Introduction
If
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 behavior.
Lead 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.
Originally 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 1178.
In more recent years,
lead sheet - manufactured by the continuously cast method - has been
introduced for roofing purposes in the UK
Repair Work...
Small
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.
Precautions
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 Flashing's
The
characteristic behavior of lead sheet needs to be taken into account
when designing or renewing details.
Thermal Movement
The main cause of failure is due to over sizing often coupled with over
fixing. 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).
It
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 under cloak 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.
Wind 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.
Flashing's and weathering's
are often insecurely fixed and during recent years, high winds have
shown up many weaknesses in fixing details. Cover flashing's should be
wedged into brick or stone walls with lead wedges at a maximum distance
of 500 mm apart. Step flashing's 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).
Lead
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 over fixed. 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
publication (3).
Moisture Corrosion
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
underlay.
Particular 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.
Conclusion
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 lead workers available who are
members of the Lead Contractors Association.
The
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.
