The maximum allowable width of mortarless head joints is 5 mm. Any wider gaps that might occur during bricklaying must be closed up with mortar right away, most certainly before a layer of plaster is applied to the wall.

Other exceptional cases that require mortaring of joints include:

• laying up lintel block (or U-block)
• exterior basement walls, depending on their loading
• non-loadbearing interior walls, closed on three sides with an open upper edge