, and on a garden wall it matters much less than on a pier carrying a beam.

How much does a frog or a core change the mortar quantity?

Enough that a takeoff based on joint dimensions alone will be short. A conventional estimate multiplies bed and perpend joint volume by the number of joints, and that assumes a flat bed face. A single-frogged brick laid frog up adds the frog's volume to every bed joint, which on a common frog is a meaningful fraction of the brick — enough to push a job's mortar requirement up by a noticeable percentage over a large wall. Cored brick behaves differently again: the cores are not usually filled in a veneer, so they add nothing to mortar, but they do change the unit's weight and its thermal mass. The practical rule is that the brick's geometry is an input to the mortar quantity and not a detail, and that an order calculated for a solid brick will run out on a frogged one.

Does a frogged brick differ from a perforated one structurally?

Yes, and the standards separate them by how much of the unit is void. A FROG is a recess in the bed face that does not go through, so the brick has a continuous solid section from face to face and behaves substantially like a solid unit once the frog is filled. PERFORATIONS go right through, so the unit's net section is reduced permanently and it is classified by the percentage of void — which decides whether it counts as a solid, a perforated or a hollow unit, and that classification governs its strength, its fire performance and whether it may be used where a solid unit is specified. The difference shows up in practice when drilling: a perforated brick offers a fixing far less material to hold, which is why a heavy fixing into one needs a resin anchor rather than a plug.