When diagnosing damp problems, the age and type of construction of a house can give important clues as to what to expect. In the first of a new series, Hudson Lambert of Safeguard looks at houses built before 1918.
Accurate damp diagnosis is the key to any remedial treatment programme, and is ordinarily why a specialist property care surveyor should be employed to accurately identify any damp issues.
However, some understanding of how air and moisture move and behave in dwellings, combined with an appreciation of the typical housebuilding types encountered in the UK, can give some indication of the problems to be expected – including those caused by modern extensions and modifications to period properties.
In this article, we look at houses built before 1918 and where they might be vulnerable to damp and some other masonry issues.
In an older building, there are four main sources of moisture: moisture naturally in the atmosphere, moisture created by occupants and domestic activity; rising damp and penetrating damp (particularly from wind driven rain). Natural moisture in the air need not concern us, except in the context of ventilation.
Moisture created by human activity is a huge cause of condensation and is managed by both heating and ventilation – irrespective of building type. So rising and penetrating damp are the main issues that often require corrective renovation works to be carried out in buildings of this era.
Rising damp occurs when groundwater rises through porous building materials, such as bricks and mortar, through a process known as capillarity. In simple terms, the moisture rises through the bricks and mortar in the same way that oil rises through the wick of a lamp.
It generally requires treatment because it has a number of undesirable effects on the performance of buildings, including: decorative spoiling; erosion of building fabric; increased heat loss and, for residents, negative effects upon health.
Walls that are affected by rising damp usually also suffer from contamination by hygroscopic salts. This is a secondary form of dampness that is caused by the build-up of ground salts introduced into a wall over many years by the rising damp. These salts can continue to cause problems even after the rising damp has been eradicated.
Because of the relationship between rising damp and salt contamination, the treatment of rising damp is usually a two-stage process:
1. Introducing a new DPC – using treatments such as Safeguard’s Dryrod – to prevent further dampness rising from the ground;
2. Specialist replastering to deal with any hygroscopic salts by replacing any salt-contaminated plaster with a new salt-resistant plaster.
The necessity of carrying out the specialist replastering stage will depend on the severity of the dampness and the level of salt contamination. Severely salt-contaminated plaster will need to be replaced, but where the plaster is sound and levels of salt contamination are low, it may not always be necessary.
Penetrating damp in solid wall construction occurs by two means: from rain, running through cracks, joints and defects, often driven in by wind pressure; or by absorption and diffusion through the wall’s material. Due to the prevailing wind in the UK, South and Westerly facing walls tend to be the worst affected.
For a rendered wall, penetrating damp can be kept out by maintaining it so that there are no cracks through which water can enter – although the render should allow water vapour to escape.
For a brick or masonry wall, a combination of crack sealing or re-pointing and treating with a vapour permeable or ‘breathable’ water repellent – such as Stormdry Masonry Protection Cream – will stop moisture ingress.
Another danger zone in pre-1918 houses are suspended timber floors. Poorly conceived works – such as raising exterior ground levels past, or blocking, air bricks – or energy efficiency retrofit works can create problems like decaying joists in suspended timber floors because of the reduction in natural ventilation essential to maintain timber ‘health’.
Furthermore, poor ventilation can result in dry rot and mould growth in the voids below suspended timber floors. Dry rot is the most common of the two and can cause structural problems. The spores from mould can find their way in to the house and then adversely affect people’s respiratory health.
The solution to these risks is first to ensure that planned improvements do not compromise ventilation into voids; and, second, to check that any existing retro-fit works – such as internal wall insulation installation – haven’t done so already and, if so, undertake its removal.
One other issue likely to affect these older houses with solid walls is masonry cracking as a result of subsidence – often, ironically given the damp discussion above, due to ground shrinkage resulting from the lowering of a water table. Particularly vulnerable areas are around windows and their reveals; where load-bearing lintels may not be present.
Where subsidence or masonry cracking isn’t so catastrophic as to need under-pinning; a crack-stitching system – like the one found in Safeguard’s BrickFix range – can be employed to reinforce and re-connect cracked masonry.
Next week we’ll look at some of the detailing found in, and challenges presented by, houses built between 1919 and 1945.
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