NHBC Standards 2006

6.1 External masonry walls


6.1 - M1Materials standards
6.1 - M2Bricks
6.1 - M3Blocks
6.1 - M4Stone masonry
6.1 - M5Mortar and rendering
6.1 - M6DPC materials
6.1 - M7Wall ties
6.1 - M8Lintels
6.1 - M9Thermal insulation
6.1 - M10Cladding materials
6.1 - M11Movement joints

6.1 - M1
All materials shall:
(a) meet the Technical Requirements
(b) take account of the design

Materials that comply with the design and the guidance below will be acceptable for external masonry walls.

Materials for external masonry walls shall comply with all relevant standards, including those listed below. Where no standard exists, Technical Requirement R3 applies (see Chapter 1.1 'Introduction to the Standards and Technical Requirements').

References to British Standards and Codes of Practice include those made under the Construction Products Directive (89/106/EEC) and, in particular, appropriate European Technical Specifications approved by a European Committee for Standardisation (CEN).


6.1 - M2
Bricks shall:

be capable of supporting intended loads

Requirements for the design strength of bricks are given in BS 5628: Part 1. The brick specified in the design should be used. Clay bricks to BS EN 771 with a minimum compressive strength of 9N/mm2 should be adequate for one and two storey dwellings and 13N/mm2 for three storey dwellings.

have appropriate resistance to the adverse effects of freeze/thaw and sulfate attack


BS EN 771 classifies clay bricks according to their freeze/thaw resistance and soluble salt content (see Appendix 6.1-E).

Only bricks that are freeze/thaw resistant (F2,S2 or F2,S1 to BS EN 771) should be used where there is a high risk of prolonged wetting and freezing. Such areas include:

  • all external facing work in Scotland
  • exposed parts including copings, sills and parapets and chimneys which have no overhang to provide protection. Reference should be made to Design clause 6.1 - D4(c)
  • areas of the country subject to exceptionally severe freeze/thaw exposure (see map in Appendix 6.1-B).

In areas of severe freeze/thaw exposure outside Scotland, bricks that are moderately freeze/thaw resistant (F1,S1 or F1,S2 to BS EN 771) may be used for general wall areas, provided they are classified in manufacturers' published recommendations as satisfactory for the exposure.

Bricks that are not freeze/thaw resistant (F0,S2 or F0,S1 to BS EN 771) are not acceptable for use externally, unless completely protected by a cladding which can resist satisfactorily the passage of water.

Where brickwork may become saturated, moderately freeze/thaw resistant bricks (F1,S1 or F1,S2 to BS EN 771) are not appropriate if there is a risk of vulnerability to frost. In saturated conditions, sulfate-resisting cement mortar is required for S1 designation bricks.


Bricks of compressive strength Class 20 (to BS EN 771-2) are suitable for most applications. Bricks of strength Class 30 and declared as freeze/thaw resistant (to BS EN 771-2) are recommended in areas of severe freeze/thaw exposure (see map in Appendix 6.1-B) or for use where bricks may be persistently wet (eg parapets, chimneys, sills and below dpc).

Calcium silicate bricks do not contain significant amounts of soluble sulfates and may be suitable where sulfate bearing soil and ground water conditions exist. Manufacturers' recommendations should be followed.


In concrete bricks there is a direct relationship between strength and durability, including freeze/thaw resistance. Most concrete bricks in production have a strength of 20N/mm2 and are durable in most situations. For copings and sills, bricks with a compressive strength of 36N/mm2 should be used.


Reclaimed bricks could be unsuitable for external work because of a high salt content or a lack of freeze/thaw resistance. Their use is permitted only in accordance with Technical Requirement R3. It is advisable to know where they come from, both geographically and within the previous building. Bricks used internally or fully protected may be unsuitable in external situations.

Reclaimed bricks should be considered as F1,S1 or F1,S2 to BS EN 771 and used accordingly. Independent certification of suitability may be required.


Special shaped bricks should conform to BS 4729.


6.1 - M3
Concrete blocks shall:

be capable of supporting intended loads

Blocks should comply with BS EN 771 and be used in accordance with BS 5628: Part 3.

In general, 2.9N/mm2 blocks are suitable for one and two storey dwellings.

For three storey dwellings or dwellings with storey heights over 2.7m, 7.3N/mm2 blocks are required for certain parts of the structure.

Structural design may show that strengths lower than 7.3N/mm2 are adequate.

Other factors may dictate the strength of blocks required in certain circumstances, eg sulfate-resistance may require blocks of greater strength.

The maximum loadbearing capacity of the wall should not exceed manufacturers' recommendations.

have appropriate resistance to adverse effects of freeze/thaw and sulfate attack

Concrete blocks used in the outer leaf without protective cladding or render, should:

  • have a compressive strength exceeding 7.3N/mm2, or
  • have a density exceeding 1500kg/m3, or
  • be made with dense aggregate to BS EN 12620, or
  • be lightweight aerated concrete blocks having had their suitability confirmed by the manufacturer.

Concrete blocks should not be used below dpc where there are sulfates in the ground, unless suitability is confirmed by the block manufacturer. Sulfates may attack the cement used in the block. Sulfate-resisting cement will be required in the mortar. The proportions will depend on the level of sulfates in the ground.

have an adequate thermal resistance, where required

The designer may have specified a particular type and thickness of concrete block because of its thermal insulation performance in addition to its strength. Alternative concrete blocks should not be used without the designer's acceptance.


6.1 - M4
Stone masonry shall be capable of supporting the intended loads and have appropriate resistance to the adverse effects of freeze/thaw

Stone for masonry should conform to the requirements of BS EN 771-6.

Reconstructed stone masonry units should comply with BS EN 771-5.


6.1 - M5
Mortar and rendering materials shall be of the mix proportions to achieve adequate strength and durability to comply with the design

Items to be taken into account include:

(a) cement and lime

Ordinary Portland cement should be to BS EN 197. Sulfate-resisting Portland cement should be to BS 4027. Masonry cement should be to either BS EN 197 or BS EN 413.

Limes should conform to BS EN 459.

(b) sand type

Sand and aggregate from natural sources should conform to BS EN 13139.

(c) mortar type

Ready-mixed mortars should comply with BS EN 998. For recommended mortar mixes, see Appendix 6.1-C.

(d) additives

Air entraining and set retarding admixtures should comply with BS EN 934.

Pigments for colouring mortars should conform to BS EN 12878.

(e) render

Sand for render should be sharp sand to BS EN 13139, preferably from the coarse end of the grading scale. Sand with excessive fine material will shrink and crack.

Metal reinforcement to rendering should be galvanized or stainless steel, in accordance with one of the specifications in BS EN 845.

Decorative finishes that contain asbestos are not acceptable.


6.1 - M6
Materials for damp-proofing shall resist adequately the passage of moisture

Items to be taken into account include:

(a) dpcs

The following materials are acceptable for use as dpcs:

  • bitumen to BS 6398
  • polyethylene to BS 6515 (except below copings and in parapets)
  • proprietary materials assessed in accordance with Technical Requirement R3.

Dpcs and flexible cavity trays should be of the correct dimensions to suit the detailed design.

For complicated junctions, preformed cavity trays are recommended. Care should be taken to order the correct type and shape.

(b) flashings

The following are acceptable as flashings:

  • rolled lead sheet (at least Code 4) complying with BS EN 12588
  • aluminium and aluminium alloys complying with BS 1470 (0.6mm to 0.9mm thick)
  • zinc alloy complying with BS 6561 and 0.6mm thick.


6.1 - M7
Wall ties shall be appropriate for their location and intended use

Wall ties shall be in accordance with either:

Ties should be long enough to be embedded at least 50mm into each leaf.

In England and Wales, wall ties should be stainless steel or non-ferrous. In Northern Ireland, stainless steel or non-ferrous ties should be used where the cavity is fully filled with insulation and 75mm wide or more. In Scotland, galvanised ties may be used.

Where partial cavity insulation is used, it should be held in place by retaining devices which may be clipped to the wall ties. These devices should be assessed in accordance with Technical Requirement R3 and used only with compatible wall ties.


6.1 - M8
Lintels shall be of the type and dimensions appropriate to their position within the structure

Steel and concrete lintels should comply with BS 5977.

Lintels up to 1.2m length which do not need a separate dpc tray should have a minimum 100mm end bearing and project beyond the cavity closer by at least 25mm. Normally, other lintels should be long enough to have a minimum 150mm end bearing each side of the opening.

Cavity trays may be required over the lintels. This should be specified in the design.


6.1 - M9
Insulation materials shall provide the degree of insulation to comply with the design

All retro-fill insulation materials (UF foam, blown mineral fibre and expanded polystyrene beads) should be installed by installers trained by the assessment holder and approved jointly by the assessment holder and the assessing organisation.

The installer should be a member of a surveillance scheme acceptable to NHBC.

Insulation materials should be installed in accordance with the following:

  • UF foam to BS 5617 installed in accordance with BS 5618
  • all other insulation materials, whether for full or partial cavity insulation, insulated blockwork or internal insulation may only be used if assessed in accordance with Technical Requirement R3.

The thickness of materials required by the design and Appendix 6.1-A should be used.


6.1 - M10
Cladding materials shall be of the quality, type and dimensions required by the design

Items to be taken into account include:

(a) tiles and slates

Clay tiles for tile hanging should be to BS 402.

Concrete tiles for tile hanging should be to BS 473.

Slates for vertical slating should be to BS 680.

(b) timber boarding

Timber should comply with BS 1186 and be Class 3 or better.

Timber should be a naturally durable species or pre-treated with preservative. Reference should be made to Chapter 2.3 'Timber preservation (natural solid timber)' (each section) for guidance on preservative treatments.

(c) underfelt behind cladding

Type 1F felt to BS 747 is acceptable as an underfelt behind cladding.

(d) battens

Battens should be of the size specified in the design and pre-treated with preservative treatments. Reference should be made to Chapter 2.3 'Timber preservation (natural solid timber)' (each section) for guidance on preservative treatments.

(e) proprietary cladding systems

Proprietary systems should be assessed in accordance with Technical Requirement R3.


6.1 - M11
Materials for movement joints shall be suitable for their intended purpose

When choosing materials, account should be taken of the following:

  • joint width
  • joint depth
  • anticipated movement
  • movement capability of material
  • surface preparation
  • backing materials
  • projected life span of joint.

Acceptable materials for movement joints in clay brick walls are:

  • flexible cellular polyethylene
  • cellular polyurethane
  • foam rubber.

Materials which are acceptable for use in contraction joints with concrete bricks or blocks, but not acceptable for use as expansion joints in fired clay bricks, are:

  • hemp
  • fibreboard
  • cork.

In concrete blockwork, the construction joint may be a simple vertical joint filled with mortar and sealed.