Good drainage is one of the most important factors in keeping a traditional building dry. Unlike modern houses, old buildings were designed to manage moisture rather than exclude it completely. This article explains why drainage matters so much, the defects to look for, and why understanding how water moves around a building is often the first step in solving damp problems.
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Old Buildings Need Dry Feet
Old buildings have different drainage requirements to modern buildings. Pre-1900 properties were constructed from vapour-permeable materials and generally built without damp proof courses. Their foundations were not formed in the same way as those of modern buildings and often extended only as far as the clay subsoil. Historic builders understood the ground they were building on and, provided moisture conditions remained reasonably stable, these buildings have proved remarkably durable. This is particularly apparent in parts of Bodmin Moor, where many traditional buildings continue to perform well despite being built without modern foundations or damp proof courses.
In Cornwall, much of the housing stock predates modern construction techniques. These traditional buildings are generally solid-walled structures built from vapour-permeable materials, often founded directly onto the clay subsoil and constructed without damp proof courses. Their approach to moisture management is fundamentally different from that of modern buildings.
Traditional walls are constructed from hygroscopic materials such as stone, earth, clay and lime. I often describe a solid wall as being rather like a giant candle wick standing on the ground. Under normal conditions it absorbs a certain amount of moisture and then releases it again harmlessly. If the ground around the building becomes saturated, however, the wall will absorb more moisture than it can readily dissipate and this excess moisture will eventually affect the internal environment.
Old buildings do not need to be waterproof, but they do need dry feet. Given reasonably stable moisture conditions, they can tolerate a surprising amount and continue to perform for centuries. Problems arise when water is continually supplied to the same area through poor drainage, leaking gutters or other concentrated sources of moisture.
Why Have People Forgotten How to Fix Damp in Old Buildings?
For thousands of years, all buildings were solid walled and vapour permeable. They were better understood by their occupants, who would have been accustomed to their maintenance needs. The materials available for repair and maintenance were well suited to the type of building, as were the skills and knowledge of the tradespeople of the time.
In the early to mid 20th century, concrete foundations, damp proof courses, and non-permeable finishes became the norm in the building industry. The knowledge, skills and availability of suitable materials needed to maintain solid walled structures died away. Construction professionals became used to dealing with new buildings, and old buildings started to get a bad reputation for being cold and damp as they were subjected to the use of non-permeable materials and poorly designed maintenance regimes.
What Effects Will Poor Drainage Have on a Historic Building?
The inclusion of a damp proof course and concrete foundations in new buildings mean that their drainage requirements are much lower than those of their historic counterparts. A modern building will tolerate poor drainage, but in an old building it will lead to symptoms such as damp internal conditions, structural movement, and reduced thermal efficiency of the walls.
Because of the lack of a damp proof course, a solid wall with poor drainage will absorb water on the external face and allow it to evaporate to the internal living space where the relative humidity is lower. This can cause localised symptoms of damp such as flaking paint or failing plaster, but can also cause more widespread condensation in certain conditions.
Foundations on old buildings are prone to movement when exposed to liquid water over a long period. Water could originate from a poorly routed downpipe, an underground culvert, or from ground that falls toward the building. Poorly drained foundations are probably the most common cause of structural movement in historic buildings.
When a solid wall becomes wet, its thermal efficiency decreases. This means that it will wick heat away from the internal living space of the building, creating cold areas. Moisture present in the air condenses on these cold spots, and this creates the ideal conditions for the growth of mould and fungi.
How Can I Tell if I Have a Drainage Issue?
The most obvious thing to do is to observe how water moves around the building. Standing water, puddles or surface runoff flowing towards the property should always be regarded as warning signs. The ideal time to inspect drainage is during or immediately after prolonged rainfall, when water movement can be seen. Buildings can be surprisingly good at hiding the source of moisture problems, but observing them in wet weather often reveals clues that are invisible during dry conditions.
Externally, look for green staining, algal growth, moss, excessive vegetation and isolated areas of bubbling or flaking paint close to ground level. These are all signs that moisture is accumulating in a particular area. Darkened mortar and persistently wet patches can also provide useful clues.
Internally, poor drainage often causes localised symptoms, with damage concentrated towards the lower two feet of the walls. Low-level deterioration of plaster and paint finishes, cupping timber floorboards and deterioration of carpets around the perimeter of a room are all common indicators. However, old buildings often suffer from more than one issue at a time. A localised source of moisture may raise the general humidity of the building and contribute to more widespread symptoms such as condensation and mould growth. For this reason, damp problems are not always as clear-cut as they first appear.
In many cases, the building itself has not changed, but the way water behaves around it has.
What Common Defects Should I Look For With My Drainage?
Misdirected downpipes: the most common root cause of a drainage issue is a poorly routed downpipe. Downpipes deal with a massive amount of runoff from the roof, upwards of 2,000 litres per year for a small family house. They should discharge into a suitable drainage point, ideally with an upstand detail so that blockages can be easily identified. Many downpipes discharge straight to the foot of the wall; this is liable to cause damp and mould within the property.
Failing drain outlets: the drainage points at the base of downpipes require occasional maintenance. The drain at the base of a downpipe may appear well detailed at first glance, but a closer inspection often reveals that this is not so. The mortar surrounding the drain cracks and erodes over time, allowing runoff to escape and penetrate the wall. This is exacerbated by blockages, which can cause the drain to overflow in periods of heavy rain. Drains should be regularly inspected and cleared.
Poor ground drainage: ground levels should fall away from the property. If water is seen to be pooling at the base of the walls, its origin should be identified and measures to improve the situation should be taken. If water is flowing toward the building (for example from a large area of hard standing), a bund (a raised hump) should be created to divert it. Faulty gutters or drains near the building are other potential sources of runoff, and should be checked.
What Effect Do Patios, Drives and Hardstanding Have?
Large areas of hardstanding are rarely beneficial around old buildings, although they do not necessarily need to be removed. The severity of the problem depends largely on the falls and the way water behaves on the surface. Hardstanding that slopes towards a building can become a major driver of moisture ingress by repeatedly delivering large volumes of water to the same area.
One way of thinking about hard surfaces is that they provide a one-way journey for water. Rainwater can enter through cracks, joints and the edges of the hardstanding, but evaporation is restricted. Moisture can therefore accumulate around the footings over time.
Gravel surfaces are only marginally better. Although water passes through them easily, they also restrict air movement and reduce evaporation from the ground beneath. For this reason, gravel should not be regarded as a cure for damp problems.
Will a French Drain Fix the Damp Problem?
French drains have become closely associated with damp problems, but they were originally developed to deal with waterlogged ground and to intercept water moving below the surface. They are not a universally useful measure when it comes to improving drainage around old buildings.
In practice, many damp problems are caused by poor management of surface water rather than water moving underground. Faulty gutters, badly routed downpipes and hard surfaces that direct rainfall towards the building are often the real problem. In these situations, it is usually better to keep water where it can be seen and controlled rather than allowing it to disappear below ground.
French drains are designed to move surface water away from an area. The most basic form consists of a trench which has a perforated pipe laid on its base and which is then backfilled with gravel or shingle. The pipe is set to fall (be angled) away from the area which is to be drained, and is led to a soakaway.
French drains can be a useful tool in improving the drainage of an old building. They can also exacerbate drainage issues and cause structural problems if not designed and installed correctly. Rather than move water away from a wall, a poorly designed French drain will collect water and create a damp stagnant area at the footings. If this type of drain is used, it should be situated a metre away from the base of the wall so as to avoid disturbing the footings, and should be dug to a depth of at least 300mm. The trench should be lined with a permeable geotextile membrane, which should be folded over at the top to enclose the gravel backfill. The top of the geotextile membrane may be dressed with a thin layer of gravel for aesthetic reasons. If the gravel backfill is not covered, it will clog with silt over time. If the backfill is covered with geotextile, it will allow for cleaning and maintenance as debris builds up in the top layer of gravel.
French drains are too often regarded as simple and problem-free remedial measures. I regularly encounter properties where they have been installed in an attempt to cure damp, only to find that the underlying problem lay elsewhere. A poorly designed French drain can simply create a saturated moat around the building, keeping the surrounding soil wet and increasing the risk of structural movement. Successful repairs begin with understanding how water is reaching the building in the first place. A French drain should follow a proper diagnosis rather than be used as a default solution, and its design should be carefully considered by a suitably experienced professional.
An Example From Practice
I was once asked to investigate a particularly attractive property situated low in a valley in Cornwall. A comprehensive network of French drains had already been installed around the building and uphill from it. The intention was to intercept water and lead it away from the property. In practice, the opposite had occurred.
Surface water moving down the valley was intercepted by the drains uphill and directed towards perforated pipes situated immediately beside the walls. Rather than solving the problem, the system had effectively transformed a surface water issue into a subsurface one, creating a damp moat around the footings of the building. It was only through careful localised excavation and tracing of the drainage runs that the problem became apparent.
This experience reinforced an important lesson. It is usually preferable to keep water where it can be seen and controlled rather than allowing it to disappear below ground. A drainage system should shed water away from a building, not collect and store it around the footings.
Final Thoughts
Old buildings do not need to be waterproof, but they do need dry feet. Most drainage problems are not caused by mysterious sources of moisture, but by water being repeatedly delivered to the same place through faulty gutters, poorly managed surface water, inappropriate hardstanding or ill-considered remedial measures.
Successful drainage is usually simple. Water should be encouraged to remain where it can be seen, understood and directed safely away from the building. In my experience, the most effective solutions are often the least complicated. Diagnosis should come before treatment, and any intervention should aim to help the building shed water rather than become part of its drainage system.
Traditional buildings are remarkably durable. Given reasonably stable moisture conditions, they can continue to perform well for centuries. They do not need to be waterproof, but they do need dry feet.
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