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 plaster coating is applied.

Other exceptional cases that require mortared joints include:

• laying up horizontal lintel units (
• exterior foundaton walls, depending on amount of loading
• non-loadbearing interior walls, closed on three sides with an open upper edge