Problems with external masonry walls come high on the list of recorded defects, according to home insurance claims and industry statistics. When designing and constructing walls, it is vital that not only is the structural stability considered but also thermal performance, moisture resistance and, where applicable, fire separation and acoustic properties. Many of the defects often attributed to masonry construction could be prevented by improved design, materials and workmanship. Nick Cuffe, technical manager at Zurich Insurance Building Guarantee – a latent defects warranty provider for predominantly domestic housing – answers three of the more common problems and looks at possible solutions.
Q. How do you ensure that adequate thermal insulation is provided to the external walls?

A.

Causes:

  • Poor quality or poorly installed wall insulation
  • Non-continuity of insulation materials causing thermal leakage or cold bridging

Solutions:

  • Ensure the correct specification of block and insulation is used and that the whole house is designed for optimum thermal performance. However, when considering thermal values, due care is required to prevent flanking sound transmission when terraced or attached properties are to be constructed.
  • Ensure masonry joints are fully filled, that there are no gaps between insulation boards and that continuity of adjoining insulation materials is maintained.
  • Ensure lintels are insulated and proprietary cavity closures are used in the formation of structural openings to prevent cold bridging.
  • Service penetrations through external walls should be reduced and where necessary made good to prevent thermal leakage. Also, timber joists should not be built into external walls as shrinkage often occurs, again resulting in thermal leakage.

Q. How is cracking to masonry prevented?

A.

Causes:

  • Lack of control for thermal movement
  • Incorrect mortar mixes
  • Insufficient structural stability

Solutions:

  • Expansion joints are required in masonry walls. Typically these are at 6 m centres for concrete blockwork and 12 m centres for brickwork. Suitable materials must be used in expansion joints to allow adequate movement of the masonry. The joint must also be finished either by the use of suitable gaskets or mastic – or both – to prevent water penetration. Additional wall ties must be used as part of the expansion joint construction.
  • Too strong a mortar mix increases the risk of cracking. Too weak a mix can result in excessive weathering of the joints between masonry components. The mix should take account of the exposure of the development, the type of masonry and the positioning of the masonry on the dwelling. It is vital, if mixing on site, that there is adequate supervision to ensure the specification is adhered to. When ready-mixed mortars are used, manufacturers' guidelines must be followed, with particular reference to curing times. Use of lime mortars can provide sufficient strength and movement. However, as with cement-based mortars, specification and mixing practices are critical.
  • Cavities in external walls are increasing in order to accommodate the greater thickness of cavity wall insulation. Sufficient wall ties must be used when masonry leaves are constructed to maintain structural stability.

    Typical centres for wall ties on masonry cavity walls are 900 mm horizontal × 450 mm vertical for cavity widths up to 75 mm, and 750 mm horizontal × 450 mm vertical for cavities over 75 mm (see diagram 1, left).

    Wall tie numbers should be increased to 300 mm vertical centres for reveals of structural openings and at expansion joints. Proprietary two-part ties may be necessary for increased cavity widths to allow for thicker insulation materials. In all instances the preformed drip of the tie should be in the centre of the cavity with the tie sloping down towards the external leaf.

    Wall ties need to provide the strength necessary to bond leaves of masonry together and must be corrosion-resistant. They must also be suitable for use with other materials and, when used with cavity insulation, they should provide adequate support.

    When constructing terraced or attached housing, consideration must be given to sound transmission. Not only should the correct tie be used in cavity-separating walls but also in external walls to prevent flanking sound.

Q. How do you prevent water penetration through an external wall?

Causes:

  • Bridged cavities
  • Incorrectly specified construction for development location
  • Poorly constructed or designed details

Solutions:

  • Avoid mortar build-up on wall ties when cavity walls are being constructed. Regularly clean cavities and preferably use cavity battens to prevent mortar from entering the cavity.
  • When using cavity insulation, ensure that it is installed following the manufacturer's guidelines (see diagrams 2A and 2B, left). Normally when fully filled insulation is used the construction sequence is outer leaf, insulation, inner leaf; for partial-fill the reverse sequence is employed. In all instances components of the cavity wall are progressed, as close as is practical, together. However, both leaves of masonry must not be allowed to rise together above the insulation. This sequencing will ensure that mortar is not allowed to build up between insulation materials. Equally, mortar on the inner faces of cavity leaves should be kept smooth, allowing correct positioning of the cavity insulation.
  • When designing cavity widths it is imperative that the development exposure is taken into account. For example, a 50 mm fully filled insulated cavity with brickwork external leaf would be totally inappropriate if constructed in an exposed seafront position. When considering partial fill insulation, a minimum 50 mm residual cavity should be formed.
  • Sufficient overhangs for windowsills, parapet copings and so on must be adequately detailed and constructed. Cavity trays, with associated stop ends and weep holes, must be used above structural openings and above external wall penetrations. Cavity trays shall also be used above abutments, on parapets and in chimneys. Gable elevations that are partially insulated must also have cavity trays installed to prevent water penetration and protect the exposed edge of the insulation in the cavity.