C Stevens Roofing

Excellence in Roofing

Lead Work...

Lead Bossing

Bossing, Welding, Jointing, Fixing, Corrosion Bossing and Welding Methods

Lead sheet should be carefully worked or welded so that the lead is not thinned by more than 25% or weakened by creasing or stretching. Bossing and welding lead should be carried out by experienced (preferably registered) operatives using the proper tools and equipment.

It is important to check for compatibility when specifying or using certain types of the following materials:-

Jointing Methods

Joints in leadwork are weathertight but not watertight. Joints in leadwork are designed to keep water out whilst allowing for thermal movement. It is therefore important that the type of joint used is appropriate for the type of application or situation.

Support and Fixing Methods

Lead sheet must be adequately supported on a smooth base sufficient to take the weight and to allow for thermal movement to take place. It may be laid on timber, plywood, concrete or masonry together with a suitable underlay. The lead sheet should be held in position with fixings of a type, strength and position which:
  • Support the weight of the lead sheet
  • Are resistant to corrosion
  • Allow for thermal movement to take place
  • Resist wind uplift

Preventing Corrosion

Lead sheet is resistant to most forms of corrosion likely to be encountered in a roofing situation, but certain precautions need to be taken against the following:

Mortar: Unprotected lead damp proof courses and cavity trays may corrode in the presence of moisture

Lichen Growth: The acid run-off from lichen or moss on a roof may cause small holes to appear in the lead sheet under the drip-off point from tiles or slates. A sacrificial flashing may be fitted to the lead gutter or roof, or the growth may be treated with a chemical fungicide.

Condensation: In certain conditions the underside of lead sheet may corrode and it is important to design the roof to reduce the risk of condensation and provide adequate ventilation. 

Properties of Lead Sheet Physical and Mechanical Properties of Lead Sheet

Malleability - Lead is soft, malleable and capable of being manipulated with hand tools and, by the technique of bossing, can be worked into the most complicated of shapes or dressed to fit the many types of profiled roofing tiles.

Fatigue and Creep Resistance - Attention to correct sizing and fixing of individual pieces minimizes any risk of fatigue cracking or creep. The chemical composition of rolled lead sheet is strictly governed by the provisions of BSEN 12588:1999, which effectively control the grain structure to make the lead sheet more resistant to thermal fatigue without affecting malleability.

Durability and Thickness - When specified and fitted correctly, rolled lead sheet can continue to provide a maintenance free weather shield for 100 years. Rolled lead sheet is therefore extremely resistant to atmospheric corrosion. The ideal thickness and size of each piece is determined by the application.

Thicker sheets may be required:

  • when dressing lead into deeply profiled tiles in areas of frequent foot traffic
  • on flat roofs and gutters where extra rigidity is required to resist wind uplift
  • Thermal Movement; Lead sheet has a high coefficient of linear expansion and therefore due allowance for thermal movement must be made in the design, layout, sizing of panels and fixing details to maximize performance.

Fire Resistance - Lead is incombustible but melts at 327.4°C.

Recyclability - Rolled lead sheet is totally recyclable. Scrap material is recovered from end of life buildings in the UK by a national network of reclamation merchants and channeled back to the manufacturers where it is carefully refined back to new feedstock for the rolling mills. The established recovery network, constant recycling of old material and low melting point with resultant energy efficiencies in manufacturing means the lead sheet industry has a solid reputation for environmental awareness and as a conserver of natural and energy resources.

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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 Flashings

The characteristic behaviour of lead sheet needs to be taken into account when designing or renewing details.

Thermal Movement

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).

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 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.

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.

Flashings 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).

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 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 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 leadworkers 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, further details can be found on page 131.

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