Rain Penetration

Douglas Kent


  Thatched building in catastrophic disrepair
  Rain played a major role in the deterioration of this listed building, later demolished.

With the British climate it is no surprise that the penetration of rain into buildings is often a problem. Leaks occur above ground at various points in the external envelope of the construction: through roofs, walls, chimneys and openings such as windows and doors. In old buildings, the cause of rain penetration can simply be poor maintenance or perhaps previous inappropriate work. Rain penetration, however, is only one of a variety of sources of excess moisture that include condensation, rising damp and plumbing leaks. Mis-diagnosis is rife and many modern treatments that are designed to stop rain penetrating actually worsen problems by attempting to seal the fabric rather than letting it breathe.


  Pentice boards fitted to gable
  Pentice boards are intended to throw off rainwater.

Old buildings need to breathe. While modern buildings rely on excluding water with a system of barriers, buildings that pre-date the mid-19th century are usually constructed of absorbent materials that allow any moisture that enters to evaporate back out: the 'raincoat' and 'overcoat' effects respectively.

Although water can enter the fabric of old buildings, dampness is not inevitable. Before central heating, heat from open fires drew in large quantities of air through loosely fitting windows and doors. This ventilation quickly evaporated moisture from permeable internal surfaces and from damp timbers in roofs without felt. Rain penetration, therefore, tended to be in a state of equilibrium with evaporation, a balance that must be retained.

To reduce rain penetration, walls were widely lime-rendered and limewashed. In more exposed parts of Britain, rendering took the form of 'rough-cast' (or 'harling' in Scotland), a heavily textured finish that reduced run-off but increased the surface area for evaporation. Other forms of wall protection included tile-hanging and weatherboarding. Weatherings such as projecting cills, string courses (projecting bands of stone or brick) and pentice boards (see illustration) have long been employed too. Earth or chalk walls are particularly vulnerable to decay when saturated so these were protected by broad eaves and stand off the ground on brick or stone plinths.

Many features that contribute to the beauty and interest of old buildings thereby serve a practical purpose by helping to keep them dry.


Water penetrates buildings through various mechanisms, including gravity, wind pressure and capillary action. It not only damages the fabric but also creates unhealthy conditions for occupants, together with less visible problems like poorer thermal performance.

Roofs, chimneys, parapets and other exposed parts of buildings are most susceptible to rain penetration, especially where access for maintenance is difficult. Something as straightforward as a slipped tile can cause timber decay or damage to plaster ceilings. Smaller holes lower down a roof will tend to admit more water than larger ones higher up. Defective verges and inadequate overhangs have the gravest consequences where directing water into the head of earth or rubble-filled walls. Junctions in roofs are further trouble spots, with water exploiting defective lead flashings, mortar fillets, ridges or hips. Roofs on porches or extensions may be of inferior quality, with poor detailing and rainwater disposal.

Leaks from parapet and valley gutters can cause significant damage to structural roof timbers, and overflows from rainwater fittings that have not been cleared of autumn leaves can cause concentrated and prolonged wetting of walls and external joinery such as window cills. Inadequate drainage or rainsplash commonly soaks walls where patios and paths are laid up to them.

The removal of render from Victorian times onwards left countless walls vulnerable to driving rain. Such practice was so vehemently opposed by the Society for the Protection of Ancient Buildings (SPAB) at the time, that the society was nicknamed 'Anti-scrape'.

Where the original lime render has been repaired or replaced with cement mortar, which is much less permeable than lime, any cracks that form inevitably admit and trap moisture, a problem frequently exacerbated by the use of impervious modern paints. Pointing stone or brick walls with a dense cement mortar also causes similar problems.

Projections which are inadequately protected by 'weatherings' (lead flashings for example) also allow water in, as do windows that are not recessed but set flush with the wall face, if poorly detailed, as is the case with many immediate post-war replacements.


  Thatched cottage with weatherboarding
  Weatherboarding can offer extra protection against the rain.

Scientific analysis aids diagnosis but interpreting results demands care, and the importance of the human senses must not be undervalued. Mistakes commonly arise from an over-reliance on electrical moisture meters. Elevated readings occur not infrequently in old buildings that are virtually dry, due to salt deposition from evaporation. Also, readings do not indicate the direction of movement, and rising damp is often misdiagnosed on the basis of high figures at the base of a wall and used to justify unnecessary damp-course treatment. They are just as likely to indicate a leak higher up, as damp descending through the core of a wall will pond when it reaches the damp course. Equally, rain-splash or the pooling of surface water might cause damp here.

A variety of clues help to distinguish penetrating damp from other sources. Wet patches on ceilings and walls following rain are symptomatic of rain penetration, as can be mould. Unlike with rising damp, clues typically include green staining externally to walls, broken roof tiles on the ground and leaking gutters or downpipes. Testing for nitrates and chlorides may also indicate the source: if the results are negative, moisture is unlikely to be rising from below ground. Carbide meters can help confirm whether a wall is wet within its thickness rather than suffering from surface condensation.

Although chartered surveyors have a duty to follow a trail of suspicion, some just note the occurrence of high meter readings and pass all responsibility on to remedial treatment contractors with a vested commercial interest in encouraging over-specification. It is worth questioning seemingly misguided work, particularly damp-proofing, demanded by mortgage lenders, and, if necessary, seeking a second opinion in writing from an independent chartered surveyor or consultant, not a contractor.

  Impervious paints are unsuitable on old walls that need to ‘breathe’.  


Remedies must aim to cure dampness by addressing the cause or, failing this, managing it by treating the symptoms. In some cases, dampness can be considered insignificant and will require no remedy.

Applying staged remedies can help to accurately diagnose the cause of dampness. Before embarking on extensive work, therefore, the first step may entail nothing more than basic maintenance, such as clearing a blocked gulley.

Be wary of written guarantees, which are often loaded with 'get-out' clauses and may have no insurance backing. The right approach from your contractor coupled with good workmanship is your best warranty.


Defective roof coverings should, of course, receive attention as soon as possible. Tarpaulin or felt can afford temporary protection although this is no substitute for full repair where required. Bear in mind also that tarpaulin can damage thatch that could often otherwise be repaired, despite pressure frequently for complete renewal. Additionally, thatch may succumb to decay if covered by tarpaulin when wet.

Church repair work being carried out as part of the SPAB's national maintenance week
The importance of good upkeep cannot be overemphasised.

Spray-on foams for the underside of roofs should not be used to secure slipping tiles or slates, nor should external bitumen coatings. Foams are being heavily marketed at present but can promote serious decay by effectively 'wet poulticing' timbers. Both methods tend to reduce ventilation and cause condensation problems, and they invariably hinder the reuse of slates or tiles.

For localised repairs, the method of re-fixing slates or tiles depends on the material. True slates, for instance, can be re-secured with copper 'tingles' (strips cut from pipe or lengths of wire) or equivalent proprietary products.

With more extensive work, the general principle is to reinstate roofs on a like-for-like basis. This helps retain character and can have other advantages. For example, replacing cleft shingles with the same, rather than sawn ones, will ensure both a close visual match and better resistance against rain penetration because the surface fibres suffer less disruption, with the grain tending to shed water quicker. Some historic embellishments are not particularly water-resistant but remain important vernacular details. For example, the timber cappings provided to the bargeboards of pantiled roofs in Suffolk, although prone to decay, give sacrificial weather protection to the verges.

Modifications are sometimes justified. When renewing mortar fillets or mitred hips, for example, it is often prudent to introduce concealed lead soakers as additional protection against moisture ingress. Mortar fillets, though, should not automatically be totally replaced with lead flashings. Roofs renewed with inadequate eaves overhangs or oversailing verges should be extended if jeopardising earth or timber-framed walls.

Modifications may also be necessary to protect redundant flues from driving rain, while still maintaining ventilation. Also, where an existing flue remains in use, old parging absorbs water, but new impervious linings may convert chimneys into drain-pipes. In this case a slab top chimney terminal may therefore be desirable to keep out direct rain.


Good maintenance is essential and in recent years the SPAB's annual National Maintenance Week has highlighted how uncomplicated tasks such as clearing gutters and rainwater pipes can save more extensive work later. Snow should be cleared from parapet and valley gutters to prevent moisture seeping through joints (minding personal safety). Alternatively, duckboards and electric heating tapes can be provided to keep gutters free of snow. At ground level improved drainage helps reduce penetration of the wall base.

Defects in rainwater disposal systems obviously require making good promptly, possibly in conjunction with modifications where these are visually acceptable, such as the introduction of overflow pipes above hopper heads.


  Plants growing in gutter
  Vegetation should not impede rainwater disposal.

Deeply eroded mortar joints should be raked out and repointed, but this is unlikely to cure serious water penetration problems. A lime:sand mix (preferably without cement) is suitable for most old buildings pre-dating c1900. Daub, lime mortar or oakum (ship's caulking) are useful for closing gaps that may develop around panel edges in timber-framed buildings.

Although remedial action should ideally involve the removal of inappropriate cement pointing and renders or modern paints entrapping moisture, this may cause further damage to the fabric and trials are always recommended. All methods of paint removal carry some risk of damage to the substrate, including sand-blasting in particular, and cement can adhere so tightly to masonry that it is almost impossible to remove without also removing the original face. Sometimes a compromise may be possible, perhaps just by removing the render at the base of walls or hollow-sounding patches where localised detachment has occurred. If it is decided to leave cement render in place, cracks can be repaired using hydraulic lime.

Where rain penetrates an exposed wall, limewash, lime render and slate- or tile-hanging are traditional solutions but cannot be employed without changing the external appearance. A measure such as partial rendering at first floor level (including any gables) may be more acceptable aesthetically. Alternatively, some circumstances permit installation of a ventilated dry lining system internally. Avoid, though, tanking, plastic-based paints and colourless water-repellent treatments on old masonry, except in certain circumstances below ground. Where projections cause rain penetration these might be given a weathering, possibly of lead, or, if minor, the problem may simply be tolerated. Typical situations include cornices, roof-top wall projections between houses in a late-Victorian terrace, and inward sloping bed joints in buttresses or 'tumbled-in' gables.


Paintwork and putty should be maintained to exclude moisture and prevent rapid deterioration. Thermal movement and cracking in timber cills may be minimised by avoiding dark paint colours. Rather than using a modern sealant, gaps between window frames and walls may be pointed with lime mortar and finished with a fillet of sand and boiled linseed oil. Although occasional minor leakage alone will not warrant replacement, new windows in the original material might be contemplated in place of poor quality replacements, particularly if these are insufficiently recessed or lack properly projecting cills.

Run-off on walls may enter openings other than doors and windows. For example, airbricks designed to ventilate sub-floor voids and reduce condensation can provide pathways for penetrating damp. This may be overcome by forming small lead hoods above to act as drips.


Recommended Reading

  • R Burkinshaw and M Parrett, Diagnosing Damp, RICS Books, Coventry, 2003
  • P Hughes, The Need for Old Buildings to Breathe, SPAB Information Sheet 4. SPAB, London, 1986
  • SPAB, Proprietary Colourless Water-repellent Surface Treatments on Historic Masonry, SPAB Statement 2. SPAB, London, 1995
  • A Thomas et al, The Control of Damp in Old Buildings. SPAB Technical Pamphlet 8, SPAB, London, 1992
  • P Trotman et al, Understanding Dampness: Effects, Causes, Diagnosis and Remedies, BRE Bookshop, Watford, 2004



This article is reproduced from The Building Conservation Directory, 2005


DOUGLAS KENT BSc (Hons), MSc, MRICS is Technical Secretary at the Society for the Protection of Ancient Buildings. The society operates a technical helpline (020 7456 0916), produces advisory publications and runs various courses.

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