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. point loads
  4. bonding
  5. movement joints.

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’.
Intermediate floors, roofs and roofs designed to provide lateral restraint to external walls BS 8103 ‘Structural design of low-rise buildings’.
Ancillary componentsBS EN 845-1 ‘Specification for ancillary components for masonry’.
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, 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.

Point loads

Where padstones and spreaders are required, they should be located beneath areas of 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 aerated 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.

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 for movement joints

MaterialJoint width (mm)Normal spacing (m)
Clay brick1612 (15 maximum)
Calcium silicate brick107.5 – 9
Lightweight concrete block and brick
(autoclaved or using lightweight aggregates)(2)		106
Dense concrete block and brick (using dense aggregate)(2)107.5 – 9
Any masonry in a parapet wall10Half the above spacings and 1.5 from corners (double frequency)

Notes

1 Manufacturer’s guidance for the provision of movement joints and bed joint reinforcement should be considered.
2 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.

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.

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.

The following materials are acceptable for use in contraction joints in concrete brickwork:

  • Hemp.
  • Fibreboard.
  • Cork.

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

Sealant should be a minimum of 10mm deep to ensure a good bond. Where the joint is in a freestanding wall, the filler will require sealant at:

  • both exposed edges
  • the top, where the joint is carried through any coping.