Gutters are one of the most important defences against damp in traditional buildings, yet they are often overlooked until problems develop. This article explains why functioning rainwater goods are essential, how gutter defects cause damp and structural decay, and what to look for when assessing an old property.
Why do we need gutters at all?
The average annual rainfall in the UK is roughly 1300ml per square metre. For the roof of a small family house that equates to roughly 45 cubic metres, which would fill 180 bathtubs. If that water is not intercepted by a gutter it spills down the walls and soaks into the ground at the base of the building. If it is intercepted by a faulty gutter that is spilling in one small area, the runoff from the roof can be concentrated and then have the potential to cause more serious damage.
Although modern buildings are designed to resist water penetration, no building is intended to withstand a constant supply of water concentrated in one area. Water is the engine of decay in buildings and, given enough time and volume, it will eventually find its way through almost any construction. Persistent wetting can raise internal humidity, encourage mould and fungal growth, reduce thermal performance and cause damage to finishes as moisture evaporates and salts are deposited. It is difficult to overstate the amount of damage that can be caused by something as simple as a faulty gutter or blocked downpipe.
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Why are gutters so important for old buildings?
Gutters are a key element in preventing old buildings from becoming damp. Their importance is often underestimated by homeowners and tradespeople alike. Over time a faulty gutter can cause thousands of pounds worth of damage to render, wash out clay mortar from a stone wall, and cause damp conditions in a property.
Old buildings are particularly dependent on functioning gutters because the buildings themselves are generally water resistant rather than waterproof. Unlike a modern cavity wall with an effective damp-proof course, a traditional solid wall is designed to manage moisture rather than exclude it completely. If rainwater is repeatedly allowed to spill down the face of the wall or concentrate around the footings, that moisture will eventually find its way into the structure.
Although no building tolerates concentrated water indefinitely, modern buildings are generally more forgiving. A damp-proof course prevents moisture from moving upwards from the ground, and impervious finishes help to resist wind-driven rain. Although overflowing gutters will still cause damage over time, the consequences are often less immediate. Traditional buildings have far less tolerance for concentrated sources of water.
This is particularly important in Cornwall, where many pre-1900 buildings were constructed with earth and clay mortars. These materials perform remarkably well when kept within their normal moisture range, but they are vulnerable to prolonged saturation. Repeated water ingress from faulty gutters can gradually wash mortar from within the wall, creating hidden voids and causing defects that may not become apparent until many years later.
Correcting a guttering system is one of the most cost effective measures that can be taken to keep an old or listed property warm and free from damp. Investing in having gutters assessed and corrected could save the homeowner thousands by protecting the building and avoiding the need for costly remedial works in the future.
Why are gutter defects difficult to diagnose?
One of the reasons gutter defects are so difficult to diagnose is that they are often intermittent. A gutter may perform perfectly well during ordinary rainfall but overflow during periods of intense rain or when the wind is driving rain from a particular direction. In exposed parts of Cornwall, I frequently encounter gutters that only spill under particular weather conditions. Conversely, some defects only become apparent during unusually easterly winds, despite the gutter appearing satisfactory during more typical south-westerly storms.
Because these conditions are unpredictable, a gutter that appears to function normally for most of the year can still be responsible for a considerable amount of dampness and decay. By the time staining, algal growth or internal symptoms become apparent, the connection with the guttering system is often far from obvious.
This is one of the reasons gutter defects are frequently overlooked. The problem is not that they are especially complicated, but that they only reveal themselves when a particular combination of weather conditions occurs.
Following the path of the water
I attended a property in Mousehole where the owner was puzzled by an overflowing section of gutter which, from ground level, appeared to be serving nothing more than a small porch roof. The gutter itself was clean, correctly aligned and in good condition, and there was no obvious reason why it should be struggling to cope. However, after tracing the route taken by the rainwater, it became apparent that several roof slopes and valleys discharged one into another before finally arriving at this seemingly insignificant section of guttering. What appeared to be a modest length of gutter serving a porch roof was, in reality, collecting water from a surprisingly large part of the building.
There was nothing wrong with the installation of that particular piece of gutter. The problem was one of system design rather than workmanship. Over many years the building had evolved, with additions and alterations changing the route taken by the rainwater. By the time it reached the final downpipe, a substantial proportion of the roof area was discharging through a single outlet. It was a useful reminder that guttering systems should always be considered as a whole rather than as a collection of individual components.
How should I go about assessing my gutters?
Although gutters appear simple, installing them correctly is much harder than many people realise, particularly on old buildings where fascia boards may be twisted, roof lines uneven and the original falls difficult to establish. Faulty gutters can also be difficult to diagnose because they often only spill during certain weather conditions. It is common to find systems that perform perfectly well during light rainfall but overflow during intense downpours or when the wind is driving rain from a particular direction. The majority of old buildings that I am called out to survey have at least one guttering issue, and much of the remedial work I carried out as a lime mason had a faulty rainwater system at its root cause.
If you do not have safe access to the gutter itself, the easiest way to identify a fault is to look for the symptoms caused by water spilling. These may include dark marks on the wall, plants sprouting from the mortar joints, or patches of algal growth at the base of the wall where runoff is promoting vegetative growth. On the fascia boards, look for patches of peeling paint or rot developing. On pointed stone buildings, the mortar appears darker where it is wet; this is often visible below a faulty running outlet (where the gutter joins to the downpipe).
Where it is safe to do so, a more thorough inspection can be carried out from a ladder. The run of the gutter should be assessed by pouring water into it and observing whether it is flowing well, whether any of the joints are leaking, and if the water is discharging freely from the downpipes. The alignment of the gutter to the roof should also be checked; it is important that the roof tiles overhang the gutter enough to prevent water from being blown back behind the gutter. The fascia board should be checked to make sure it is true on the vertical plane. If the fascia leans back toward the roof then the rear edge of the gutter will be low, which will allow it to spill during heavy rainfall.
Faults also commonly exist in the specification of the system itself. A gutter may be perfectly installed but still spill water if it is not of sufficient capacity or is not served by an adequate number of downpipes. With extreme weather events in the UK on the increase, gutters that were adequate in the past may no longer be so.
Faulty gutters can also produce symptoms that are commonly attributed to 'rising damp'. When water is repeatedly discharged at the foot of a wall, moisture levels within the structure increase and the internal humidity of the building rises. This can lead to damp lower walls, mould growth and salt deposits, all of which are frequently described as 'rising damp'. In practice, it is important to identify the source of moisture before considering remedial treatments. The rainwater goods should always be examined carefully, as a leaking gutter or overflowing downpipe can contribute significantly to the problem.
The subject of 'rising damp' deserves an article in its own right, but it illustrates the importance of diagnosing the source of moisture rather than focusing solely on the symptoms.
What shape (profile) of gutter is best?
The profile (shape) of a gutter is more than an aesthetic consideration. The more capacity a gutter has, the less likely it is to block. The most common profiles are half round, square, and ogee (pronounced oh-gee). In theory, the half round profile has sufficient capacity for smaller roofs. In practice, it spills easily if it is not perfectly installed or if any blockages occur. The square and ogee profiles have much higher capacity and are more able to deal with imperfect installation or minor blockages. As gutters are such an important element in protecting an old building from damp, I recommend that in most cases a high capacity ogee gutter is installed.
Should I choose a plastic or metal gutter?
In the UK, plastic is by far the most common material used for domestic guttering. Modern plastic systems are much improved compared with earlier products and, when correctly installed and adequately supported with clips, they can give many years of satisfactory service. Over time, however, plastic gutters are prone to distortion and movement, particularly if they are poorly supported or allowed to become blocked and overloaded with water. A lifespan of fifteen to twenty years is a reasonable expectation.
Elsewhere in Europe, copper and zinc guttering are more commonly encountered. Non-ferrous metal systems have a much longer lifespan and generally require less maintenance. It is not unusual for well-installed copper or zinc gutters to remain serviceable for many decades. The downside is the initial cost, which can be several times higher than that of plastic.
Steel guttering is also available and is considerably cheaper than copper or zinc. In the maritime climate of Cornwall, however, it should be treated with caution. Any damage to the protective coating or untreated cut edges can quickly lead to corrosion, which may significantly shorten its lifespan.
In practice, both plastic and non-ferrous metal systems can perform well if they are properly designed, adequately supported and maintained. Plastic guttering requires more attention over its life, whereas copper and zinc demand a much greater investment at the outset. The choice is therefore often a balance between initial cost, maintenance requirements and expected lifespan.
Maintenance
A guttering system is not something that is installed and forgotten about. Like any other part of a building, it requires periodic inspection and maintenance. Plastic gutters move over time, clips fail, joints deteriorate and trees continue to shed leaves and moss onto the roof. Even a well-designed system will eventually develop faults if it is neglected.
In my experience, relatively minor defects in rainwater goods are responsible for a surprising amount of dampness and decay. Correcting these defects is often one of the most cost-effective measures that can be taken to protect an old building. Buildings do not need to be waterproof, but they do need to be properly weatherproofed, and few elements are more important in achieving this than the humble gutter.
Perhaps the most important lesson I have learned is that water should be followed rather than assumed. The symptoms may appear inside the building, but the cause often lies elsewhere. Understanding how rainwater moves through and around a building, and maintaining the systems designed to manage it, can prevent a great deal of unnecessary damage and expense.
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