External masonry walls

6.1
Also see:
Chapter 3.2

6.1.14Mortar

Mortar shall be of the mix proportions necessary to achieve adequate strength and durability and be suitable for the type of masonry. Issues to be taken into account include:

  1. sources of sulfate
  2. admixtures and additives
  3. preparing mortar
  4. joints.

Unless recommended otherwise by the brick manufacturer, the mixes in Table 6 should be used for clay bricks. In the case of concrete or calcium silicate bricks, particular attention should be paid to the manufacturer’s recommendations.

Table 6: Mortar mixes using ordinary Portland or sulfate-resisting cements

LocationRecommended cement:lime: sand mixRecommended cement:sand mix with air entraining plasticiserRecommended masonry cement: sand mixMortar designation to BS EN 1996-1-1Equivalent Mortar Class to BS EN 1996-1-1
General wall area above the DPCIn areas of severe or very severe exposure – high durability

Other exposure categories – general use		1:½:4½


1:1:5½		1:3½


1:5½		1:3


1:4½		(ii)


(iii)		M6


M4
Below DPC level and in chimney stacksHigh durability1:½:4½1:3½1:3(ii)M6
Cappings, copings
and sills		Low permeability1:0 to ¼:3--(i)M12

Air-entraining plasticiser can be incorporated in the following general use and high durability mortars:

  • 1:1:5½, cement:lime:sand, or
  • 1:1:4½, cement:lime:sand.

Retarded mortar

Retarded mortar and most premixed mortars can be used over a longer period of time than site-mixed, cement:lime:sand mortars. When using retarded mortar:

  • follow manufacturer’s recommendations and timescales
  • do not use it beyond the time for which it is effective
  • protect it against freezing prior to use
  • temporary bracing of larger walls, e.g. at gable peaks and long walls, may be necessary due to delayed setting times.

Sources of sulfate

Mortar is vulnerable to deterioration by sulfates, especially when masonry is saturated for long periods of time. Clay bricks contain soluble sulfate (S1 designations have no limit on their sulfate content) and so a suitable mortar should be used.

To reduce risk, cement types listed in BS EN 998:2 NA1.2 with sulfate resisting properties alternatively CEM II cements based on blast-furnace slag are in widespread use where sulfate resistance is required should be used:

  • below the DPC level when sulfates are present in the ground
  • when clay bricks (F2,S1 and F1,S1 to BS EN 771) are used
  • when there is a high saturation risk (examples below).

High saturation risk situations are:

  • below the DPC
  • areas of severe or very severe exposure to driving rain
  • parapets
  • retaining walls
  • freestanding walls
  • rendered walls
  • chimney stacks.

Admixtures and additives

Admixtures should:

  • only be used where authorised
  • not contain calcium chloride
  • be dosed and used in accordance with the manufacturer’s recommendations.

Mortars containing an air-entraining plasticiser are more resistant to freeze and thaw damage when set, but do not prevent freezing before the mortar is cured.

White cement to BS EN 197 and pigments to BS EN 12878 may be used, but pigments should not exceed 10% of the cement weight, or 3% where carbon black is used.

Preparing mortar

When preparing mortar:

  • ensure the mix is appropriate for the use and location
  • plant and banker boards should be kept clean
  • mixers should be kept clean to operate efficiently
  • the colour should be consistent.

When laying bricks and blocks:

  • mortar which has started to set should not be retempered
  • they should have a solid mortar bedding and fully filled perpends, to reduce the risk of rain penetration and dampness in the wall.

Notes

Thin layer mortars are supplied in bag form and should be mixed with water on site strictly following the manufacturer’s recommendations.

Joints

Jointing is preferable to pointing because it leaves the mortar undisturbed. Struck (or weathered) and bucket handle joints are preferable for external walls. Unless the design states otherwise, only bucket handle or weathered joints should be used.

Recessed joints should not be used where:

  • bricks are not frost-resistant, e.g. clay F1,S1 or F1,S2 to BS EN 771, unless the brick manufacturer has confirmed their use for that particular location in writing
  • the home is built on steep sloping ground, facing open countryside or within 8km of a coast or large estuary
  • bricks are perforated closer than 15mm to the face
  • there is no reasonable shelter from driving rain, e.g. from buildings or groups of trees within 50m and of similar height to the home
  • the cavity is to be fully filled with cavity insulation.