||Simple parapet gutter at Corpus Christi College
Lead linings are commonly found in the
valleys between pitched roofs and behind
masonry parapets. Defects here often seep
quietly for months or even years, providing
the ideal conditions for dry rot, wet rot, insect
infestations and other forms of decay. The
correct use of lead is crucial to their success,
and if correctly detailed, these linings can last
Lead sheet is the earliest known metal to be
used as a roofing material, and with a proven
lifespan in excess of 100 years it continues
to be used today. From early medieval times
many churches and fine houses used sheet
lead which had been cast on a bed of sand
for covering their roofs, and sand cast sheet
continued to be used exclusively until the
18th century, when milled lead (now known
as rolled lead) began to be used for roofing.
Today, there are three types of lead
sheet manufactured for use in construction,
with significant differences in each end
product that can affect performance.
Rolled lead sheet (previously referred
to as milled lead) is the most commonly used
and widely produced. As its name suggests,
it is made on a rolling mill that controls the
thickness of the finished sheet to within fine
tolerances (any point on the sheet must be
within +/- 5% of the standard thickness).
Rolled lead is the only type of sheet that is
produced to a European Standard, BSEN12588,
which as well as regulating dimensional
tolerances also states the amount of permitted
inclusions (of copper, tin, antimony, etc) in
the lead that provides the feedstock to the
rolling mill. Thickness are defined by codes
with Code 4 (1.8mm) to Code 8 (3.55mm) the
most commonly used. Rolled lead’s uniform
metallurgical grain structure helps it to resist
fatigue cracking from thermal movement.
Sand cast sheet is still made using the
oldest method of manufacture, as it remains
in demand for restoration and refurbishment
work where like-for-like replacement is
required. This traditional method cannot
produce the fine thickness tolerances achieved
by modern rolling mills and is normally
produced in the thicker codes of sheet.
Nevertheless, the British Standards Institute
recognises the product in BS6915 (Design
and construction of fully supported lead sheet
roof and wall coverings – Code of Practice) as
it has proved during its centuries of use to
perform just as well as rolled lead sheet.
Machine cast sheet is the latest
manufacturing form, originally introduced
to the UK as a sound attenuation product and
adapted for roofing in the 1980s. Machine cast
sheet is not made to a European Standard as
it is not produced to within the +/-5 per cent
thickness tolerances required of BS/EN12588
material. In addition, the metallurgical
consistency of machine cast is different to
rolled sheet as it does not have a uniformly
distributed grain structure within the sheet.
Users of machine cast sheet should seek
working and fixing recommendations from
the manufacturers which have been shown
to be adequate through evidence of use.
Lead is a silvery metal which rapidly turns a
dull grey when exposed to the atmosphere.
This patina is relatively inert and extremely
durable, and is one of the two main reasons
why lead sheet makes such an excellent roof
covering. The other is that the sheets are
easily shaped by basic hand tools. The result
is an unrivalled weatherproofing material
which can, in the hands of a skilled and
knowledgeable craftsman be shaped to fit,
protect and preserve any detail of architectural
significance, no matter how complex.
However, this same malleability creates
problems for the unwary. Being a soft
metal, lead sheet reacts to thermal changes,
expanding in warm sunlight and contracting
when temperatures cool. Properly allowing
for this thermal movement when sizing and
fixing any detail is critical to the long term
performance of the sheet, which is why
only a skilled installer should be used.
Lead sheet in general has an excellent
performance record, but when failures do
occur, it is normally the result of incorrect
sizing and fixing, restricting natural thermal
Most lead details involve a piece of
lead sheet ‘hanging’ in some way on its
fixings; even a roof bay fitted to a fall of
only 1 in 80 tries to ‘creep’ down the slope,
restrained by the fixings at the head of
the panel. Head fixings prevent creep,
and intermediate fixings (clips) prevent
wind lift, both of which need to be fitted
correctly to allow free thermal movement.
||The Lead Sheet Association's recommended detail for
an overflow pipe from a sump (Reproduced from
The Lead Sheet Manual by kind permission of the
Lead Sheet Association)
||An alternative overflow detail where the parapet gutter discharges horizontally through the parapet wall: the
step ensures that under normal conditions the spout
discharges into the hopper below. In the event of a
blockage, water will cascade over the step and clear
of the fabric safely, in a highly visible way, alerting
owners to the problem.
||A typical section through a parapet gutter (not to scale)
Thermal movement is a natural occurrence
in lead sheet, caused by the variation in day
and night temperatures. Extreme variations
occur in spring and autumn, when a hot
sunny day can be followed by a very cold
night. If a long sheet of lead is fixed at both
ends, natural expansion and contraction will
result in the sheet buckling regularly at the
same point, causing fatigue lines to appear
as the material begins to loose its elasticity.
Cracks follow, allowing water ingress.
Other issues affecting the performance of
the sheet which the architect or specifier needs
to be aware of include the type of substructure
on which the sheet is laid, the positioning of
the drips (steps) resulting in trapped bays, the
sizing of joints (drips and laps) and general
faults in the design of the installation.
A good quality substructure is essential.
Rough sawn, square edged softwood board is
the preferred material. The boards should be
regularised to ensure that the surface is even
as, once the lead settles onto the substrate,
any large surface discrepancies will tend to
show through the sheet.
Green oak can cause corrosion as tannin
(tannic acid) contained within the oak attacks
the lead. If the lead is laid directly onto
green oak a separating membrane should
be used such as a Class A building paper
or, in certain circumstances, a bitumen
paint may be suitable, particularly under
lead flashings. If green oak is fixed above
leadwork, tannic acid can leech onto the
lead, and while in this situation the tannic
acid does not harm the lead, it can cause
unsightly staining and discolouration.
Where lead sheet is laid on masonry,
it is essential to ensure that the surface
is smooth, as sharp edges could perforate
the lead when thermal movement
occurs. An underlay of Class A building
paper or geotextile matting is best.
It is important to remember that
lead sheet will settle into any surface
imperfections in the substrate. It must
therefore be laid to a constant fall with no
depressions which would allow rain water to
collect. Where ponding occurs, high winds
can then force the collected water back up
and over steps and rolls into the substructure.
When considering parapet gutters or tapering
valley gutters on historic buildings, it is
usual to specify Code 7 or Code 8. Very often,
however, the existing configuration will
have bay lengths in excess of the current
recommended limits in the BS6915 Code
of Practice. It is also regularly found that
drip heights do not conform to the Code of
Practice and it is therefore essential that the
contractor engaged to carry out the leadwork
has sufficient knowledge and experience to
recognise such problems and provide an
alternative design to enable the installation to
be completed correctly.
Failures caused by oversized gutters
normally present themselves first as a ridge
across the sole of the gutter before a fatigue
crack actually appears. A similar effect is
caused by over fixing correctly sized sheet.
Gutter failures can therefore be the result
of any of the following common causes.
Over-sizing (in width and/or length) It should be remembered that the overall
maximum width of a gutter can be
increased, provided the overall length
from drip to drip is reduced. However,
the maximum recommended length
of a bay cannot be increased, even if
the width is reduced (guidance from a
specialist is strongly recommended).
Over-fixing Gutter bays should be fixed
at the head, into the rebate of the step. If the
fall exceeds 3° additional fixings will also be
required at the base of the drip. Gutter bays
can also be fixed for the first third of the bay
length through the upstand (to the parapet
wall for example). However, if the fixings are
taken beyond this point, then even a correctly
sized bay will ultimately fail, with fatigue
cracks appearing across the sole of the gutter.
The fixing heads should be sealed by either
welding lead patches over or, in the case of a
hot work ban on the site, using a lead patch
fixed by a suitable ‘liquid metal’ resin.
Stone slates If the pitched roof
covering is stone slate, such as York stone, then it is possible the weight of the stone
on the lead where it turns onto the tilting
fillet will act in much the same way as if
there were mechanical fixings down the
length of the bay. It is advisable in this
situation to stop the gutter wall just below
the top edge of the fillet and introduce a
separate cover flashing lapped over the
gutter wall from under the stone slates,
thus allowing the bay to expand freely.
Trapped bay Perhaps a slightly less
obvious potential failure is a trapped bay.
This simply means a gutter bay that is
fixed into the rebate at one end and turns
a corner at the bottom end before the drip
detail. In this situation, if at all possible,
the drip should be relocated before the
bay turns the corner. However, where this
is not possible it is essential (although
not ideal) to leave the bay short of the
corner so that during thermal expansion
the lead can move up to the wall freely.
Run-off from lichen and moss It is
advisable for any roof or gutter bay detail
to have a sacrificial flashing installed to
catch the run-off from a slate or tiled roof
that has a covering of lichen or moss, as
it is mildly acidic and corrodes the lead at
the point of contact. The sacrificial flashing
should be fixed by clips below the edge of
the slates so that it can be easily replaced.
The sizing and positioning of the outlet is
another area that needs careful consideration.
In historic buildings with a parapet, the
water from the parapet gutters or valley
gutter collects behind the parapet in a sump.
Traditionally, there is only one outlet, usually
a lead pipe passing through the masonry to
an external rainwater downpipe, but in some
cases there is an internal rainwater downpipe
instead. In both situations it is essential to
introduce an external overflow pipe or welded
chute, as leaves and other matter that collect
here cause restrictions and partial blockages:
the gutter area fills to the point where water
starts seeping into the underlying roof
|Above left: A flashing which was inadequately fixed into the parapet wall has fallen out,
exposing the upstand of the gutter lining. Above right: A flashing which is fixed into masonry must have lapped joints at least every
1.5 metres, as otherwise it will be unable to tolerate the amount of thermal
movement, causing it to buckle and eventually crack, as here.
Parapet gutters invariably have horizontal
flashings on the parapet side, lapped over
the upstand. The flashings are wedged or
screw fixed into joints in the parapet, with
the result that they are unable to expand or
contract along this side. Lengths of flashing
should therefore be kept short to avoid
excessive movement, with laps every 1,500mm.
Movement in longer lengths will result in a
crease forming when the material expands.
Over time vertical fatigue lines form and
eventually fracture, allowing water ingress.
In such circumstances it is not necessary to
renew the entire flashing, as a small piece
of lead can be wedged and pointed into the
flashing joint, lapping over both sides of the
crack by at least 100mm.
Further problems can occur when a
flashing that is turned into a masonry joint
springs out, allowing water ingress. This
can happen through inadequate fixings or
because the turn in to the masonry joint
is too shallow. BS6915 Code of Practice
requires a 25mm turn-in, but this should be
regarded as a minimum. When preparing
flashing pieces with a fold to be turned
into a masonry joint, the deeper it is
turned in, the better the fixing will be.
DEEP FLASHINGS AND VERTICAL CLADDING
A flashing of 250mm ‘girth’ (the full width of
the sheet, including bends) can have anything
up to 150mm of unprotected substructure
behind it, once the 25mm minimum turn-in
and the 75mm minimum lap over the upstand
to the roof or gutter is taken into account. A
lap of 100mm between lengths of flashing
would not be adequate in this instance
because wind driven rain will travel sideways
at roughly 45° in a lap and water will therefore
reach the substructure before it reaches the
top of the gutter upstand. The installer should
always check that the lap is slightly more than
equal to the exposed substrate, and in this
instance a 150mm lap should be used. This
is usually an unseen potential failure point,
but easily checked by lifting the overcloak and
measuring the lap.
If the flashing over an upstand is
required to be more than 250mm, then the
experienced installer will use a combination
of a standard cladding detail with a cap
flashing. The cladding should be jointed by
vertical rolls, or more commonly welts, in
order to protect the substructure. The detail
is finished with a cap flashing, turned and
lapped into the chase and then pointed.
It is essential with vertical cladding to
ensure the fixing is correct. A common fault is
the head fixing, which should be determined
by the size and weight (thickness) of the
panel. Normally Codes 4 and 5 are used for
vertical cladding, and panels under 500mm
high can be adequately secured with one row
of nails fixed 50mm apart. For larger panels
two rows of nails are required, 75mm apart
with staggered centres, aligned 25mm and
50mm from the top edge. Panels using Code 7
or 8 should be fixed with three rows of nails.
Where vertical cladding has been
inadequately fixed at the head, problems
present themselves in the form of
slipped panels revealing elongated fixing
holes just below the bottom edge of the
upper panel or cap flashing. Over-fixing invariably results in bulging panels
where movement has been restricted.
Clips are an essential element of vertical
cladding and should be fitted to the bottom
edge to prevent wind lift (rather than to
provide additional fixing). The lower 2/3 of
a vertical cladding panel must be left free to
expand and contract with natural thermal
movement. Vertical clips should therefore
be turned so that a minimum gap of 6mm
is left to allow the panel to expand.
A clip that is fitted without this gap is
easy to spot. If it is a lead clip, the panel
will simply push the clip open. If the clip
is of a hard metal, such as copper, then a
bulge will form in the lead immediately
above the clips, clearly showing where
its movement has been restricted.
In addition to the bottom edge clips
on the panel, the vertical welts also have
clips fitted within them. It is therefore
essential not to make the welts too tight, as this will not only restrict thermal
movement between panels, it will lock the
hard metal clips to both panels and further
aggravate the movement restriction.
|Above: A roll which terminates short of the drip, allowing moisture
to penetrate under the lap: the fault is immediately visible
from even a cursory inspection of a roof. Below: Underside lead corrosion can occur where lead
which has not formed a stable patina is exposed to
|Main picture: A valley gutter with lead rolls in each bay, correctly detailed so that the roll terminates neatly at the drip. The section
nearest the eaves is often sufficiently narrow not to need a lead roll.
Rolls are commonly used to reduce the width
of lead sheets in wide gutters. In effect a roll
is a small upstand made of a rounded wooden
core: sheets of lead on either side of the core
are lapped over this upstand. In a valley
gutter, for example, as the gutter rises in steps
from the outlet to the back of the valley, the
width widens, making the introduction of a
roll necessary on the sections furthest from
Incorrect roll ends are a common cause
of failure in both flat roofing and gutters.
A roll end really must finish at the very
edge of a drip or step, but very often roll
ends are left well short, allowing water to
track under the lap, around the end of the
roll and into the substructure. Short roll
ends are possibly one of the most obvious
visible defects to spot on a lead roof.
The final area to be considered with regard
to common defects in lead installations is a
problem that has only recently been properly
identified and understood. It is caused by
condensation on the underside of the lead
sheet where, in the absence of weathering,
the lead may not have formed a stable patina,
leaving it vulnerable to decay.
One source of condensation is warm
moist air from rooms below, such as a kitchen,
bathroom or even a heavily used function
room, which condenses on the cold underside
of the lead if there is no airflow. Another
common cause is thermal pumping: if the
lead sheet is laid on an unventilated warm
roof (with boards laid directly onto insulation
sealed from below by a vapour barrier), a rain
shower on a warm day will cause the air inside
the roof to cool rapidly and contract, sucking
in air and water through the laps in the
lead above. Once inside, it evaporates in the
warmth and condenses when the temperature
drops against the cool underside of the lead.
In both cases, in the absence of a stable patina, the water causes the lead to
corrode through flaking (much the same as
rust on steel) until holes begin to appear
in the sheet from below. Dry or wet rot
can also be created in the substrate.
Where underside corrosion is identified,
the solution typically involves relaying
the lead over a new ventilated substrate.
In summary, lead sheet is no different in many
respects to most roofing materials in that it
will not work if it is not detailed and fitted
When working on buildings of
architectural or historic significance, many
main contractors decide the most cost
effective route is to place the whole roofing
package in the hands of a general sub
contractor. This is often a critical and costly
mistake which can lead to an outbreak of dry
rot requiring extensive repairs to interior
roof structures, and the all too familiar
refrain: 'It was only a piece of flashing
– I didn’t think I needed a specialist'.
Rolled Lead Sheet – The Complete Manual (Reprinted June 2007), The Lead Sheet
Association, Tonbridge, Kent