Dampness in building elements
Main walls
This text is chiefly concerned with traditional load bearing masonry houses. Walls will generally be of solid or cavity construction. These 2 construction forms behave differently both in terms of how they support loads, and how they shield the elements.
It can be difficult to confirm whether walls are built in solid or cavity form, and until you have established the wall construction type, it is hard to confirm a dampness diagnosis or to recommend a remedy with confidence. The pattern of the external bricks may give you a clue to construction. The external skin of a cavity wall is usually built in 'stretcher bond', but there can be exceptions (for example, an attempt is sometimes made to create Flemish bond in a half-brick outer skin, to match up new to existing construction). Measuring wall thickness at openings will give another clue, but this may still not confirm whether a wall is of solid or cavity construction. Drilling a 10mm diameter hole would be a last resort, but would certainly help you to ascertain construction and if the construction has a cavity, this can be viewed using a borescope.
There has been much research into the detrimental effects of poor design detailing and shoddy building of cavity walls. The BRE Report Rain Penetration through Masonry Walls describes how cavity walls behave when subjected to driving rain, and shows examples of poor construction practice such as unsatisfactory installation of insulation materials within the cavity. The report says that in persistent heavy rain the outer skin of a cavity wall will 'allow water to leak into the cavity'. This water may run down the back face of the outer leaf until it encounters poor cavity wall detailing that enables it to run or soak through to the inner leaf, where it will cause damp patches inside the building. The introduction of retro-fit cavity wall insulation is also causing great concern, as some types of loose blown insulation fibres tend to get soaked with water due to rainwater penetration, creating dampness through to the inner leaf wall. Many cavity wall voids are found to be defective from construction processes and alterations to buildings post construction, which also provides a conduit for moisture penetration and cold spots in walls.
Poor detailing of damp membranes, lintels and cills around window openings also provides a route for water to enter the building. Advice on remedy of cavity wall defects is also included in the report. In more extreme cases the remedy for poor cavity wall construction may be a rebuild because defects within the cavity wall construction are inevitably hidden from view (apart from limited inspection by borescope) and inaccessible.
Figure 1: Cavity tray problems
Solid walls generally enable rain to penetrate into the outer thickness of the wall, but they usually dry out between wettings (see figure 2). In most weather conditions a 1-brick-thick wall (225mm) will cope by being able to take in and give out moisture – breathing. However, solid masonry at high level and exposed elevations could suffer rain penetration problems, even if in sound condition. Good wall detailing of copings, drips and so on will help to minimise undue moisture penetration, and a good eaves overhang helps to shelter the upper parts of an elevation.
Solid walls exposed to persistent driven rain may eventually become significantly damp to their inner thickness. This could put built-in timbers at risk of rotting in older constructions, and could result in staining and damage to decorative finishes. Damp penetration from the outer wall face to inner wall parts is more likely to occur if the wall is in an unsatisfactory condition (with missing pointing or perhaps spalled bricks, for example).
Figure 2: A 'breathing building'. This late 19th century terrace accepts and releases penetrating moisture to achieve a 'safe' moisture equilibrium in its external walling
Having established whether walls are of cavity or solid wall construction, it is useful to know whether the subject wall is performing as an 'overcoat' or a 'raincoat'.
If a brick wall is finished with a cementitious render (such as plain render, pebble-dash or roughcast), the wall will act like a raincoat, preventing much water from soaking into the building fabric (see figure 3). In this case more water may be seen running down the building face in heavy rain, and this water will seek out any holes, cracks or imperfections. Once behind the impervious finish, the water will not easily evaporate away and a significant dampness problem could be building up.
Figure 3: Dense render prevents rain penetration very effectively, but diverts rainwater to potentially vulnerable detailing. Such a finish makes it difficult for a wet wall to dry out
Walls that are built of more porous materials that remain exposed as the building finish will become damp where rain-soaked, appearing darker, but will dry out between the rain spells. These walls function as 'overcoats'. A small area of defective pointing would often not adversely affect the overcoat wall, unless it was subjected to a heavy cascade of water (for example from an overflowing gutter above). However, it is quite likely that a similar area of defective pointing in a wall built of dense bricks could suffer more serious consequences.
Masonry previously coated with water repellent may be subject to a build up of trapped moisture where the water repellent also acts as a water retainer, preventing masonry from breathing.
Figure 4: What materials lie behind the plaster? In this case a timber frame with brick infills
You may also encounter external cladding. Face sealed systems are intended to provide a perfect outer seal to prevent any water penetration, whereas rainscreen cladding systems are designed with the expectation that some water will find a way through but can be drained harmlessly away.
Before beginning your investigation of the walls, make sure you know which elevations face the prevailing winds, note the condition of the wall pointing, and think about how much rain will be inflicted on the elevations. Look for soaked patches of masonry around faulty rainwater goods (dark damp patches may only persist for a limited time after wetting). In open woodland it is easy to work out where north is from the build-up of green staining (moss) on that side of the tree. With buildings, you can discover which elevations are the most damp and shaded by noting the build-up of green staining – the moulds, algae and mosses that are present due to a moisture source – particularly at the bases of walls and adjacent paths.
Your attention will often first become focused on a particular wall where visible symptoms of dampness are present – mould development, stains or defective finishes. We have probably all come across the characteristic white staining, often for full height, on a building that has suffered from a long-standing cistern overflow drip. It often has a heavily soaked green zone of masonry at the base of the wall, typically fringed white from salt evaporation. Such problems may indicate a poor maintenance programme or the owners' or occupiers' sheer lack of resources. Either way, failure to carry out such simple building maintenance over time often causes the most expensive of building repairs.
The survey will begin at the outer finishes and decorative coverings of the walls (external and internal), but you may need to extend your inspection deeper into the wall construction. Dampness to surface plasters may only be indicative of a surface condensation problem, but confirmation of significant dampness deeper into the wall could indicate associated penetrating or, occasionally, rising dampness.
Figure 5: Rising damp? The picture above shows staining due to raised flowerbeds abutting the front elevation solid bay wall. Additional water ingress around the bottom rail of the rotting external timber vertical sliding sash windows
Checking timber skirting with a moisture meter is often a quick way of homing in on particular walls with damp problems, where there is visible evidence to the finishes. Remember to always cross-check external and internal condition by noting conditions on each side of a wall when a defect is observed. A damp stain to plasters internally directly adjacent to an external leaking rainwater pipe is a sure sign of penetrating dampness. You will need to assess whether the stain is recent, or reflects building condition before an external fault was rectified.
As a general rule, think hard and creatively about the journey rainwater follows from its initial contact with the building. In your mind's eye follow the water's journey downwards as it soaks into porous materials, gets pulled ever downwards by gravity or forced in almost any direction by the wind. Try to work out any weakness of the external fabric that it could seek out. Vertical joints and gaps are less likely to present an entry opportunity than horizontal joints and gaps. Think of the horizontal window putties and paintwork to transoms and cills that must be in near perfect condition to keep the rain out. Be mindful of abutting boundary walls that could cause penetrating damp or bridging problems.
Figure 6: The above picture shows severs cracking to face brickwork of a solid wall. The property suffered from serious rainwater penetration and drafts
Projections should be provided with a drip detail to help shed water away from the wall face, but this theory is not always carried through in practice, as seen in the following example.
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Example The end walls of a new low-rise housing development in south London were designed by the architect to be topped with a stone coping flush with the external wall face. The contractor pointed out the need for copings to have a drip detail to prevent water from running down the brick face and causing damp penetration, but the architect insisted on sticking to the original design for 'aesthetic considerations'. Just weeks after finishing one phase of the development there was an unsightly sawtooth-shaped green staining developing down these end elevations from rain penetration exacerbated by poor coping detail. At the same site, the contractor also argued most forcibly about the brickwork jointing profile, where the architect wanted the brickwork jointing to be rubbed flush with the brick face. The contractor said that such a jointing method might not be durable or weatherproof enough for the British climate. The first phase of the development was constructed as designed, but the client for the second phase of work agreed with the contractor and asked them to 'iron in' the joints with a 'weather-struck' profile. |
A weather-struck joint will probably not project from the brick face, but will angle down from slightly behind the upper brick arris to flush and level with the lower brick arris. In contrast, weather pointing has a markedly different profile, where the bottom edge of the pointing will project slightly below and away from the lower arris. Properly executed weather pointing is not common these days, as the time-served bricklayers to execute such a pointing profile have all but disappeared. Weather pointing requires the use of a straight edge to produce a neat slightly projecting lower edge to the work. There is nothing nicer to perceive than freshly produced quality weather pointing, and the lining up of the lower edge of the pointing can neaten up old irregular brickwork to produce sometimes surprisingly wonderful results on an old wall. The work will be lightly brushed when it has begun to harden up to further enhance its appearance.
The difference in the performance of a simple bucket-handle compared to a weather-struck joint in preventing water penetration is probably less than you might expect, and it may be the ironing in that is the crucial factor. It is unlikely that a weather-struck joint or weather pointing will help shed water away from the wall face as some authors suggest, because the projection of the weather-pointed profile is only a matter of millimetres and there is no drip detail as such.
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