to Lead Sheet Roofing and Flashings
C Coote, Lead Sheet Association
lead work is to be properly detailed and fitted it is essential to
have a good understanding of the nature and uses of the material,
and to follow certain rules of good practice which have been developed
from long, and sometimes costly experience. Some of the common causes
of failure, fault recognition and appropriate remedial action are
explained together with an overview of the material's characteristic
sheet, one of the oldest and most durable roofing materials, has been
known to last for over two hundred years. Some lead sheet is still
made by the original method of casting molten lead on a bed of sand.
This cast lead sheet is produced by specialist firms and is largely
used for replacing old lead sheet on cathedrals and churches where
authenticity is important.
manufactured on rolling mills and known as milled lead sheet, it began
to replace cast lead sheet at the beginning of the 19th century, and today,
nearly all the lead sheet used in building is in this form. Modern milled
lead sheet is made to the specification laid down in British Standard
more recent years, lead sheet - manufactured by the continuously cast
method - has been introduced for roofing purposes in the UK
SURVEYS AND REPORTS
contractor must have good understanding of the most likely faults
and how to identify and diagnose the causes of defects before making
a recommendation on appropriate repairs or renewals. If the inherent
design faults go unnoticed or ignored, the problems will inevitably
recur within a short space of time.
a general rule, if lead roofing, cladding or flashings are mostly
in good condition with just a few minor splits, then it is advisable
to carry out appropriate repairs. However, more serious and extensive
failures will require careful consideration to the renewal of the
leadwork. Before beginning any repair or renewal work, various elements
should be assessed such as: the condition of the lead and how long
it is likely to last; work required to adjacent materials; and inherent
design faults. Other additional factors should also be considered
before deciding upon the right course of action:
the existence of serious ripples or splits
the use of oversized pieces
the poor location of fixings restricting the normal thermal movement
of the lead
inadequate or worn out fixings
the use of unsuitable underlays
conditions such as heating, insulation, etc, which could affect
the roof decking
signs of corrosion either on top or on the underside of the lead
decision on whether to repair or renew could also be influenced by
other factors. A parapet or central valley gutter lining may be in
a fair condition and on its own, worth repairing to extend life by
another ten to fifteen years. But if the adjacent slated or tiled
areas are to be renewed, the relatively low cost of re-laying the
lead gutters would be well worth accepting.
patches of lead sheet may be carefully welded over any splits to form
permanent repairs. The lap joints should be used to strengthen and
to prevent penetration of the flame through the lead. Roll ends are
a common point of failure and the whole section may be cut out so
that a new pre-fabricated roll end can be welded into position.
should be taken against the risk of fire when making repairs using
a blow-torch. Where a hot working ban is enforced, repairs can only
be made by either taking the defective panels out and welding patches
off site, or sealing the cracks with a patent sealing tape. We do
not normally recommend the latter because repair tapes do not usually
last very long.
THE CHARACTERISTICS OF LEAD SHEET ROOFING AND FLASHINGS
characteristic behaviour of lead sheet needs to be taken into account
when designing or renewing details.
The main cause of failure is due to oversizing often coupled with
overfixing. Lead sheet on buildings is usually fixed externally and
is thus subjected to conditions of changing temperature. Lead has
a high coefficient of linear expansion and when the difference between
the winter and summer temperatures are taken into account the result
of a simple calculation will show an increase in the size of the sheet.
If thermal expansion and contraction cannot take place freely there
will be a risk of distortion and stress which in time will cause the
lead to buckle and crack. It is of first importance with lead sheet
fixed externally, as with all sheet metals, to limit the size of each
piece so that the relatively small amount of thermal movement is accommodated
within the jointing and fixing details. Recommendations on the maximum
sizes of pieces of lead sheet are shown in tables published by the
Lead Sheet Association (LSA) and in the British Standard 6915 (2,
5 and 6).
is also important that fixings should not restrict thermal movement
but must be adequate to support the lead and, depending upon the degree
of exposure, retain it in position. Bays on flat roofs should only
be fixed at the top third of the roll undercloak only and on pitched
roofs and cladding across the head under the lap joints. Copper retaining
clips fixed within the joints should allow for thermal movement to
take place and fixings along the free edges should hold the lead freely
against wind lift.
Lift and Weight
Inadequate head fixings allow lead sheet to slip and fall out of position
- sometimes wrongly referred to as 'creep'. This type of failure is
caused by using fixing methods and materials without consideration
to the weight of the lead or the degree of exposure to wind lift.
The weight of lead will cause the sheet to tear away from any fixings
which are positioned too close to the top of the sheet. The correct
method of fixing to a timber substrate (at the head of panels of lead
sheet on roofing and cladding, over a three degree pitch) is with
a double row of copper clout nails staggered at 75 mm apart, with
the top row a minimum of 25 mm from the top edge. All head fixings
should be covered by a lap joint appropriate for the degree of pitch.
In general, fixings should be included in jointing details and the
panel sizes should be reduced so that intermediate fixings are unnecessary
and weatherings are often insecurely fixed and during recent years,
high winds have shown up many weaknesses in fixing details. Cover
flashings should be wedged into brick or stone walls with lead wedges
at a maximum distance of 500 mm apart. Step flashings should be fixed
with a wedge to each step. Fixing clips should be detailed for all
free edges of lead sheet. These should be detailed to suit the degree
of exposure of the lead flashing to wind lift. All clips should be
fixed with sufficient tolerance for thermal movement. Further details
on joints and fixings are contained in LSA publications (1, 2).
sheet should have a continuous support of a smooth decking material.
This should have a suitable underlay between the lead and substrate.
An unsuitable underlay will cause the lead to buckle and split - sometimes
even where the panels are not oversized or overfixed. Roofing felts
with a bituminous surface or organic fibres with a bonding agent can
become sticky in hot weather and cause the lead to be bonded firmly
to the substrate. Further information is contained in a recent LSA
In well heated buildings, it is possible for warm moist air to filter
through to the roof structure and, unless prevented, condense on the
inner face of the sheet lead. If there is insufficient air circulation
to form a stable patina, corrosion of the lead sheet is probable.
The usual signs of corrosion from condensation are white streaks running
out from under lap joints (not to be confused with run-off stains),
and a white powder forming under the lead. The corrosion process is
sometimes advanced by the presence of oak timber or an organic fibre
attention should be paid to the conditions inside the building and
also within the roof structure itself. Moisture will migrate from
one place to another beneath a roof decking. Regardless of a vapour
barrier and dry site conditions during construction, condensation
may still form on the underside of the lead sheet. This can never
be accurately predicted and it is therefore recommended that a ventilated
air space be detailed below the decking material.
Lead sheet is a reliable material. In the hands of a person trained
and experienced in lead working skills it will not only enhance the
aesthetic appeal of a building, but will keep the building dry for
many years. There is a list of specialist leadworkers available who
are members of the Lead Contractors Association.
technical officers in the Building Section of the LSA provide technical
information and advice by telephone or by post and will comment on
any drawings or specifications for lead sheet projects. When in doubt
about any lead work details it is always best to obtain the latest
information and advice. The Lead Sheet Association offers technical
advice on all aspects of lead sheet use, further details can be found
on page 131.
by the Lead Sheet Association, Lead Sheet Manuals: Volume 1, Lead
Sheet Flashings; Volume 2, Lead Sheet Roofing and Cladding;
Volume 3 Weathering
by the Lead Contractors Association: Directory of Specialist Leadworking
Contractors, available free Tel 01342 317888
Standard Code of Practice 6915