The maximum allowable width of mortarless head joints is 5 mm. Any larger 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 mortaring of joints include:

• Laying up of lintel block (U-block)
• Exterior basement walls, depending on their loading
• Non-loadbearing interior walls, closed on three sides with an open upper edge