|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 are intended to throw off rainwater.
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.
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.
that contribute to the beauty and interest of old buildings thereby
serve a practical purpose by helping to keep them dry.
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.
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.
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.
||Weatherboarding can offer extra protection against the rain.
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
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.
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
|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.
FITTINGS AND DRAINAGE
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.
||Vegetation should not impede rainwater disposal.
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
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.
WINDOWS AND OTHER OPENINGS
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
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.
- 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
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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