External masonry walls

6.1
Also see:
Chapter 7.1
Chapter 9.1
PD 6697:2010

6.1.11Construction of masonry walls

Construction shall ensure a satisfactory standard of brickwork and blockwork. Issues to be taken into account include:

  1. finished appearance
  2. bonding
  3. construction
  4. openings
  5. corbelling
  6. chasing for services
  7. protection of ancillary components.

The construction of masonry walls should comply with the following:

  • requirements of BS EN 1996-2 Eurocode 6. Design of masonry structures. Design considerations, selection of materials and execution of masonry
  • and BS 8000-3 Workmanship on construction sites. Masonry. Code of practice

Finished appearance

A site reference panel should be constructed to agree a benchmark for workmanship and products.

The appearance of a masonry wall depends upon the:

  • materials used
  • setting out
  • workmanship.

When setting out masonry:

  • avoid cutting bricks or blocks, except when it is essential
  • irregular or broken bonds, particularly at openings
  • fair faced masonry bricks should be set out dry, prior to commencement of works.

All work should be reasonably level and true, and:

  • the bond detailed in the design used
  • perpendicular joints kept in line and plumb
  • courses kept level by using lines and spirit levels
  • meet the tolerances defined within chapter 9.1.

To keep courses to the correct height, use a gauge rod marked with the height of windows, doors and floors.

Where a number of openings of similar width are being formed, use a rod cut to the required size to check the width of openings as the work rises.

Brickwork and blockwork should not be subjected to vibration until the mortar has set.

Bonding

A regular bonding pattern should be maintained. External walls should be bonded to partitions and party walls as required by the design. Either:

  • tooth every alternate course, or
  • tie with wall ties, expanded metal or equivalent at maximum 300mm vertical centres.

Where joist hangers are not used, joist filling should be brickwork or blockwork and without excessive mortar joints.

Joist filling should be:

  • 12mm below the top of flat roof joists to allow for timber shrinkage
  • the recessed portion of timber joists should be treated
  • checked to ensure the cold roof ventilation is not blocked.

Clay bricks and concrete blocks should not be mixed. Where a different size of masonry unit is needed to ensure correct coursing, small units of the same material should be used to reduce cracking and problems due to different thermal insulation properties.

Where the inner leaf of a cavity wall is being used for thermal insulation, and where a different size of masonry unit is used to ensure correct coursing, the unit should have similar thermal insulation properties to the masonry used for the rest of the wall.

Construction

The difference in heights between the two leaves of a cavity wall under construction can be up to six block courses, provided the ties are sufficiently flexible to ensure coursing is achieved without breaking the bond. To keep the wall plumb, do not over-reach at changes of lift; wait for the next scaffolding lift. With thin layer mortar construction having an assessment which complies with Technical Requirement R3, it is normally permissible to build the inner leaf to storey height ahead of the outer leaf. In such cases the recommendations of the assessment and the manufacturer’s recommendations should be followed.

Natural stone should be washed and wetted prior to construction.

Cavities should be constructed so that:

  • they are uniform and in accordance with the design, including wall tie specification and cavity width
  • mortar is struck from all joints as work proceeds
  • cavity trays and wall ties are clear of droppings and debris
  • mortar droppings are removed
  • where cavity insulation is used, mortar droppings are removed from the top edge
  • where partial cavity insulation is used, it is against the inner leaf of the cavity.
  • the thickness of the external leaf is consistent, any stone used should not project into the cavity
  • cavity barriers are installed as the work progresses.

Openings

Masonry may be built around either:

  • the frame in-situ, or
  • a profile or template to enable the frame to be fitted later.

Openings should be the correct size, square and:

  • the spacing between the masonry and frame should allow for movement and tolerance
  • the frame should not be distorted by forcing bricks against the jamb.

When window and door frames are built-in, they should be fixed with:

  • frame cramps
  • proprietary cavity closers, or
  • plugs and fixings.

Where opening sizes and locations do not match brickwork setting out, brick bonding pattern should be set out at the base of the wall to ensure that cut bricks occur below openings.

Protection of cavity walls during construction

Masonry cavity walls shall be protected whenever work stops e.g. for inclement weather or overnight. The tops of both leaves, as well as the cavity and any insulation, should be covered with sacking or plastic sheet and appropriately secured in place.

Corbelling

For feature brickwork sections the masonry should only be self supporting.

Where courses are corbelled outwards in ordinary masonry, one above another; the extent of corbelling should not exceed that shown in the diagrams on the right.

Where reinforcing is used, corbels should be designed by an engineer in accordance with Technical Requirement R5.

Chasing for services

Chases should:

  • not be cut with impact power tools, as they can damage the wall
  • not be cut into hollow blocks unless specifically permitted by the manufacturer
  • be cut with care
  • be limited to 1/6 of the depth of the leaf where horizontal
  • be limited to 1/3 of the depth of the leaf where vertical
  • maintain a residual thickness of 15mm between the chase and the void for hollow or cellular blocks unless otherwise recommended by the manufacturer.

Protection of ancillary components

Table 4 contains guidance for a selection of ancillary components for use in buildings up to three storeys in height, in a non-aggressive environment.

Table 4: Protection of ancillary components

Notes

1 Material/coating reference in accordance with the relevant part of BS EN 845.
2 These products are not suitable for use in contact with the outer leaf of an external cavity wall or a single leaf cavity wall.

Components in contact with, or embedded in, an inner leaf which is damp or exposed to periodic wetting (e.g. below the DPC) should be protected in the same way as components in contact with, or embedded in, an outer leaf.