Metal Sheet Roofing
|Lead stripped from the roof of St Andrew’s, Boothby Pagnell, Lincolnshire (Photo: E-Bound AVX Limited)
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
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.
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
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
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 on the roof of the friary church of Blackfriars, Norwich
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
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. Depending on the world market
price, copper can be vulnerable to theft,
although the method of fixing tends to
render this less likely than with lead.
This mineral was also known to the
Romans and the oxide was used in the
production of brass, but it was not
until the mid-16th century that it was
recognised as a metal in its own right.
The industrialisation of the smelting
process began in Bristol in 1743 but it was
not until further developments in Belgium
and Germany in the middle of the 19th
century that the production of a cheap
zinc roofing material got under way. The
metal itself is comparatively cheap to
produce and to lay but more vulnerable to
corrosion in damp, polluted atmospheres
than copper or lead. In the UK it has been
prone to perforating due to acid rain and,
when laid over oak boarding, tannic acid.
Further developments in 19th-century
France introduced galvanising by smelting
zinc and tin together and coating this
onto steel using an electrolytic process.
More common in mainland Europe
and the Far East, zinc sheet roofing
has recently become fashionable in
the UK and it is available in a limited
variety of coatings and colours
produced by companies such as
Nedzink in Germany and Elzinc in
Spain, although its use in a conservation
context is likely to be limited.
||Corrugated iron roof and walls of a ‘tin tabernacle’: All Saints, Brokerswood, Wiltshire (1904)
Henry Palmer, an architect for the
London Dock Company, patented a
die-rolling process in 1829 to corrugate
metal sheeting, which gave it strength
to support itself over greater lengths,
thus reducing the cost of construction.
Originally Palmer used wrought iron,
but thinner steels were lighter and
more transportable. Palmer's patent ran out
in 1843 and the first pre-constructed
buildings using the material were
exhibited at the Great Exhibition in 1851.
The use of corrugated iron for cheap
buildings spread rapidly leading to
what William Morris called in an 1890
pamphlet: ‘a pestilence spreading over
the country’. Enterprising companies
marketed packaged buildings for
community uses, and the ‘tin tabernacle’
was born. Some early survivors have been
listed, but it is now largely restricted to
temporary structures and protection.
Long-term protection depends on
maintaining a good repainting regime.
Iron and steel
Iron roofing was an early substitute for
other materials due to its fire-proofing
qualities but surface corrosion was always
a problem. Cast iron was less prone to
corrosion than ordinary iron, but still
required protection by paint or anodic
protection. One of the most famous
buildings which retains a cast iron roof is
the Palace of Westminster.
Steel performed slightly better due
to the addition of carbon to the molten
iron but zinc coatings and lead paint were
much improved forms of protection,
although each came with a significant
health risk. A big leap forward came in
1913 with Harry Brearley’s discovery that
adding a small proportion of chromium to
steel during smelting greatly improved its
|Stainless steel being laid on the aisles of the parish church of Tisbury, Wiltshire: the high parapets made the lower roofs vulnerable to theft, but they also meant that a change in appearance would not be visible.
Stainless steel, or ‘corrosion-less
steel’ as it was first called, was originally
developed to prevent corrosion in gun
barrels by the addition of small amounts
of chromium and nickel. Its use for
roofing didn’t begin until the 1970s,
when the Less Steel Strip Company
promoted the material as a substitute for
lead roofing. It was relatively light, being
about half the thickness of copper, and
easy to apply, usually with a device which
produced a folded-over standing seam.
Since it does not oxidise or discolour,
stainless steel roofing remains bright,
which is a disadvantage on historic
buildings. It also has a relatively low
puncture-resistance and is difficult to
mend. Repairs on historic structures tend
to involve glue or mastic. As it ‘drums’
when rain falls on it, a deeper isolation
material must also be used than is
necessary for most other forms of roofing
to muffle the sound.
There is a wide range of specifications
of stainless steel on the market
varying according to the proportion
of other metals they contain, which
affect workability and durability. They
are divided into three types by their
crystalline structure: ‘austenitic’, ‘ferric’
and ‘martensitic’. Two basic types are
commonly specified for roofing. The most
commonly specified is Type 304, which
contains 18 per cent chromium and 8 per
cent nickel. Type 316, which is used in
harsher environments, also contains a
small proportion of molybdenum, added
to increase its resistance to corrosion.
Both are ‘austenitic’ types.
The earliest roofing felts were papers
covered in tar to make them waterproof
and were used to create temporary
waterproof structures. In the 1930s their
use developed and rapidly replaced
horsehair felts after health scares
about anthrax. Various base layers were
developed including asbestos fibre, sisal,
fibreglass and hemp, all coated in bitumen
derived from the petrochemical industry.
The great benefits of roofing felts are
that they can be applied to any roof no
matter the pitch, and can take a variety
of surface finishes. They can last up to 30 years depending on the quality of
the solar finish applied. However, they
need to be bedded with bitumen which
makes them both problematic to lay
over a historic building structure and
relatively unsustainable as a long term
solution. Their ‘cost in use’ over the life of
a building is relatively high compared to
metal roofing due to the short life-span
before the felt succumbs to embrittlement
due to sunlight (UV radiation).
Glass fibre was developed by the
American Corning company in the
1930s after a technician accidentally
blew compressed air through a vat
of molten glass. DuPont developed
the resins needed to strengthen the
material and further development in
the 1960s produced glass reinforced
plastic or GRP, which is now widely
used in a range of applications.
||The restored tower of St Augustine’s in the City of London, now part of St Paul’s Cathedral School: the steeple is a fibreglass replica of the Nicholas Hawksmoor original (Photo: Deror Avi)
Its great selling point is its versatility
and ability to mimic aesthetically any other
material including lead roofing, tiling
and even thatch, and it has often been
used to replicate difficult or inaccessible
roofing details. However, while the glass
content itself is probably indestructible,
the polymer matrix is vulnerable to
embrittlement, fading and scorching
by sunlight, affecting early examples.
Despite recent improvements, its long-term
weathering remains uncertain.
As a rigid, lightweight material,
GRP is also vulnerable to wind-lift, and
although the material is impervious to
moisture, its ability to prevent water
penetration is only as good as its fixings.
Being impervious, GRP roofs can also
suffer from mould growth underneath the
surface without adequate ventilation.
Single ply membranes
These come in five basic types: PVC (Poly
Vinyl Chloride), TPO (Thermoplastic
Polyolefin), TPE (Thermoplastic Polyolefin Elastomer), EPDM (Ethylene
Propylene Diene Monomer) and PIB
Of these, PIB and EPDM are forms
of synthetic rubber. PIB has been in
production the longest, is the more
versatile of the two, and is recyclable.
EPDM is a synthetic rubber used for pool
liners and although it is the cheapest
of the single ply membranes it was not
specifically designed for use in roofing.
TPO, TPE and PVC are hot-air
weldable, repairable, durable and
recyclable materials, but come in a
limited range of colours. Lead-grey
materials may be used with accessories
that mimic lead batten roll and standing
seam profiles (see illustration).
BBA-certificated life expectancy
ranges from 20 to 40 years depending on
the quality of the product, so cost in use
varies significantly. Sika Ltd has developed
a range of improved PVC products with
the refurbishment market in mind under
their ‘Sarnafil’ trade mark which claims 50
None of the single ply membranes
are breathable to any great extent so
carefully detailed ventilation needs to be
Less common as a roofing material,
aluminium can now be enhanced with
the addition of a zinc layer. ‘FalZinc’,
‘Titansilver’ and ‘KalZinc’ are typical
trademarks. The benefits are lightness
and workability. The fixings are usually
standing seams or batten rolls which makes
lead replacement finishes problematic
from an aesthetic point of view. The
material’s 95 per cent recyclability is
stressed in product literature but its green
credentials are somewhat undermined
by the high energy consumption of the
smelting process. Life expectancy is given
as 40 years by most manufacturers.
Terne-coated stainless steel
|Above left: Sarnafil, a polymeric sheet material, in this case detailed in imitation of lead at St Joseph’s Church, Wool, Dorset (Photo: Owlsworth Roofing). Above right: Terned steel on the roof of a new extension to Wymondham Abbey, Norfolk, by Freeland Rees Roberts: bright silver when first laid, the metal will continue to dull as it ages.
Terne coating has been on the market
since 1968, ‘terne’ referring to the
application of an alloy containing lead
and tin which gives steel the appearance
of lead without the weight, being about
equivalent in weight to Code 1 lead sheet.
In the 1990s the Follansbee steel
company developed terne coatings of
zinc and tin with a superior weathering
to lead (TCS/ZT) and, in recent years,
with a thicker and more durable coating
‘TCS’, a registered trade mark, is
also available with an aluminium and zinc
coating (TCS/AZ) which performs better
in marine environments.
A limited number of other
producers market terne-coated stainless
steels in a wide range of colours
and textures. Uginox is probably
the European market leader and its
‘Patina’ range of ferric stainless steels
and ‘Top’ range of austenitic terne-coated
steels have been specially
formulated for roofing applications.
Careful selection is essential to
achieve the best stainless steel for the
appropriate environments at the best cost
in any given application.
British Stainless Steel Association
Copper Development Association
Federation of Traditional Metal Roofing
Lead Contractors Association
Lead Sheet Association
National Heritage Roofing Contractors