External masonry walls

6.1

6.1.3Structural design

External masonry shall be designed to support and transfer loads to foundations safely and without undue movement. Issues to be taken into account include:

  1. compliance with relevant standards
  2. lateral restraint
  3. concentrated loads
  4. bonding
  5. movement joints
  6. dpc.

Compliance with relevant standards

Design of masonry walls should comply with relevant standards:

Structural designBS EN 1996-1-1 ‘Eurocode 6. Design of masonry structures. General rules for
reinforced and unreinforced masonry structures’.
PD 6697 Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2.
Intermediate floors, roofs and walls designed to provide lateral restraint to external walls BS 8103 (all parts) ‘Structural design of low-rise buildings’.
Ancillary componentsBS EN 845-1 Specification for ancillary components for masonry. Wall ties,
tension straps, hangers and brackets
BS EN 845-2 ‘Specification for ancillary components for masonry. Lintels and
BS EN 845-3 Specification for ancillary components for masonry. Bed joint
reinforcement of steel meshwork’.
Walls of homes, or buildings containing
homes, over three storeys high		Designed by an engineer in accordance with Technical Requirement R5.

Lateral restraint

Lateral restraint provided by concrete floors:

Concrete floors, with a minimum bearing of 90mm onto the wall, can provide adequate restraint. Concrete floors running parallel to, and not built into, walls require restraint straps to provide restraint to the wall.

Lateral restraint provided by timber floors:

Timber joisted floors can provide adequate restraint when joists are carried by ordinary hangers to BS EN 845-1, and connected to the wall with restraint straps. In buildings up to 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 BS EN 845-1 restraint-type hangers with performance equivalent to a restraint strap spaced at a maximum of 2m centres.

Lateral restraint provided by buttressing walls

The ends of every wall should be bonded or otherwise securely tied throughout their full height to a buttressing wall, pier or frame. Long walls may be provided with intermediate buttressing walls, piers or support dividing the wall into distinct lengths within each storey with each distinct length being a supported wall for the purposes of this section.

The intermediate buttressing walls, piers or supports should provide lateral restraint to the full height of the supported wall, and they may be staggered at each storey.

Lateral restraint and acoustics

The ends of separating walls are only tied into the inner leafs and do not have multiple ties across the separating wall cavity at the end of the wall.

Concentrated loads

Concentrated loads should be designed by a suitably qualified Engineer, for example at the bearing of trimmers, lintels, multi ply trusses or steel beams.

Where bearing stresses under concentrated loads are greater than the strength of the supporting masonry wall, padstones and spreaders should be provided.

Padstones and spreaders may be required, to support concentrated loads.

Bonding

Where partition walls abut an external wall constructed of similar materials, fully bonded or tied joints are acceptable. To reduce the risk of cracking, a tied joint is preferable where:

  • materials have dissimilar shrinkage or expansion characteristics, e.g. dense concrete and aircrete concrete
  • there is a connection between a load-bearing wall on foundations and a non load-bearing wall supported on a ground-bearing slab.

Tied joints should be formed using expanded metal, wire wall ties or a proprietary equivalent, spaced at maximum 300mm intervals. Dissimilar materials should not be used in the same wall (e.g. clay bricks as “make up” courses in concrete blockwork walls).

Movement joints

Movement joints should be included in long lengths of walling to reduce unsightly cracking, and detailed so that stability is maintained. Where possible, joints should be hidden in corners, or behind rainwater pipes, and:

  • run the full height of the superstructure masonry wall
  • continue from those provided in the substructure to the superstructure (movement joints may be needed in the superstructure and not in the substructure, providing suitable allowance is made for relative movement).

Vertical movement joints should be provided in the outer leaf, in accordance with Table 1.

Table 1: Suitable dimensions and spacings for movement joints

MaterialJoint width (mm)Normal spacing (m)
Clay brick16210-12
Calcium silicate brick107.5 – 9
Lightweight concrete block and brick (autoclaved or using lightweight aggregates)(2)1064
Dense concrete block and brick (using dense aggregate)(2)107.5 – 9(3)
Any masonry in a parapet wall10Half the above spacings and 1.5 from corners (double frequency)
Natural stone masonry105
15 - 20 6

Notes

1 Manufacturer’s guidance for the provision of movement joints and bed joint reinforcement should be considered.
2 For clay bricks, the joint width in mm should be spacing in metres+30%. i.e at 8m movement joint spacing the joint width should be 10mm.
3 Lightweight concrete masonry units are generally made of aggregates that have a gross density not exceeding 1,500 kg/m³. Dense concrete masonry units are generally made of aggregate that have a gross density exceeding 1,500 kg/m³.
4 The ratio of length to height of the panels should generally not exceed 3:1
5 As defined within PD 6697
6 Located no more than 7.5m from an external corner.

The spacing of the first movement joint from a return should not be more than half of the dimension in Table 1.

Movement joints are not generally necessary in 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
  • bed joint reinforcement as an alternative to movement joints in areas of risk, e.g. under window openings.

Wall ties should be provided on either side of movement joints, in accordance with Clause 6.1.18.

Where masonry walls form panels in a framed structure, movement joints should be provided in accordance with BS EN 1996-2 and PD 6697.

Movement joints should be formed using the correct materials, and account taken of:

  • joint width and depth
  • anticipated movement and capability of the material
  • surface preparation and backing materials
  • likely design life of the joint.

Clay bricks expand and require movement joints formed from easily compressible materials, such as:

  • flexible cellular polyethylene
  • cellular polyurethane
  • foam rubber.

Concrete bricks and blocks contract, and the following materials are acceptable for use in contraction joints:

  • hemp
  • fibreboard
  • cork.

The joints should be formed using semi-rigid, closed cell polyethylene or other suitable materials.

To perform effectively a sealant in a movement joint should be applied against a suitable debonding joint filler board/backing rod
so that the sealant only adheres to the two opposing masonry faces

Damp-proof courses

Damp-proof course (DPC) materials should conform to BS 8215 and PD 6697 Table 1.

Designers should pay adequate attention to the characteristics of the materials chosen for DPCs. Materials that squeeze out or are impaired on highly stressed walls are undesirable and should not be used as DPCs. DPCs adhesion to mortar and their ability to resist sliding and/or shear stresses should be considered especially in relation to lateral loading.

Where DPCs are required to provide resistance to shear e.g. in frame or cross walled structures, the design of wall panels should reflect this and the deemed to satisfy rule of Approved Document A or equivalent regulation not be appropriate.

In general, advice on the resistance to compression, tension, sliding and shear should be sought from the manufacturers of DPC.