Striking a Joint

Timing is everything. Strike a joint too soon, while the mortar is still too wet, and the tool tears and smears it instead of compacting it. Wait too long, once the mortar has started to set, and it won't compress at all, leaving a weak, porous joint that fails early. A mason works down a wall striking joints in the same order they were laid, checking that each one has firmed up just enough to hold a clean tooled line.

A properly struck joint is noticeably denser than an untooled one, because the tool presses the mortar tight against both brick edges and closes hairline gaps a trowel alone leaves behind. That density is what makes a concave strike the default here: a well-compacted joint resists water intrusion far better through a winter of hard freeze-thaw cycles than a joint that was just wiped flush and left alone.