6.1 External masonry walls


This Chapter gives guidance on meeting the Technical Requirements and recommendations for external masonry walls.


6.1 - D1 Design standards
6.1 - D2 Statutory requirements
6.1 - D3 Structural design
6.1 - D4 Exposure
6.1 - D5 Mortar
6.1 - D6 Damp-proof courses and cavity trays
6.1 - D7 Wall ties
6.1 - D8 Stone masonry
6.1 - D9 Lintels
6.1 - D10-D11 Thermal insulation
6.1 - D12 Fire safety
6.1 - D13 Sound insulation
6.1 - D14 Claddings
6.1 - D15 Rendering
6.1 - D16-D17 Provision of information

6.1 - D1
Design shall meet the Technical Requirements

Design that follows the guidance below will be acceptable for external masonry walls.


6.1 - D2
Design shall comply with all relevant statutory requirements

Design should be in accordance with relevant Building Regulations and other statutory requirements.


6.1 - D3
External cavity and solid walls shall be designed to support and transfer loads to foundations safely and without undue movement

Items to be taken into account include:


Structural design should be in accordance with BS EN 1996-1-1.

Intermediate floors and roofs should be designed to provide lateral restraint to external walls, in accordance with BS 8103 and ancillary components to BS EN 845-1.

Walls of dwellings or buildings containing dwellings over three storeys high should be designed in accordance with Technical Requirement R5.

lateral restraint provided by concrete floors

Concrete floors, with a minimum 90mm bearing onto the wall, provide adequate restraint.

Concrete floors running parallel to and not built into walls require restraint straps to provide restraint to the wall (reference should also be made to Chapter 6.4 'Timber and concrete upper floors' (each section)).

lateral restraint provided by timber floors

Timber joisted floors can provide adequate restraint when joists are carried by ordinary hangers to BS EN 845 and connected to the wall with restraint straps.

In buildings of not more than two storeys, timber joisted floors can provide adequate restraint without strapping when:

  • the minimum bearing onto masonry is 90mm (or 75mm onto a timber wall plate), or
  • joists are carried by restraint type hangers, as described in BS EN 845-1 with performance equivalent to a restraint strap.
point loads

Padstones and spreaders may be necessary and, where required, should be located beneath areas of concentrated loads.


The position and depth of chases for services should be considered. Horizontal chases should be limited to one-sixth the thickness of a single leaf, and vertical chases to one-third the thickness of a single leaf.

Particular care is needed where hollow blocks are specified. Hollow blocks should not be chased, unless specifically permitted by the manufacturer.

Chases should not be formed in external walls within 650mm of a sound-resisting wall.


When partition walls abut the external wall and are of similar materials, they may be either fully bonded or tied together. Where materials have dissimilar shrinkage or expansion characteristics, eg dense concrete and aerated concrete, a tied joint is preferable as this will reduce the risk of random cracking.

In the case of a connection between a loadbearing wall on foundations and a non-loadbearing wall supported on a ground bearing slab, it is preferable to tie, not bond, the walls. This will reduce the risk of cracking due to differential vertical movement.

Tied joints should be formed using expanded metal, wire wall ties or a proprietary equivalent, at maximum 300mm intervals.

movement joints

Movement joints should be provided, where necessary, and in such a way that stability is maintained. If no provision is made for both initial and long term movements, masonry walls may crack.

Vertical movement joints should be provided in the outer leaf to minimise the risk of major cracking, as shown in the following table:

Material Joint width
Normal spacing
Clay brick 16 12m (15m maximum)
Calcium silicate brick 10 7.5 to 9m
Concrete block and brick 10 6m
Any masonry in a parapet wall 10 half the above spacings and 1.5m from corners (double the frequency)

The spacing of the first movement joint from a return should not be more than half of the above dimension.

When different materials are used together, consideration should be given to potential differential movement. Wall ties are needed on either side of movement joints (reference should be made to Clause D7 and Sitework clause 6.1 - S5).

Movement joints should run the full height of the masonry wall. Any movement joints provided in the substructure must be carried up into the superstructure. Movement joints may be needed in the superstructure where none are required in the substructure - however suitable allowance should be made for relative movement.

Where masonry walls form panels in a framed structure, movement joints should be provided in accordance with BS EN 1996-2.

Details of suitable materials to form movement joints are given in the Materials section of this chapter.

Where movement joints are provided to control shrinkage in concrete blockwork, they may be simple vertical joints filled with mortar and sealed.

To ensure the sealant is effective, there should be a good bond with the masonry. The sealant should be at least 10mm deep or in accordance with manufacturers' instructions.

Movement joints are not normally necessary to the inner leaf of cavity walls but consideration should be given to providing:

  • movement joints in rooms with straight unbroken lengths of wall over 6m. This is unnecessary for fired clay bricks
  • bed joint reinforcement as an alternative to movement joints in areas of risk, eg under window openings.

To reduce cracking and to maintain the level of thermal resistance:

  • bricks and blocks, or blocks of different densities, in a wall should not be mixed
  • a joint should be formed where dissimilar materials abut
  • the joint should be tied (eg with expanded metal in the bed joint) unless the joint is to act as a movement joint.

Where cracking is likely, walls should be dry lined or clad (reference should also be made to Sitework clause 6.1 - S2(g)).

calcium silicate brickwork

Design of calcium silicate brickwork should follow the the brick manufacturer's recommendations.

cladding to framed structures

Allowance should be made for differential movement between cladding and frame.

The following precautions should be taken to prevent buckling and fracturing of masonry panels:

  • flexible movement joints should be provided at the underside of each horizontal support member
  • the masonry outer leaf should have at least two-thirds of its width supported securely by the concrete frame or a metal angle
  • the inner leaf should be adequately tied to the structural frame. Forked plate ties held in dovetail slots, cast into the column or an equivalent are acceptable
  • vertical movement joints should be provided at corners.

For timber framed construction, reference should be made to Chapter 6.2 'External timber framed walls' (Design).


The size of corbels should not exceed the dimensions given in Sitework clause 6.1 - S2(j).


6.1 - D4
External walls shall be suitable for their exposure and resist the passage of moisture to the inside of the dwelling

In this Chapter, reference is made to exposure to:

  • wind driven rain
  • frost attack.

Details of how these are defined are contained in Appendices 6.1-A and 6.1-B.

Items to be taken into account include:

general aspects affecting durability

Masonry in the following locations is particularly likely to become saturated and may remain so for long periods. Precautions as necessary should be taken to resist frost damage and sulfate attack in:

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

The selection of bricks and mortar should follow the recommendations given in BS EN 1996-1-1 and manufacturers' recommendations.

In addition to the mortar designations given in BS EN 1996-1-1, the following mortar mixes can be used with ordinary Portland cement or sulfate-resisting cement:

  • air-entrained 1 : 1 : 5½ cement : lime : sand, or
  • air-entrained 1 :½ : 4½ cement : lime : sand.

Sulfate-resisting cement should be used where clay bricks with S1 designation are used as follows:

  • below dpc where there are sulfates present in the ground
  • below 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 be used only in accordance with Technical Requirement R3.

rain penetration

Rainwater will penetrate the outer leaf of a masonry wall in prolonged periods of driving rain. Resistance to rain penetration of masonry walls can be improved by cladding the wall. Total resistance can only be achieved with an impervious cladding.

The following should be taken into account to minimise the risk of rain penetration:

  • determination of the exposure to wind driven rain
  • a suitable wall construction and insulation method
  • design detailing for the local exposure, taking into account the likely quality of workmanship on site.

A very high standard of workmanship is required to ensure that cavities are not bridged. Where full or partial cavity insulation is proposed, the installation should follow the recommendations of any assessment and the manufacturer.

The most exposed part of the building should be given particular attention when selecting a suitable construction method as this may affect the choice for the whole building.

The following aspects of design can reduce the risk of rain penetration:

  • providing cladding (other than render) to the wall. Even if cladding is only added to gable walls and upper floors, it reduces rain penetration
  • increasing the clear cavity width or the width of full cavity insulation. Increasing the cavity width for full cavity insulation from 50mm to 75mm or more greatly reduces the risk of rain passing through the cavity. A nominal cavity of 50mm is always required on the outside of partial cavity insulation
  • rendering the wall (reference should also be made to Clause D15). Specify backing material carefully to avoid cracking which can reduce the effectiveness of render against rain penetration
  • designing protective features to keep the wall dry, eg projecting sills and deep overhanging eaves and verges
  • mortar joints. All joints should be fully filled. Where full cavity insulation is proposed, recessed joints should not be used. Reference should also be made to Clause D5(c).

In areas of Very Severe exposure to driving rain and in Scotland the cavity should not contain full fill insulation.

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.

Rebated or check reveal in areas of very severe exposure

Proprietary cavity closers may be an acceptable alternative provided they have been assessed in accordance with Technical Requirement R3. For information on doors and windows, reference should be made to Chapter 6.7 'Doors, windows and glazing' (each section).

In Scotland and areas of Severe or Very Severe exposure to driving rain, cavities should be continuous around enclosed porches and habitable areas.

Cavity trays should be used at junctions with roof (reference should also be made to Clause D6).

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

frost attack

The main factors affecting frost attack are:

  • 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.

Areas of severe frost exposure are shown on the map in Appendix 6.1-B.

In areas of exceptionally severe frost exposure, which is defined as a location which is in a severe frost exposure area and, in addition, faces long stretches of open countryside, only frost-resistant bricks (F2,S2 or F2,S1 to BS EN 771) are acceptable for the superstructure.

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

  • clay facing bricks which are frost-resistant (F2,S2 or F2,S1 to BS EN 771). Reference should be made to Appendix 6.1-E for the freeze/thaw resistance classification of bricks to BS EN 771
  • clay bricks which are classified in manufacturers' published recommendations as satisfactory for the exposure
  • calcium silicate bricks (in accordance with BS EN 771)
  • concrete bricks with a strength not less than 20N/mm2
  • concrete blocks with a density not less than 1500kg/m3 or of strength not less than 7.3N/mm2
  • most types of aerated concrete blocks with render.

In Scotland, all clay bricks used as facings should be frost-resistant (F2,S2 or F2,S1 to BS EN 771).

If there are doubts about the suitability of a facing brick for sites in areas of exceptionally severe frost exposure classification, written confirmation should be obtained from the brick manufacturer that the brick is suitable for:

  • its geographical location, and
  • its location in the structure.

This applies particularly to bricks such as fletton facings which are moderately freeze/thaw resistant (F1,S2 or F1,S1). In addition, follow manufacturers' recommendations on suitability, including the choice and use of mortar and the type of pointing.

Recessed joints should only be used in compliance with Clause D5(c).

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.

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

For example:

  • paths should drain away from walls to avoid saturating bricks near the ground
  • sills, copings and the like should have a weathered upper surface
  • a coping 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 have been used. Copings should have a generous overhang, throatings at least 40mm clear of the wall and a continuous, supported dpc underneath which projects beyond the line of the wall. Single leaf parapet walls should not be rendered on both sides.

    Where there is a risk that brickwork may be persistently wet, bricks should be specified that are low in soluble salts (if clay, F2,S2 or F1,S2 to BS EN 771).


    Only clay bricks designated L by BS EN 771 have a low limit on their soluble salt content. In persistently wet conditions, clay bricks of S1 designation may create sulfate attack on the mortar.

    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 S1designation bricks without the brick manufacturer's written agreement.


    6.1 - D5
    Mortar shall be of the mix proportions necessary to achieve adequate strength and durability and be suitable for the type of masonry

    Items to be taken into account include:

    geographical location and position within the structure

    Recommended mortar mixes for different locations are given in Appendix 6.1-C.

    sources of sulfate

    Ordinary Portland cement mortar can expand, crumble and deteriorate badly if attacked by sulfates. Sufficient soluble sulfate to cause this problem may be contained in clay bricks. Clay bricks with an S1 designation have no limit on their sulfate content. The problem is most acute when brickwork is saturated for long periods; mortar is vulnerable to attack by any soluble sulfates present.

    To reduce the risk, sulfate-resisting Portland cement to BS 4027 should be used:

    • below dpc level when sulfates are present in the ground
    • when clay bricks (F2,S1 and F1,S1 to BS EN 771) are used, and there is a high saturation risk, for example in the following situations:
      • parapets
      • chimney stacks
      • retaining walls
      • freestanding walls
      • rendered walls
      • areas of Severe or Very Severe exposure to driving rain.

    Struck (or weathered) and bucket handle joints are preferable.

    Recessed joints should not be used where:

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

    Jointing is preferable to pointing because it leaves the mortar undisturbed.

    admixtures and additives

    Calcium chloride should not be used as an admixture to mortar. The contents of admixtures should be checked to ensure that they do not contain calcium chloride.

    Admixtures should only be used in accordance with manufacturers' recommendations/instructions.

    Mortars containing an air-entraining plasticiser are more resistant to freeze/thaw damage when set.

    White cement to BS EN 197 and pigments to BS 1014 may be used, but pigments should not exceed 10% of the cement weight or 3% if carbon black is used.

    Further advice concerning admixtures is given in Appendix 6.1-C.


    6.1 - D6
    Damp-proof courses and related components shall be provided to prevent moisture rising or entering the building

    Items to be taken into account include:

    (a) dpcs

    Damp-proof courses should be provided in accordance with the Table in Appendix 6.1-D.

    At complicated junctions, clear drawings should be provided and preformed profiles specified. Isometric drawings can sometimes be clearer than the combination of plan and section/elevation drawings.

    (b) cavity trays

    Cavity trays should be provided at all interruptions to the cavity, eg window and door openings, air bricks, etc, unless otherwise protected, eg by overhanging eaves.

    A cavity tray should:

    • provide an impervious barrier and ensure that water drains outwards
    • project at least 25mm beyond the outer face of the cavity closure or, where a combined cavity tray and lintel is acceptable, give complete protection to the top of the reveal and vertical dpc where provided
    • provide drip protection to door and window heads
    • have an overall minimum upstand from the inside face of the outer leaf to the outside of the inner leaf of 140mm
    • be shaped to provide at least a 100mm vertical protection above a point where mortar droppings could collect.
    Shape of an effective cavity tray

    In Scotland, Northern Ireland, the Isle of Man and areas of Very Severe exposure to driving rain, the upstand part of the damp-proof protection should be returned into the inner leaf of masonry except at sloping abutments. In all other areas, the upstand should be returned into the inner leaf unless it is stiff enough to stand against the inner leaf without support.

    Where fairfaced masonry is supported by lintels:

    • weep holes should be provided at 450mm (maximum) centres with at least two weep holes per opening
    • cavity trays or combined lintels should have stop ends.

    Where full fill insulation is placed in the cavity, a cavity tray should be used above the highest insulation level, unless the insulation is taken to the top of the wall. (Manufacturers' recommendations should be followed.)

    (c) abutment details

    Cavity trays should be provided at abutments of roofs and cavity walls. This will ensure that any water penetrating into the cavity does not enter the enclosed area. This only applies where the roof is over an enclosed area, including an attached garage, but does not apply to open car ports and open porches.

    Where the roof abuts at an angle with the wall, preformed stepped cavity trays should be provided.

    Stepped cavity trays

    (d) parapet details

    Dpcs below the coping should be supported over the cavity to prevent sagging. A dpc should be specified that can achieve a good key with the mortar.

    Dpc supported over cavity

    (e) materials

    Materials that are suitable for use as dpcs are given in Materials clause 6.1 - M6.


    6.1 - D7
    Wall ties shall be provided, where required, to tie together the leaves of cavity walls

    The spacing of wall ties in masonry walls should be in accordance with Sitework clause 6.1 - S5.

    Details of suitable wall ties are given in the Materials section of this Chapter.


    6.1 - D8
    Elements constructed of natural or cast stone masonry shall comply with the performance standards for brick and block masonry, where applicable

    Stone masonry (natural or cast stone) should be designed to meet the requirements of BS EN 1996 'Design of masonry structures'.


    6.1 - D9
    Lintels shall safely support the applied loads

    Items to be taken into account include:

    structural support

    Concrete, steel and reinforced brickwork are acceptable materials for use as lintels.

    Timber lintels should not be used, unless:

    • they are protected from the weather, and
    • they do not support masonry or other rigid or brittle materials.

    Lintels should be designed in accordance either with Technical Requirement R5 or manufacturers' published data. A lintel should be provided where frames are not designed to support superimposed loads.

    Lintels should be wide enough to provide adequate support to walling above. Masonry should not overhang the lintel support by more than 25mm. A lintel should extend beyond the opening at each end by at least the following lengths:

    Minimum bearing length (mm)
    Span (m) Simple
    Lintel combined
    with cavity tray
    Up to 1.2 100 150
    Over 1.2 150 150

    To avoid overstressing, composite lintels should have the required depth of fully bedded brickwork stipulated by the manufacturer above the lintel, before point loads are applied. Where necessary, padstones and spreaders should be provided under the bearings of lintels. Reference should be made to Chapter 6.5 'Steelwork support to upper floors and partitions' (Design) for details of padstones.

    adequate durability against corrosion and resistance to water entering the dwelling

    Cavity tray/damp-proof protection should be provided over all openings, either as a combined part of the lintel or separately. Reference should be made to Clause D6(b).

    Steel and concrete lintels should comply with BS EN 845-2.

    Separate cavity tray protection should be provided when:

    • when the profile of the lintel is not as shown in Clause D6(b), or
    • steel lintels have materials coatings references L11, L14 and L16.1 and are used in external walls

    Lintels used in aggressive environments (e.g. coastal sites) should be austenitic stainless steel.

    In Scotland, Northern Ireland, the Isle of Man and areas of Severe or Very Severe exposure to driving rain, separate damp-proof protection should be provided over all lintels in accordance with the guidance for cavity trays given in Clause D6(b).

    Lintels should be of such a size and be located so that the external edge of the lintel projects beyond, and therefore offers protection to, the window head.

    cold bridging and condensation

    The BRE Report 'Thermal insulation: avoiding risks' discusses aspects of insulation relevant to external masonry walls. In England and Wales account should be taken of Accredited Details.

    The risk of condensation at reveals and soffits becomes more likely as the wall insulation increases. Cold bridge paths should be avoided.

    To avoid a cold bridge, the wall insulation should ideally abut the head of the window frame.

    Clause D4(b) details methods of preventing rain penetration which may also be required.

    adequate fire resistance

    Where steel lintels are used, manufacturers' recommendations for providing adequate fire resistance, particularly to the lower steel flange, should be followed.


    6.1 - D10
    External walls shall be designed to provide the required standard of thermal insulation

    The insulation value of the wall must meet the requirements of the relevant Building Regulations.

    Design should avoid cold bridging at openings and at junctions of external walls with roofs, floors and internal walls.

    6.1 - D11
    External walls shall be designed to ensure the correct use of insulation materials

    Items to be taken into account include:

    acceptable insulation materials

    Insulation, or lightweight aerated concrete blocks, or blocks with face bonded insulation, or voided blocks with insulation infill should be used in accordance with:

    full cavity 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, it is not permissible to fill the full width of the cavity with any thermal insulants at the time of construction.

    The type of insulation, its thickness and the wall construction should be suitable for the exposure of the dwelling in accordance with Appendix 6.1-A.

    Materials clause 6.1 - M9 sets out the range of acceptable insulation materials and the relevant British Standards.

    Render on an external leaf of clay bricks  (F2,S1 or F1,S1 designation bricks to BS EN 771) in Severe or Very Severe exposures is not permitted where the cavity is to be fully filled with insulation.

    The following design points should be noted:

    • stop ends should be provided to cavity trays or combined lintels
    • weepholes should be provided at 450mm (maximum) centres with at least two per opening
    • mortar joints should not be recessed
    • painted finishes on bricks or render are not acceptable if they are likely to cause frost damage or sulfate attack or other damage.
    partial cavity insulation

    Partial cavity insulation should be fixed only against the cavity face of the inner leaf. The clear cavity width between partial cavity insulation and the outer leaf should be at least 50mm nominal. In areas of Very Severe exposure in England and Wales, a residual cavity of 75mm is required where the outer leaf is fair faced masonry.

    Wall ties should be long enough to allow a 50mm embedment in each masonry leaf.

    inner leaf of insulated blockwork

    Types of blockwork include:

    • lightweight aerated concrete
    • lightweight aggregate blocks
    • voided blocks with insulation infill
    • blocks faced with insulation material.

    Manufacturers' recommendations should be followed and particular note taken of the following:

    • a clear 50mm wide cavity should always be maintained
    • the blocks should be capable of supporting concentrated loads
    • the correct type of joist hanger for the type and size of block and size of joist should be used
    • long unbroken lengths of blockwork should be avoided
    • precautions should be taken to reduce risk of shrinkage cracking
    • dry lining should be used where shrinkage cracking might be unsightly and to avoid long term pattern staining at mortar joints
    • restrictions on chasing for services when using voided blocks should be noted (reference should be made to Clause D3(e)).
    dual insulation

    Where partial cavity insulation is used in addition to an insulated block inner leaf (reference should be made to Clause D11(c)), the whole composite construction should have been assessed in accordance with Technical Requirement R3.

    insulated dry linings

    Where an insulated dry lining contains a combustible insulant, the plasterboard should be at least 12.5mm thick and mechanically fixed to the masonry inner leaf. This is to prevent early collapse of the lining in a fire.


    6.1 - D12
    Cavity walls shall adequately resist the passage of fire

    Cavities should be closed with cavity closers in accordance with statutory requirements.


    6.1 - D13
    External walls adjacent to sound-resisting walls shall be designed to adequately resist flanking sound transmission

    Acceptable levels of sound reduction between dwellings may be achieved by:

    • the inner leaf of an external cavity wall having sufficient weight, and
    • sealing of air paths
    • allowing appropriate spacings between openings in external walls.

    The density of external walls and the position of openings adjacent to sound-resisting walls should be in accordance with statutory requirements and, where relevant, an assessment which complies with Technical Requirement R3.


    6.1 - D14
    Cladding shall satisfactorily resist the passage of moisture

    Items to be taken into account include:

    (a) battens

    Where battens are required, they must be pre-treated with preservative, as described in Chapter 2.3 'Timber preservation (natural solid timber)' (each section).

    (b) joints

    Joints between claddings and adjacent materials should be detailed to be watertight under the particular exposure conditions of the site. Where necessary, provision should be made for differential movement.

    (c) moisture barriers

    Unless specifically not required for a proprietary cladding, moisture barriers should be provided between walls of solid masonry and any boarding, slating, tiling or other similar claddings. The moisture barrier may be roofing underfelt or equivalent.

    Vapour barriers such as polyethylene sheet are not an acceptable moisture barrier as they can trap moisture in the structure.

    For timber framed walls clad with masonry, reference should be made to Chapter 6.2 'External timber framed walls' for details.

    (d) vertical tile or slate cladding

    Every tile or slate should be nailed with two nails. Nails should be aluminium, copper or silicon bronze.

    Bottom edges should be finished with an under-course and tilting batten.

    (e) timber cladding

    Timber claddings should be pre-treated with preservative in accordance with Chapter 2.3 'Timber preservation (natural solid timber)' (each section).


    6.1 - D15
    Rendering, in conjunction with the surface to which it is applied, shall satisfactorily resist the passage of moisture

    Items to be taken into account include:

    rain penetration

    External rendered finishes should comply with BS EN 13914 'Design, preparation and application of external rendering and internal plastering' and the guidance given below.

    It is important to prevent rainwater penetrating behind the rendering. Design features around openings and at the head of the rendering should provide shelter, where possible, and help to shed water away from the surface below.

    exposed elements

    It is not advisable to render exposed parts of a building, such as parapets and chimneys constructed of clay bricks of S1 designation, without the use of sulfate-resisting cement.


    Movements can occur at a change in material. In such cases, the render should be either stopped at specially formed movement joints or, if the expected movement is small, be reinforced by metal lathing carried across the joint. If metal lathing is used, three rendering coats should be applied.


    To achieve a good bond, the masonry backing should be moderately strong and porous to give some suction and a mechanical key. Dense masonry with a smooth surface should not be rendered.

    Aerated or lightweight aggregate concrete blocks can be used, as a background, but more care is needed when selecting a rendering mix and surface treatment. Strong render mixes should not be used. Roughcast and dry dash finishes that require a strong mix are not recommended for use on aerated or lightweight aggregate blocks. Block manufacturers' recommendations should be followed.

    In Scotland, render should be applied only to bricks:

    • which are keyed, or
    • where a spatterdash coat has been applied before the first render undercoat.

    In other areas, render should be applied only to bricks where either:

    • keyed bricks are used, or
    • the joints are raked out at least 15mm deep.

    Render may be applied to bricks (if clay F2,S1 or F1,S1 to BS EN 771) only if the following conditions are met:

    • cement for brickwork mortar is sulfate-resisting to BS 4027
    • the brick manufacturer has confirmed, in writing, that the brick is suitable, taking account of the brickwork detailing and the particular exposure of each rendered element. If sulfate-resisting cement is used in the mortar, it should also be used in spatterdash coats and base coats of the render.

    Care should be taken when specifying render to walls with full cavity fill. The lack of a ventilated cavity can slow down the rate at which the wall dries out.

    Rendered finishes should not be used over fully filled cavity walls if:

    • the outer leaf is built in bricks with S1 designation (F2,S1 and F1,S1 to BS EN 771), and
    • the site is in an area classed as Severe or Very Severe exposure to driving rain (see Appendix 6.1-A).

    Rendering may be used on brickwork with partial cavity insulation provided a clear cavity width between insulation and outer leaf of at least 50mm nominal is maintained.


    The rendering mix should be appropriate to the strength of the background. No render coat should be stronger than the background or richer than the preceding coat. The render should be of adequate strength to achieve durability.

    Mixes should comply with the recommendations of BS EN 13914 'Design, preparation and application of external rendering and internal plastering'. When rendering on bricks that are F1,S1 or F1,S2 to BS EN 771, the Table to Sitework clause 6.1 - S8(b) should be followed. The manufacturer of the background masonry should be consulted regarding particular requirements for the mix or its application.

    Pigments complying with the requirements of BS 1014 may be added to the finishing coat up to a limit of 10% of the cement weight or 3% in the case of carbon black. White Portland cement may be used.

    number and thickness of coats

    The number of coats should be chosen with regard to the background and the exposure conditions of the site.

    For rendering on masonry cavity walls, one undercoat and one finishing coat is acceptable. On metal lathing or on solid wall construction, two undercoats and one finishing coat are required.

    Initial undercoats should not be less than 10mm and not more than 15mm thick. Any further undercoat should be thinner than the preceding coat. Finishing coats should be generally between 6mm and 10mm thick.

    Undercoats should be allowed to shrink and dry out before applying following coats. When rendering onto dense concrete blocks, adhesion can be improved by use of proprietary bonding agents or a spatterdash coat.

    detailing of timber/brick/render

    Dwellings which incorporate rendered panels between timber boards should have at least one coat of render applied over the whole wall face before the boards are fixed. The second coat may be applied between the boards.

    Rendering and timber can shrink causing gaps. Precautions should be taken to prevent rain from penetrating the junction as this might cause the render to fail as a result of frost damage.

    All exposed timber, except naturally durable species, should be treated in accordance with Chapter 2.3 'Timber preservation (natural solid timber)' (each section).

    Where timber is used on brick or render, it is essential that all cut ends, mortices, etc made after treatment are flood coated with preservative.

    Large section timbers should be fitted with suitable weather bars, flashings, etc to prevent moisture penetration through joints with adjacent materials.

    Non-ferrous fixings should be used. Aluminium is not suitable when the preservative is Copper/Chromium/ Arsenic.

    proprietary and local rendering systems

    Proprietary rendering finishes should be applied in accordance with manufacturers' recommendations.

    Traditional local rendering should comply with the above guidance, as appropriate, and with established local practice.


    6.1 - D16
    Designs and specifications shall be produced in a clearly understandable format and include all relevant information

    For external masonry walls, the drawings should show:

    • wall layout with all dimensions shown
    • position and size of openings
    • coursing of the bricks and blocks in relation to storey heights and opening positions
    • details at all junctions, indicating position of dpcs and cavity trays. Isometric sketches are recommended for complicated junctions
    • position and type of lintels
    • position of restraint straps
    • details of cavity closers
    • details at reveals
    • details of how support is given to other elements, eg padstones and wall plates
    • position and detail of movement joints
    • acceptable methods of pointing or mortar joint finish
    • type of insulant to be used
    • type and location of wall ties.
    6.1 - D17
    All relevant information shall be distributed to appropriate personnel

    Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.

    Where proprietary products are to be used, manufacturers usually have specific requirements for fixing and/or assembly of their products. This information should also be made available for reference on site so that work can be carried out satisfactorily in accordance with the design and specification.