How to Tool Concave Mortar Joints

Strike fresh mortar joints concave with a jointer at thumbprint-hard, so the joints compress and shed rain. The timing matters more than hand skill.

What you need

Press a thumb into the joint. It is ready when it takes a clean print and no wet paste lifts onto the thumb.
Match the jointer end to the joint: 3/8" for standard modular brick, 1/2" for block and older, wider brick joints.
On a repair, match the old wall's joint width, profile and depth.
Put on gloves. Strike the short vertical head joints first.
Run the long bed joints through the heads in one steady pass, with firm, even pressure.
Stop when the joint is faintly sheened, tight and uniformly curved, a hair recessed from the brick face.
Wait for the mortar fins to dry crumbly, then knock them off with a stiff bristle brush stroked diagonally across the joints.
Keep the joints damp with a light mist or damp burlap, out of direct sun and wind, for the first days.

The window is about an hour on a hot, dry, windy wall and half a day on a cool, damp one. Early joints stiffen first.
Compress, do not carve. One smooth pass beats three.
Concave is the default for exterior brick and block.
If the tool tears or chatters, the mortar is past the window. Fix the timing next time.

Wet mortar is strongly alkaline, pH 12 to 13, and burns skin with no pain to warn you. Wear gloves from the first joint.
Do not wipe or brush fins while wet. It smears cement haze onto the brick that rinsing will not remove.
Mortar tooled too late and marked can only be fixed by cutting out and repointing.

Questions about the right tool for your job? Email cs@hydestore.com or call 508-916-6442.