Rainwater

Jonathan Taylor

 

A rural church with storm clouds gathering in the background  

The root cause of most deterioration in historic buildings and their fittings and furnishings is moisture. Whether trickling down from a blocked gutter, penetrating by wind pressure and capillary action, or condensing out of warm air onto cold surfaces, the original source is usually rainwater.

Most materials are susceptible to decay in damp conditions. Mechanisms include:
biological decay of organic material, such as the decay of timber by woodworm or dry rot, and the decay of textiles by carpet beetle;
physical damage of rigid materials such as iron, stone and glass as a result of the expansion of ice crystals, salt crystals or rust; and
chemical damage such as the corrosion of iron exposed to air and water, or the reaction of materials with atmospheric pollutants in acid rain.

If left unchecked, penetrating rainwater can cause extensive damage to the structure and the fabric of a building, particularly if dry rot takes hold. Once established and in the right conditions, deterioration can be rapid, costing more to put right the longer it is left unchecked.

Damp in the exterior fabric of a building also reduces its insulation value, conducting heat out of the building and cooling the interior through evaporation. High relative humidity inside the building can also make it feel cooler. The result adds up to higher fuel bills and carbon dioxide emissions.

The visual effects of damp should not be ignored either. Finding the funds necessary for conservation can be difficult enough for an enthusiastic congregation, but it can be far more difficult where the interior has been disfigured by patches of damp, mildew and the accumulation of past repairs.

For all these reasons, it is vital to keep the rain out of a building and to react promptly at the first signs of damp.

SOURCES OF DAMP

Rainwater ingress can be direct, such as through a leaking roof or parapet gutter, or through gaps in walls and eaves; or ingress may be indirect, such as ground water accumulating as a result of inadequate drainage or faulty drains.

Damp can move from one source to affect fabric far from the original source, either by gravity or through cycles of evaporation and condensation. The rubble-filled core of a solid wall, for example, often forms the path of least resistance for water to trickle down from a blocked gutter or slipped roof tile, or from wind-driven rain soaking the exterior of a west-facing wall far above. Travelling unseen through the masonry core, it descends, picking up soluble minerals along the way, until it reaches the base of the wall or is forced to the surface by an obstruction such as an impervious lintel. Damp patches at the base of a wall or column, often accompanied by efflorescence (the light-coloured bloom of soluble minerals re-crystallising), are often assumed to be caused by rising damp, when the source is not even close.

Nevertheless, damp at the base of a wall can be the product of high water levels in the ground caused by blocked drains or heavy rainfall, or higher ground levels externally. A functioning system of downpipes, ground-level drainage channels and belowground drains is essential.

High level maintenance is carried out to church masonry with the aid of a cherry picker
Specialist maintenance firms can provide cyclical maintenance such as gutter clearance and lime pointing. This cherry-picker retracts to the width of its tracks, enabling it to access most churchyards. (Photo: Forrester Access)
Newly installed lead roof Stone-slated roof and parapet over a masonry wall
A new lead roof over the 15th-century south aisle of a church in Somerset following its second lead theft (Photo: Chedburn Dudley) A stone-slate roof over a church porch in Wiltshire: historically, pitched roofs often did not have gutters, relying on good ground drainage to avoid erosion to the wall base

Water pressure and capillary action are insufficient to cause damp to rise much above external ground level. Vapour, however, is more mobile, and can be carried by convection currents and dissipation. The warmer the air, the more moisture it can carry, so heating a large church space with high moisture reservoirs just before the service begins can be disastrous. Moisture evaporating from the base of the walls can be carried by the warmed air to condense on those parts of the structure that have not yet warmed up. In the worst cases, cold damp walls can run with condensing moisture, reducing their insulation value further in a self-perpetuating spiral through the cooler months, so that the fabric never dries out.

Condensation also occurs out of sight within structures and porous fabric, wherever the temperature of moist air falls below a certain level (the ‘dew point’). In a large building there are many elements that are particularly vulnerable to this type of condensation, such as the metal fixings in the roof structure, and porous insulation such as glass fibre or sheep’s wool.

A particular problem is condensation on the underside of lead-sheet roofing, as this can lead to lead corrosion.

MAINTENANCE AND INSPECTION

Churches that fail to carry out regular maintenance are likely to face the largest repair bills, and The National Churches Trust Survey, published in 2011, found that approximately one in four churches that do little or no maintenance are already in poor or very poor condition.

Gutters should be cleared after the autumn leaf-fall, and downpipes checked to ensure that they are flowing properly. Any obvious problems in the roof can be identified at the same time, and a cursory inspection of the interior can highlight any obvious problems. Key requirements are:

  • high-level access – if roofs are not accessible from a staircase, a small mobile cherry picker may be hired or a rope-access specialist may be contracted to carry out the work
  • specialist expertise – the contractor must be familiar with traditional construction methods and materials, and their expected performance
  • supply of materials – a stock of appropriate materials must be available for carrying out minor repairs, whether kept on site (old roof tiles for example) or by the contractor (such as lime mortar to match the existing, stainless steel fixings, etc)
  • regularity – a cyclical maintenance regime is essential, with annual gutter clearance and other routine work as specified by an architect or surveyor, and preferably managed under contract with a local or regional specialist.

Most historic places of worship are given a more thorough survey every five years by an architect or surveyor who is trained in conservation work. This ‘quinquennial’ inspection, if carried out well, will provide a useful guide to those areas of fabric that are most at risk of failure. The occasional slipped slate, for example, may indicate that the fixing nails are rusting and that the roof needs to be re-laid. In the event of a leak or the appearance of an unexplained damp patch, the inspection report should be consulted.

ROOF COVERINGS AND FLASHINGS

Unless a patch of damp is clearly related to a particular feature that is known to be vulnerable (a patch of damp immediately below a leaded light window, or a blocked downpipe for example) the roof is the first thing to check whenever and almost wherever damp is found. The second is gutters, hoppers and downpipes, but also look for defects in pointing and projections that might need flashings.

The crisp edge of a thatched church roof overhanging a recessed window with green exterior shutters  
A rare example of thatch on the Quaker meeting house at Come-to-Good, Cornwall (c1720): the stone plinth of the cob walls is protected from erosion by a French drain  

Until the later Middle Ages, thatched roofs predominated in rural areas, and often in urban areas too. However, as the most important building in the community, churches were usually the first buildings to be reroofed with more durable materials. Slates (including ‘stone slates’) were commonly used to roof churches in areas where the stone was fissile, while clay tiles became common in other areas. In their simplest form these materials were used in much the same manner as thatch, with broad eaves shedding rainwater clear of the walls below, without gutters or flashings.

Sheet lead was the most expensive of the traditional roofing materials, but also the most adaptable and durable with a life span of over 200 years. It could be used sparingly, with tiles or slates, in the form of small flashings where gaps might arise, such as where the tiles butt up against a tower. It also enabled complex roof forms because it could be used as a liner for a valley or parapet gutter, or as a flashing for an upstand or abutment.

Because it could be laid on roof slopes that were almost flat, sheet lead also enabled side aisles to be added to existing churches without compromising the ceiling height, and for abbeys, cathedrals and the most wealthy parishes it gave free rein to the development of magnificent gothic interiors.

Several other sheet metals have also been used, particularly since the mid-19th century, including copper and zinc, and stainless steel has become a popular choice recently, particularly for roofs prone to lead-theft.

Where roof-level leaks are suspected and access is readily available, immediate inspection may reveal a blocked gutter that can be quickly and simply remedied. However, where access requires the use of a cherry picker or access platform, it is best to co-ordinate access with a visit by the inspecting architect or surveyor in case more complex problems emerge. Principal issues include:

  • ponding, usually caused by blocked gutters or valleys
  • tile or lead slippage due to inadequate or decayed fixings.

Other problems include:

  • wind damage causing metal roof coverings and flashings to lift
  • stress cracking in lead due to thermal expansion and contraction, often connected with poor specification or age
  • lead corrosion caused by acids leaching from bird droppings or oak
  • spalling roof tiles due to frost damage, often under the lap
  • incidental damage to the roof covering due to the failure of other components
  • accidental damage by, for example, heavy boots or clumsy workmanship
  • ice blocking gutters and chutes causing overflows from snow melt
  • lead theft.
U-section stone rainwater chutes below gothic tower balustrade Richly decorated lead hopper surmounted by relief coat of arms featuring lion, unicorn and imperial crown
Above left: simple rainwater chutes on the 14th-century church tower of St Thomas a Beckett, Box, Wiltshire. Centre: spectacular late-15th-century gargoyles at North Petherton, Somerset. Right: a fine example of a lead hopper on a church tower near Chippenham, Wiltshire, enriched with cast lead coats of arms and dated 1725.

RAINWATER DRAINAGE SYSTEMS

The earliest roofs had broad eaves to project rainwater clear of the walls. However, with the introduction of parapets, the water had to be channelled behind the parapet in lead-lined gutters then out through a chute which projected clear of the wall. The chute, if constructed in stone, provided the medieval craftsman the ideal opportunity for carving a terrifying beast or demon disgorging rainwater from its open mouth – the gargoyle.

Both open eaves and chutes rely on having well-drained ground around the church to take the water away, and work best in conjunction with a French drain. In essence this is a drainage trench around the base of the wall filled with free-draining gravel or chippings, preferably with a field drain at the bottom to conduct the water away from the building.

  Cast iron gutters and square-section downpipes supported off slender strap brackets and with off-white paint finish
Late Victorian cast iron gutters and down-pipes at the Church of the Holy Cross, Tramore, Northern Ireland, with a new section cast to match the original on the left, and repaired section on the right (Photo: Alumasc).
 
  Stainless steel chute fixed over hopper: the chute has high sides with a curved profile fixed to a low-fronted tray with a large circular aperture in the centre feeding into the hopper
A new stainless steel chute from a parapet gutter feeding into a cast iron hopper: the front edge is lower than the sides so that, if blocked, rainwater is projected clear of the walls below, avoiding ponding.
 
  Broken stone channel for rainwater run-off at base of masonry wall
  A defective ‘ground gutter’ in a Somerset church: blocked soak-aways, pipes that miss drains and damaged runs are common defects (Photo: Chedburn Dudley)

Lead hoppers and downpipes were used on the Tower of London as early as 1240 to protect its whitewashed walls from spray, and it is likely that they were used on churches at around the same time as an alternative to gargoyles and other rainwater chutes. The downpipes connected with drains or ran into ‘ground gutters’ (drainage channels) at the base of the wall.

Eaves gutters could be created using timber boxes fixed to the ends of the rafters and lined with lead. However, the timber required regular maintenance and replacement, and many medieval rural churches may never have bothered with them.

It was not until the rise of cast iron in the late 18th century that the modern system of gutters and downpipes eventually emerged. The development offered a much simpler alternative to bespoke leadwork, and was robust and far easier to install, and by the mid-19th century these systems predominated.

Below ground, drains usually run to soak-aways which become silted up over the years, as do French drains. Ground gutters were usually paved and are prone to changes in level due to plant growth and settlement, and rarely work as intended. Both lead to increasing moisture levels around the building.

Faulty rainwater disposal can lead to saturated walls and erosion of the mortar pointing, as well as frost and salt damage to the masonry. In addition to keeping downpipes, hoppers and gutters clear of debris, inspections should check for slipped or broken downpipes, and climbing plants such as ivy should be controlled as they can act as sails in high winds, pulling off anything they are attached to. Over-chutes should be incorporated in any new work wherever there is a risk of ponding from a blocked downpipe (see photo, middle right, for example), and in areas of high snowfall, trace heating should be introduced to maintain ice-free pathways for drainage, particularly through valley gutters and behind parapets.

CLIMATE CHANGE

According to the Met Office, rainfall patterns are changing across the globe, and in the UK recently, rainfall has been decreasing during summer months and increasing in winter months. Extreme weather patterns are also a concern, with 2012/13 seeing periods of drought followed by floods and one of the coldest winters on record.

It is clear that historic buildings in the UK need to be well prepared for extreme weather conditions, including exceptional wind strength and intense rainfall and snowfall. Quinquennial inspection reports should reflect this by recommending shorter time periods within which repairs become urgent and all churches should keep extreme weather conditions in mind when carrying out routine maintenance. The consequences of failure are not only greater expense in the long term, but also substantial losses of heritage.

 

 

Historic Churches, 2013

Author

JONATHAN TAYLOR MSc IHBC is joint editor of Historic Churches and executive editor of The Building Conservation Directory. A co-founder of Cathedral Communications Limited, he studied architectural conservation at Heriot-Watt University, Edinburgh and has a background in architectural design, conservation and urban regeneration.

Further information

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