External masonry walls

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External walls shall be suitable for their exposure and resist the passage of moisture to the inside of the home. Issues to be taken into account include:

  1. durability
  2. rain penetration
  3. frost attack.


Masonry can become saturated, and may remain so for long periods. Therefore, precautions should be taken to resist frost damage and sulfate attack affecting:

  • parapet walls and copings
  • sills and projections
  • masonry below the DPC at ground level
  • freestanding walls.

Bricks and mortar should comply with BS EN 1996-1-1 and the manufacturer’s recommendations. In addition, the following mortar mixes can be used with ordinary Portland cement or sulfate-resisting cement:

  • Air-entrained 1:1:5½ cement:lime:sand
  • Air-entrained 1:½:4½ cement:lime:sand.

Sulfate-resisting cement should be used where S1 clay bricks are used in the following situations:

  • Below the DPC where there are sulfates present in the ground.
  • Below the DPC where there is a high risk of saturation.
  • Retaining walls.
  • Parapets.
  • Freestanding walls.
  • Rendered walls.
  • Areas of severe, or very severe, exposure to driving rain.

Reclaimed bricks should only be used where in accordance with Technical Requirement R3.

Rain penetration

In prolonged periods of driving rain, water will penetrate the outer leaf of a masonry wall. The following should be taken into account:

  • Site-specific exposure to wind-driven rain
  • Suitability of the wall construction and insulation method
  • Design detailing for the local exposure, and the likely quality of workmanship on site.

Exposed parts of the building should be given particular attention when selecting a suitable construction method, as this may affect the choice for the whole building.

Complete resistance can only be achieved with an impervious cladding. However, the following approaches can reduce the risk of rain penetration:

  • Providing cladding to the wall.
  • Increasing the clear cavity width (minimum 50mm) or the width of full-fill cavity insulation (increasing the cavity width for full-fill cavity insulation greatly reduces the risk of rain passing through the cavity).
  • Rendering the wall and specifying crack-resistant backing material.
  • Designing protective features to keep the wall dry, e.g. projecting sills and deep overhanging eaves and verges.
  • Ensuring mortar joints are fully filled. Where full cavity insulation is proposed, recessed joints should not be used.
  • Following the recommendations of any assessment of the insulation and the manufacturer’s recommendations.
  • Ensuring that cavities are not bridged.

Cavities should be continuous around enclosed porches and habitable areas.

Insulation should be in accordance with Clause 6.1.7 and Table 2. In Scotland, the cavity should not contain full-fill insulation. In Northern Ireland and the Isle of Man, it is not permissible to fill cavities with pumped thermal insulants at the time of construction.

In Scotland, Northern Ireland, the Isle of Man and in other places where the exposure to driving rain is very severe, masonry should form a rebate at the reveals of openings to avoid a straight through joint where the frame abuts the masonry, or a proprietary cavity closer assessed in accordance with Technical Requirement R3 should be used.

Sills, copings and similar features should be weathered and throated unless adequate alternative provision is made to protect the brickwork from saturation, frost damage and staining.

Variations to the exposure shown on the map can only be made by site-specific calculations using BS 8104 ‘Code of practice for assessing exposure of walls to wind-driven rain’.

Adapted from BRE report ‘Thermal Insulation: avoiding risks’.

Figure 1: Exposure zones

Frost attack

Common factors which affect the level of frost attack include:

  • degree of exposure (incidence of frost)
  • saturation of the masonry
  • frost resistance of the masonry
  • localised protection of the masonry by roof overhangs, trees and other buildings.

Good detailing can limit persistent wetting and reduce the risk of frost attack:

  • Paths should drain away from walls to avoid saturating bricks near the ground.
  • Sills, copings and similar features should have a weathered upper surface.
  • A coping or capping should be provided for all parapet walls, chimneys and freestanding walls, unless clay bricks of F2,S1 or F2,S2 classification to BS EN 771-1 are used.

Copings should have:

  • an overhang
  • throatings a minimum of 40mm clear of the wall
  • a continuous supported DPC which projects beyond the line of the wall.

The following should be taken into account when selecting bricks:

  • Manufacturers’ recommendations, including the choice and use of mortar and the type of joint finish.
  • Bricks that are not frost-resistant (F0,S2 or F0,S1 to BS EN 771) may not be acceptable for use externally, unless completely protected by a cladding which can adequately resist the passage of water.
  • Where there is a risk that brickwork may be persistently wet, bricks should be specified that are low in soluble salts.
  • Painted or decorated finishes can trap moisture in external brickwork and increase the risk of frost damage, sulfate attack or other detrimental effects. They should not be applied to S1 designation bricks without written agreement from the brick manufacturer.
  • In Scotland, all clay bricks used as facings should be frost-resistant, F2,S2 or F2,S1 to BS EN 771-1.

Severe exposure to frost attack
The hatched areas on the map opposite have a frost over 60 days in a year, annual rainfall over 1m and are 90m above sea level. They are therefore considered to be at severe exposure to frost.

In areas of severe exposure to frost, the following types of brick are acceptable:

  • Clay facing bricks which are frost-resistant F2,S2 or F2,S1 to BS EN 771-1.
  • Clay bricks which are classified in the manufacturer’s published recommendations as satisfactory for the exposure.
  • Calcium silicate bricks of at least compressive strength Class 30 and declared as freeze/thaw resistant to BS EN 771.
  • Concrete bricks with a minimum strength of 20N/mm2.
  • Concrete blocks with a minimum density of 1,500kg/m3 or compressive strength greater than 7.3N/mm2.
  • Most types of aerated concrete blocks with render.

Exceptionally severe frost exposure

These are locations which face long stretches of open countryside and are within an area of severe frost exposure, where only frost-resistant bricks F2,S2 or F2,S1 to BS EN 771 are acceptable for the superstructure.

Where there is doubt about the suitability of a facing brick for sites in areas of exceptionally severe frost exposure, written confirmation should be obtained from the brick manufacturer that the brick is suitable for the geographical location, and location in the structure.

Postcode areas at risk of severe exposure to frost attack

The following list identifies the postal areas within which the three criteria for severe exposure to frost attack are met.


1 Reproduced by permission of the London Brick Company Ltd.
2 Shaded boxes indicate areas which are wholly within areas of severe frost exposure. Other areas are partly within.