External masonry walls

Also see:

6.1.20Protection of the works during construction

Precautions shall be taken to protect walls from damage during construction.

Issues to be considered include:

  1. cold weather working
  2. hot weather working
  3. excessive rain working.

Cold weather working

Freshly laid mortar may fail where it freezes.

The use of air entraining agents in cold weather gives better frost resistance to set mortar but does not aid the set. The use of accelerating admixtures and other admixtures should not:

  • be relied on as an anti-freeze precaution
  • contain calcium chloride.

Ensure the setting times of additives are checked and adhered to in accordance with the manufacturer’s recommendations. Cold weather retarders increase setting times.

In cold weather:

  • brickwork and blockwork should not be built when the air temperature is below 3°C and falling
  • work can resume when the temperature is 1°C and rising with the expectation the temperature will exceed 3°C
  • walls should be protected from frost until the mortar has set sufficiently to resist frost damage
  • covers should be provided to form a still air space to insulate the wall
  • walling damaged by frost will not regain strength and should be taken down and rebuilt when conditions improve.


Thin joint mortars that can be shown to have been successfully tested for use down to 0°C are acceptable when the temperature is 0°C and rising. The mortar
should be used strictly in accordance with the manufacturer’s instructions and Chapter 3.2 ‘Cold weather working’.

Hot weather working

In very hot weather above 30°C the main concern is the rate water is removed from the mortar either by suction of the warm masonry or evaporation, the mortar will also tend to lose its plasticity at a faster rate due to the evaporation of the water from the mix.

Mortar mixed at high temperatures may have a higher water content, a lower air content and a shorter board life. The quality of the bond between the mortar and the brick or block is dependent on having the correct amount of water and so this bond may be affected.

To reduce the impact of higher temperatures:

  • store bricks and blocks in the shade to help control heat gain.
  • spraying with modest amounts of clean water can keep their temperature down and to stop the suction but, do not soak them.
  • mixing equipment can be shaded from direct sunlight prior to use.
  • mortar tubs and mortar boards should be rinsed with cool water before coming into contact with the mortar.
  • where ready-to-use mortar is being stored on site it is important to keep it well covered in the tub.
  • dry Silo Mortar - where the dry sand and cement mortar is stored on site in a hopper and mixed with water on demand, this offers the added advantage of being able to mix small batches which can be used up quickly.

Excessive rain working

Do not lay bricks or blocks in excessive wet conditions.

Ensure new brickwork and blockwork is completely covered to protect it from the elements.

If brickwork and blockwork is exposed to water for a prolonged period of time, the risk of leaching, cement residues and efflorescence will increase.

Protection of cavity walls during construction

Masonry cavity walls shall be protected whenever work stops e.g. for inclement weather or overnight. The tops of both leaves, as well as the cavity and any insulation, should be covered with sacking or plastic sheet and appropriately secured in place.