According to the processing standard DIN 1053-1, "Masonry Calculation and Execution", Section 9.2.2
Butt joint width up to 5 mm: no defect
Note: Also applies if light can be seen through the joint in the shell construction. Plastering on both sides creates a stationary layer of air. This is taken into account in the thermal conductivity of the masonry.
Butt joint widths greater than 5 mm: Repair required
Also applies to: Mortar pockets or teeth at wall ends and wall corners as well as
defects Procedure: Sealing the joints on both sides with mortar
Execution: During masonry or afterwards
Note: Repair must take place sufficiently long before plastering. The service life before plastering can be assumed to be one day per mm thickness. (e.g. joint width 8mm = 8 days service life)
Butt joint widths of approx. 20-50 mm: Repair necessary
Procedure: Fill the joints completely with lightweight mortar (if subsequently: light plaster/thermal insulation plaster).
Execution: During masonry or afterwards
Note: 40 -50 mm wide butt joints are only acceptable
in isolated cases
Butt joint widths over 50 mm: Repair necessary
Procedure: Insertion of suitable bricks
Execution: During masonry
Note: Heat loss through the joints can be neglected due to their small area. Source
: bauwion
