C Stevens Roofing

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Lead Work...

Lead Work

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...

Lead Welding

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, further details can be found on page 131.