Cavity walls: fixing defects

Cavity caveats

28 June 2019

If cavity-wall construction does not follow the relevant guidance then a number of problems may develop. In this article, John Miles looks at how to identify and remedy defects

In a previous feature, we looked at cavity-wall insulation and the defects that retrofitted insulation causes. This article looks into cavity-wall construction as a whole, and the issues that can occur if you don’t follow the Approved Documents’ guidance.

Cavity walls have become synonymous with contemporary wall construction; however they didn’t become commonplace until the 1920s. The advent of industrialised concrete block production made construction cheaper and quicker as builders could split the external wall into an inner and outer skin.

This meant that the amount of expensive brick or stonework required was halved, and it also addressed the issue of damp that was common with the more traditional 9-inch (230mm) solid wall. A cavity wall could also separate a largely aesthetic facade of brick or stonework that keeps weather at bay from an internal load-bearing wall.

The fundamental requirements for cavity-wall construction haven’t changed much since then. Wall thicknesses should be constructed in accordance with Table 3 of Approved Document A, while parapet walls should follow the design criteria in Diagram 4.

All such walls should have a 50mm minimum cavity and be supported with wall ties spaced horizontally by 900mm and vertically by 450mm, as per paragraph 2C8 of the document; paragraph 2C19 and Table 5 in turn set the required cavity tie length, as well as specifying that ties should be either stainless steel or basalt, so as to conform with PD 6697: 2010 Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2.

Although a cavity wall would appear to be a simple construction, a number of failures can occur that could entail remedial works or ultimately reconstruction. One of the most common is wall-tie failure, in particular the rusting of corroded metal ties. Expansion of ties can meanwhile cause the outer leaf to crack and distort and lead to possible collapse. Wall-tie failure can often be identified by the horizontal cracking that results from the expansion caused by rusting.

Another problem from a works quality management perspective is the incorrect setting out of a cavity wall. This can cause a number of issues, including insufficient cavity width and eccentric loading of foundations, which they are not designed to take.

A number of failures can occur to a cavity wall that could entail remedial works or ultimately reconstruction

Cavity width will also vary with location. As the purpose of a cavity wall is to keep moisture on the outside and a dry insulated load-bearing wall on the inside, the Building Regulations recognise regional differences in wind exposure and split the UK into 4 climatic zones. Each has its own risk profile and prescriptions for the way insulated cavity walls should be built. Incorrect spacing of a cavity wall construction can lead to insufficient clear space within the cavity as required by paragraphs 5.12 and 5.13, Diagrams 11 and 12 and Table 4 of Approved Document C, which set limits and minimum cavity widths depending on building location and exposure to the elements. This determines the thickness of cavity-wall construction.

Cavity-wall construction with insufficient cavity width

Figure 1: A cavity-wall construction with insufficient cavity width

Figure 1 shows a wall construction with a cavity that is insufficiently wide. Achieving the correct width was in this instance especially difficult, given the unevenly coursed stone walls of varying thickness. The main issues were:

  • residual cavity of less than 25mm; a 25mm width was permitted by the BBA certification for the ridged board insulation, but good practice in exposed locations is still to maintain a 50mm residual cavity, and this is likewise a requirement of warranties provided by the NHBC and similar organisations;
  • significant mortar deposits that completely bridged the cavity; and
  • wall ties to a back fall, meaning that moisture can run to the inner leaf rather than back towards the outer leaf.

The lack of foresight in the design process has resulted in uneven stonework built in a way that collects mortar. This mortar is termed a snot, and if not removed can eventually bridge the cavity wall, providing a pathway for moisture to the internal skin of the wall. The wall and the cavity is also not of the required thickness to enable the amount of insulation required by Approved Document L to be fitted.

It is common to see a generic specification and accompanying detail when the Standard Assessment Procedure calculation is completed. This can result in the builder trying to achieve a target U-value by fitting thick, partial-fill insulation into a cavity designed for a thinner profile – as can also be seen in Figure 1, where the insulation is too close to the external leaf.

In climatic zones where exposure is minimal and the external wall is masonry, some insulation manufacturers have tested their product to enable the residual cavity to be less than the width prescribed in Table 4 of Approved Document C. In such instances, although the guidance supplied by the manufacturer should be followed, the construction is still required to provide a cavity that should be kept clean throughout the construction process and meet the requirements of the warranty provider.

Figure 1 also shows issues with tie placement in a building that has a stone external wall and a block internal leaf. Ties as specified in Approved Document A should be used in sufficient number, but without enabling a pathway for moisture to the internal leaf.

In this instance, the provision of a full-fill insulation was discussed with the builder by building control surveyors as an alternative approach. As shown in Diagram 12 of Approved Document C the site exposure was only moderate, and such full-fill insulation would have addressed the residual cavity and mortar snot issues.

Wall ties also pose an issue when used incorrectly on timber-framed dwellings. Figure 2 not only shows debris within a cavity but also poorly bedded ties and even a hammer that has been left wedged in the cavity. Remedial works in this instance involved adding extra wall ties to support the external skin and clearing out the cavities by removing the debris and knocking the snots off the wall ties.

Debris within a cavity wall

Figure 2: Debris within a cavity, poorly bedded ties and a wedged hammer

For the surveyor specifying repair works to cavity walls, all remedial ties should be obtained from suppliers registered to ISO 9000. There are a range of ties available, so the choice of a suitable product should be discussed with the installer and, if necessary, a structural engineer.

All works should be carried out in accordance with the guidance in BRE Digest 329, Installing wall ties in existing construction, and pull-out tests should also be carried out on site to prove the suitability of the system adopted.

Remedial wall ties should be tested to ensure they comply with the specification of BRE Digest 401, Replacing wall ties of 1995. When carrying out this work, all the original wall ties should be replaced to ensure the defect does not recur.

Another common defect in cavity wall construction is that walls run out of plumb – that is, they are not strictly vertical. Typically, remedial works to walls running out of plumb will either involve providing support with additional ties, or the removal and rebuilding of the structure.

The processes discussed above can be applicable to the following competencies.

  • Building control inspections: carrying out site inspections to ensure that cavity-wall construction meets relevant performance requirements, observing, assessing and taking action against contraventions on site. This would normally demonstrate that a candidate had attained Level 2 of the competency, although where remedial works are required and reasoned advice is provided then they could achieve Level 3.
  • Building pathology: understanding defects analysis, and explaining building fabric failure. Using this knowledge to identify potential failures, offer advice, and identify cavity wall-tie failure and its causes as well as the remedial works required, to demonstrate their achievement of Level 3.
  • Construction technology and environmental services: understanding the design and construction and being aware of construction solutions to problems.
  • Works progress and quality management: knowledge of construction technology techniques and their relevance on site.

While carrying out an inspection, the surveyor should be able to identify the types of defect discussed above, and provide appropriate measurements and photographs in site reports. The detail should be reviewed appropriately using relevant guidance such as Approved Documents A and C, BRE Digests 329 and 401 and other guidance such as the NHBC technical standards to determine whether the external wall is constructed to the relevant requirements.

From this point, the surveyor would be able to take appropriate action, such as advising on remedial measures or the requirement for further review by a structural engineer. Where a candidate is identifying and resolving such site defects, they may be able to record their experience under at least Level 2, and potentially under Level 3, for the competency of Building control inspections.

John Miles is a technical and business development manager at Assent Building Control

Further information