Michael Parrett concludes his series on diagnosing causes of dampness in buildings. He discusses common issues with cast iron gutters and rainwater pipes such as lack of paint protection leading to corrosion. Drainage design is also important to properly discharge rainwater from roofs. Defective water mains frequently cause damp issues, with leaking pipes sometimes running under buildings and causing damage. A holistic approach is needed to properly diagnose the source of dampness which can have various causes beyond just a failed damp proof course.

1. RICS PROPERTY
JOURNAL
R
3 8 N O V E M B E R 2 0 1 5
RESIDENTIAL
BUILDING PATHOLOGY
Images © Michael Parrett
The
whole
story
C
ast-iron gutters and
rainwater pipes,
prevalent through
the Victorian and
Edwardian periods,
need paint protection.
Unfortunately,
painting schemes may miss the inside
faces of these goods, which then
corrode. Similar problems occur with
internal cast-iron service pipes, such
as soil stacks.
Connections are vulnerable where
flat-roof rainwater outlets meet internal
pipes, which often suffer blockages
from airborne detritus. In winter, pooling
water freezes and can crack the outlet.
Either problem may cause water to leak
outside the drainage area down the
outside of the internal pipe.
Surveying cast-iron goods can be
tricky due to restricted access. I use a
mirror on a telescopic pole to check the
rear of pipes and gutters, but this may be
insufficient if problems are beyond the
reach of a 3m surveyor’s ladder. These
issues are greater in tower blocks, which
require safe working access.
When installing PVCU systems,
allowing for linear expansion is often
missed, causing them to buckle.
Expansion is needed on all elevations,
especially when south-facing, but
following manufacturer’s instructions
should mean guttering, pipes and
connections are cut appropriately.
Guttering alignment is critical, but
falls are often incorrect, causing flows
away from the outlet and spillages over
Michael Parrett concludes his series on damp and reminds
surveyors to take a holistic approach when diagnosing issues
the gutter. Some modern systems can be
laid level, with outlets designed to create
a vortex to draw water out of the gutter.
Drainage design on buildings is vital
to properly discharge rainwater. I have
investigated spectacular failures where
the design has been ‘hydraulically
short’ and size, spacing or number
of rainwater pipes insufficient. The
calculations for correct designs are
complex, because they consider factors
such as rainfall intensity, roof area, roof
angle and parapet wall height.
I saw an example of this on a sports
centre, which had a shallow-pitched roof
surrounded by a high parapet wall. Unable
to discharge water effectively, the box
gutters filled, causing water ingress into the
building. This was exacerbated by blocked
outlets from stray sports balls. Roof access
also required a safe working platform but
the cost limited regular maintenance.
1 2

2. RESIDENTIAL
BUILDING PATHOLOGY
is essential and problems can usually
be solved by replacing or repairing
rainwater goods. Owners may be
reluctant to change the lowest section
of a cast-iron rainwater pipe because it
is resilient; bicycles may be chained to
them, for example. In reality, if the
upper sections are defective then
so will lower parts, which should be
replaced (possibly using modern
cast-iron versions).
Cast-iron pipes can let directly into
an underground branch drain, often
overlooked in replacement programmes.
But if this is defective, water can seep
into the ground and building at low level.
Branch drains should be part of survey
and renewal programmes as they often
fail through blockages or frost action.
Leaking water pipes
In the 1960s and 1970s, it was common
to run copper heating and other water
pipes through solid floors. Denso tape
or fibrous wrapping were often not used
or poorly applied to protect the bare
copper from acidic cementitious floor
screeds; as a result they corroded and
caused leaks. When mixed metals were
joined, i.e. copper and steel, this caused
an electrolytic reaction that acceleratedn
Tree and leaf litter obviously cause
blockages. I know of examples where the
bottom section of pipes (particularly when
entering a branch drain) was completely
blocked and heavy rain caused spraying of
water from each joint.
Warning signs: Leaks onto outside walls
are difficult to detect, particularly on solid
walls when dampness may only be noticed
when it has penetrated. There may be no
internal signs on a cavity wall. If the cavity
is not blocked, leaks may stay undetected
for a long period. A good way of spotting
leaks is to conduct a survey in heavy rain.
On north-facing elevations there may be
algae under a leaking gutter joint, but on
sunny south-facing elevations, leaks are
more difficult to see, although there may be
staining. One sign is if water falls straight to
the ground causing rain splash, especially
noticeable if it extends onto flowerbeds.
Solutions: Clearing gutters regularly
1 Broken collar and corroded section of cast
iron rainwater pipe. The defects occurred due
to a lack of paint protection and subsequent
wetting and frost action. Often when painting
cast-iron pipes, the rust is not properly
removed, which will cause paint rejection
and failure
2 In this flat-roof to pitch conversion, the
original rainwater downpipe is still routed
integrally through the building. However, the
new section of rainwater pipe from the gutter
external to the building was defective allowing
rainwater to leak down the outer face of the
building and cause water staining, which was
not visible during dry periods
3 Birds drop seeds onto roofs that collect
in gutters and eventually grow, blocking them
and leading to an overspill of water soaking
into the walls of the building below the eaves.
Rainwater penetration into the habital space
will occur where the external walls are solid
4 Leaking water main supply pipe under a
timber suspended floor causing extensive wet
rot decay. The defective section of water main
was underneath kitchen cupboards. Detection
was possible using a hygrometer probe and
endoscope
N O V E M B E R 2 0 1 5 3 9
3
4

3. RICS PROPERTY
JOURNAL
RESIDENTIAL
BUILDING PATHOLOGY
R
4 0 N O V E M B E R 2 0 1 5
Images © Michael Parrett
corrosion. Gun metal was once used for
joints because it is neutral and breaks
these corrosive forces.
In the 1920s, some water mains
were made of lead, but galvanised
barrel pipes were later often laid,
many without wrapping protection.
The galvanised zinc coating acted
as a ‘sacrificial anode’ in aggressive
soils to leave bare steel. The National
Physical Laboratory has mapped UK
soil types to show those containing
metal-eating microbes, which is useful
when assessing the likelihood of
corrosion. Tree roots and desiccation
can also disturb water mains.
Warning signs: A common cause of
dampness is leaking water mains.
These may undermine foundations or
cause overhydration to create heave,
especially in areas of London clay.
When investigating a property, it is
useful to know how mains are routed in
different building genres.
In Victorian and Edwardian properties,
lead water mains and drains often went
under the house (mostly from the front,
but sometimes the rear) from the isolation
valve outside the curtilage. Leaks could
soak into the fender wall of fire hearths
(that do not have damp-proof courses)
and wick up the chimney breast creating
rising damp (but not requiring a retrofit
damp-proof course to resolve it).
During the inter-war years, large
‘cottage estates’ were developed after
slum clearances. Because these houses
were publicly owned, one water main and
isolation valve typically served a terrace of
four, six or eight properties. Mains would
normally be routed alongside the buildings
(through a mid-terrace walkway or at the
end of the row), through back gardens and
then teed-off into the properties.
Solutions: Responsibility for leaks was
complicated by the Housing Act 1980,
which allowed tenants to buy their council
home. The result is mixed terraces of
public and privately owned properties
with shared water mains. Resolving repair
responsibilities can be difficult, because
costs are shared between the owners,
who blame each other. Knowing the
location and problems of these mains is
important, because millions of ex-local
authority properties are now held privately.
Surveyors may recommend that owners
apply to their water provider to install their
own separate water main and isolation
valve to avoid this shared scenario.
Once armed with knowledge of the
building genre, water main routes and
any shared supplies, a surveyor can
Defective water mains are often just
locally repaired. However, old pipes
leaking in one section may well begin to
leak in another. Best practice suggests
renewing the whole length of pipe.
Chimneys
Chimney breasts are generally wholly
internal (on a party wall, usually
back-to-back with a neighbour’s);
three-sided (wholly internal with the
fourth side on an external wall) and
projected (three sides standing out
from the building).
A chimney stack projecting through
a roof, where the roof slope terminates
at the stack, is one of the most
vulnerable parts of a construction.
Chimney stacks are usually the highest
point of a building, so are very exposed
to the elements but often overlooked
because they are difficult to survey. A
future article will focus on identifying
problems associated with chimneys and
possible solutions.
Drainage
Sudden dampness can usually be related
to a defective drainage system, caused
by tree root penetration, blockages and
physical defects. Any leaks can raise the
humidity in a building and cause dampness,
especially under suspended floors.
n
5 In this Victorian property, there was
dampness to the timber suspended flooring
in contact with the fender wall of the hearth,
wicking dampness from the oversite into
the timber flooring. The discovery warranted
further investigation to check for water
escape from the lead mains water supply pipe,
underground drainage failure, high water table,
blocked sub-floor air vents and/or rubble and
arisings piled up under the floor
6 Leaking integral rainwater water pipe to a
block of flats. A cracked swan neck section of
the pipe connecting to the flat roof drainage
outlet caused rainwater to penetrate down the
outside of the cast-iron pipe soaking into the
surrounding ducting and part of the floor slab
by way of the inter-floor fireproofing material.
Integral cast-iron pipes are prone to corrosion
from a condensate forming on the outer
surface of the pipe
7 Copper water pipes in a solid floor screed
corroded and leaked due to poorly applied
plastic wrapping. This exposed sections of the
pipe to the cementitious floor material
leading to its perforation within eight years
of being laid
8 Plastic rainwater pipe with ‘rodding eye’
access removed. The pipe was completely
blocked with a mixture of silt and rust from
the connected cast-iron drainage outlets to a
podium roof directly above
check whether a leak exists. After the
usual internal reconnaissance of timbers,
a key question is whether an immediate
neighbour has also complained of
dampness or any sudden drop in water
pressure. If so, a leaking water main
must be suspected, but where?
A surveyor may suggest that a leak
is investigated by the water company.
An inspector will visit the site at night
(when water use is low) and use a
listening stick on isolation valves. They
might ‘hear’ a leak and then use a leak
correlator, an electronic sounding
device, to pinpoint the location.
Another check is the syphon test.
After closing the water main, put the
spout of the kitchen sink tap in a full
glass of water. If the tap starts to suck
the water out of the glass, there is a
leaking water main because the water is
being pulled back through the tap by an
escape of water underground.
6
5

4. RESIDENTIAL
BUILDING PATHOLOGY
Michael Parrett is a Building Pathologist,
Chartered Building Surveyor and Founder of
Michael Parrett Associates. He is an Eminent
Fellow of RICS
info@dampbuster.com
www.michaelparrett.co.uk
Related competencies include
Health and safety, Inspection,
Building pathology
Tree root penetration is a massive
threat to underground pipes, particularly
vitrified, salt-glazed clay pipes but also
modern systems (some claim their
interconnecting joints are impenetrable
to tree roots, but I have yet to see this).
Roots may disturb drainage pipes and
alter their alignment.
Desiccation may unsettle drainage
during long dry periods and cause the
ground to move. A water main leaking
near a drainage pipe might cause
overhydration and heave. In either
scenario, where each section of pipe
once neatly and smoothly abutted
the next, pipes will either split or be
misaligned, causing material to collect
on these ledges and create blockages.
Underground drainage systems
constructed before October 1939
combined foul and surface water all
into a single drain, but these sources
have or should have been separated.
In older properties, everything
therefore goes through one drain.
Increasing rainfall intensity, bad habits
of occupants (such as disposing of
plastic nappy liners down toilets)
and larger buildings converted into
separate flats all mean higher demands
on these antiquated systems.
Warning signs: Common indications
include a ground collapse or dampness.
In Victorian properties where the
drainage system runs under the
building, there will be foul smells
(usually every time a toilet is used)
and often high ammonia. These
problems often arise suddenly and
tend to suggest something has
changed in a short space of time.
There may also be localised
flooding from blocked drains,
particularly in back inlet gullies next
to the external walls, where rainwater
pipes terminate. Blockages can mean
back surges with sewage appearing
in the bath, hand basins and toilets.
There may even be a contaminated
flood to the ground floor. Often,
blockages occur in interceptor
chambers and foul water will bubble
out of the top of the chamber,
regardless of its depth, together
with foul smells.
Solutions: Occupiers should be
educated about the impact of putting
waste into the drainage system.
Other preventative actions include
improving the refuse system to
allow people to dispose of rubbish
more easily.
CCTV camera surveys are
becoming increasingly important to
understand the state of drainage and
identify problems. If tree roots are
present inside drainage pipes, all
but the severest can be bored out,
using a special attachment to
allow relining.
With severe blockages, a
pneumatic steel arrowhead can
be pushed through to shatter the
pipe; a new channel can be bored
and a drainage pipe pulled through.
Excavation and complete renewal
may be the only other option.
A surveyor’s approach
Surveying buildings for dampness
should be a holistic process and
surveyors should move from
recognising a symptom to diagnosing
its cause and source. This series
on common causes of dampness
illustrates that a failed damp-proof
course should be the last thing
suspected. It will usually be any
one or a combination of these
other causes.
Achieving a professional opinion
on the source of dampness
means improving knowledge and
understanding of building failures
through the various building genres,
and the techniques and equipment
used as part of an overall process
that is called building pathology. R
More information
>
BS EN 12056:3-2000 Rainwater
drainage design
http://bit.ly/1KgLgFo
Diagnosing Damp; Ralph Burkinshaw &
Mike Parrett
http://bit.ly/1G2tEsb
Mike Parrett’s guide to building pathology
http://bit.ly/1zeIO7X
N O V E M B E R 2 0 1 5 4 1
87