External wall issues

Cavity walls emerged from the hollow wall bonds that developed as a response to the revised brick tax of 1803. Rat trap, Dearns and Loudan bonds were hollow walls (thought to derive from garden wall bonds), but were not effectively cavity walls as we know them. The rat trap bond was very economical but not particularly watertight, so if used for houses it has to be rendered on the outside.

Cavity walls, with separated leafs, started to appear as early as 1850, becoming popular to the end of the century. These were often constructed with tar-dipped wrought iron wall ties, or special cavity bricks. Most cavity walls were constructed 230mm (9 inches) thick, but tended to be solid into the footings. Victorian 'cavity' walls are not easy to identify, as the Flemish bond was still used. This effect was maintained by snapping the headers in half. Victorian snapped headers are not as problematic as their Georgian counterparts, but there is a link to problems with the use of iron wall ties that may rust within the wall. It is known that some cavity walls were vented, but it is not apparent that this was universal. Cast iron grills at the base and head of a wall could indicate that cavities exist, but should not be confused with the practice of venting upper floors or upper rooms that contained fires.

Some bonds, such as English cross-bond, were revived to allow for polychromatic brick patterns to make masonry more decorative and intricate. This bond would tend to indicate a solid wall rather than a cavity wall. The use of mathematical tiles to cover timber frames, popular in the 1700s, continued until the 1820s.

It is wise to examine the roof space to check the position of the wall plates, because it was not always the case that the wall plate was on the inner side to the wall. If the wall is a hollow type it may not follow the usual modern convention of the inner leaf being the load-bearing leaf.

20th century cavity walls

Some of the main problems with houses from the turn of the 20th century stem from not knowing what the wall construction is. Both cavity walls and solid walls were being constructed, and some houses had damp-proof courses while others did not. The cavity wall made with two leaves joined by wall ties was evolving from the hollow wall in bonds such as rat trap and Dearns in the late 1800s.

Early cavity walls were constructed using metal wall ties that were protected against corrosion in a variety of ways and with varying degrees of success. Some ties are known to have been laid to form a slope, so as not to encourage water to cross from the outer wall to the inner. Walls did not have weep holes or cavity barriers, as seen in the modern cavity wall, and the width of the cavity varied.

Because the method of protecting the metal ties was not formally standardised, cavity walls built between 1900 and 1940 are particularly prone to wall tie corrosion. The corrosion of iron wall ties is known to be exacerbated if they are set in black ash mortar.

The basic principle is to build two separate walls, but link them together (with ties) for stability. The outer wall (or leaf) is allowed to saturate, while the inner wall (leaf) remains dry. This principle relies on the two walls being separate.

The major failures arise when:

• the two walls come into contact with each other; or
• the inner leaf becomes wet by other means.

The key to analysing water penetration through cavity walls is to recognise:

• how complex water is;
• that during prolonged rainfall, the outer leaf will allow water into the cavity;
• the main weakness in a cavity wall is cracks between the bricks and mortar; and
• when there is more than one defect present.

Ask:

• Are the leaves actually separated?
• Can water get out from between the leaves?
• Is there more than one defect?
• Where is the water coming from?

Guide to water penetration in cavity walls

In some cases, the symptom is not in the locality of the source.

While it is a good idea to start looking around the area where staining occurs, keep an open mind on how water can track within the structure. Do not underestimate the ability of water to do this. A defect can occur up to 15m away from the point of entry.

Check for an obvious water source in the area.

This is the most obvious point, but can be dismissed too quickly. There is a great wealth of technical advice on where water may come from. Common culprits are poorly maintained gutters and downpipes, followed by a rise in ground levels but there are many more. Long-term problems persist because the cause is not always visible, e.g. only visible when it rains. Asking the occupant to keep a log of events and note when the damp becomes worse is a good way to link evidence.

What is different about the defective wall compared to apparently unaffected walls?

This point is often overlooked because the focus is usually drawn to the area affected. There is a great benefit to breaking the line of thought that only those areas where damage can be seen require attention. Check the condition of the other walls, as they may also be wet but are not stained. Such realisations can make for quantum leaps during investigations.

Has the problem been encountered before?

Consider this question carefully because there may be many directions to take based on how the evidence is viewed, and there is potential to make matters worse.

• If it is assumed that an earlier remedy was wrong, the next step might be to remove that remedy or add another one.
• If it is assumed that the previous remedy was right but badly executed, the next step might be to redo the work, or remove it and start again.
• The most complex scenario is where it is assumed that the remedy was right but badly executed, when in fact the remedy was perfectly executed but a second fault exists.

Water penetration caused by cavity fill

With the introduction of more stringent insulation requirements from 1980-2000, developers looked for ways to introduce thicker and more efficient insulation into cavities. The cavity batt was developed, with cavity closers set into openings to prevent cold bridging. Retrofit blown-fibre insulation was used by many local authorities. The installation of these components sometimes fell to a variety of trades before the advent of specialist subcontractors. This led to some build quality defects, which mainly resulted in poor continuity of these components or the positioning of the insulate on the wrong side of the cavity, i.e. the cold side.

The setting out of the walls became a site issue because a bricklayer who was not aware of the thickness of the insulation to be installed might set the wall out too narrow, which would result in the cavity gap being too thin when the insulation was introduced. This defect can be easily hidden on site and would not become apparent for some time. Often the only resolution is to remove any defective layer and add new external or internal layers.