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BCD SPECIAL REPORT ON
HISTORIC CHURCHES
22
ND ANNUAL EDITION
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METAL SHEET ROOFING
and its substitutes – a brief history
Tony Redman
T
HE ROMANS are credited with
first discovering the weathering
properties of lead, which they
called
plumbum
on account of its
resistance to rain (
pluvia
in Latin). This
also explains the origin of the name
for leadworkers (‘plumbers’) and the
chemical symbol in the periodic table, Pb.
The Romans were using small lead sheets
or tiles for roofing as early as 27 BC.
Lead is a naturally occurring mineral
gained by smelting galena ore. It was
sometimes mixed with small quantities
of silver and tin, and some authorities
suggest that small quantities of such
‘impurities’ add to its malleability and
weather resistance.
The Romans also introduced copper
roofing, in the form of small overlapping
sheets, especially on high status buildings,
but its high cost compared to that of tiles
produced from local clays limited its use
throughout the Roman world.
Both lead and copper are highly
durable, which is why they have been used
on important buildings for so long. They
are also very easily recycled, which is why
they have been prone to theft.
This article looks at the various types
of traditional metal roofing as well as
modern substitute materials.
Lead
Properly installed and maintained in an
environment which does not involve
acidic conditions, lead is probably the
longest lasting roofing material available.
It is UV stable, resists corrosion from
normal weather, and is easily repairable
by traditional lead workers. In its natural
state lead is a whitish silver colour but
it tarnishes rapidly to a dull grey in air,
although patination oil can retard this as
well as preventing staining due to run-off.
Lead has a low cost in use compared to
other forms of roof covering, largely due
to its weather resistance. But it does have
its drawbacks.
Firstly, lead has a high coefficient of
thermal expansion and needs to be given
the capacity to move without tearing its
fixings. The standard thicknesses of lead
sheet are defined by ‘codes’ that are based
on weight in imperial pounds per square
foot, so Code 4 lead sheet’s approximate
weight is 4lbs/sq ft, while Code 5’s is
5lbs/sq ft. The Victorians, proud of being
able to calculate stresses in materials and
design buildings to use materials to their
maximum tolerances, often exceeded
modern lead tolerances, not always
successfully.
Secondly, lead can be subject to
underside corrosion, especially in contact
with green oak, and associated with the
use of unvented gas heating systems.
An acidic atmosphere condensing in
an unvented environment leads to
oxidisation and the formation of white
lead oxide, a highly toxic powder.
Thirdly, lead itself is toxic and builds
up slowly in the body requiring care on
the part of those who work with it.
Sand-cast lead, run out over a table
of compacted sand, is widely regarded as
superior to milled lead, where the molten
raw material is run between two rollers and
cooled more rapidly. However, it requires
more care and craft skills in its production
and often contains more antique lead,
containing a greater proportion of silver
and tin which give it better workability and
durability. Milled lead on the other hand
may be a more consistent product and it is
often cheaper, although the cost balances
out when the value of salvaged lead is
added into the equation.
Copper
Copper roofing underwent a revival in the
1950s when its use became fashionable on
civic buildings, and the formation of the
Copper Roofing Association promoted
its development and use. Many historic
buildings have copper roofing in good
condition which is over 60 years old. It is
relatively trouble-free.
Copper sheet was usually laid
over ‘inodorous felt’, which was
traditionally made from horse hair but
modern underlays use a wide variety
of materials including geotextiles.
Surface finishes have been applied to
either encourage corrosion to a green
verdigris colour, or to prevent it.
Copper roofing is less prone to
thermal movement than lead, enabling
it to be laid using upstand seams and
clipped more securely to its substrate.
Lead stripped from the roof of St Andrew’s, Boothby Pagnell, Lincolnshire (Photo: E-Bound AVX Limited)