Can your chosen Lead Flashing contractor install the flashing correctly? In many cases this will be your chosen Roofing contractor whether they are installing slate , concrete tiles or clay tiles and unfortunately many are not capable of this and rely on mastic and the roof underlay to keep the roof weathertight and not their skill at installation. This area is often overlooked but it is a vitally important part of the roof and needs to be done correctly or can cause major problems. Flashings are often where two roof faces meet and this is where water is getting channelled so it need s to be correct and not reliant on mastic. Choose your tradesman carefully. Please view my Gallery for a number of lead welded flashings that will be watertight and are not reliant on any mastic.
Lead sheet should be carefully worked or lead welded/burned so that the lead is not thinned by more than 25% or weakened by creasing or stretching. Bossing and welding/burning lead should be carried out by experienced (preferably registered) lead operatives using the proper tools and equipment.
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.
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.
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 behaviour.
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.
Your 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.
As 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 characteristic behaviour of lead sheet needs to be taken into account when designing or renewing details.
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 available on request.
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.
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.
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.
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..
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.
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.