The Lead Sheet Association has been providing technical advice for decades across the construction industry.
Using that experience, its Technical Team have put together the top ten commonly asked questions about the use of Rolled Lead Sheet manufactured to BS EN 12588 to help you get the most out of this fantastic material.
How to ventilate the area where lead sheet is used is probably the most common question we deal with. All lead sheets should be laid onto a through ventilated substrate. That means the decking which supports the lead sheet should have a minimum of a 50mm through ventilated void underneath the decking and the ventilated pathway should have no stagnant air pockets. Even vertical applications should be ventilated although here the ventilated path can be reduced to 25mm.
2. Neoprene Expansion Joints
Do we agree with the use of these types of compensators in lead sheet gutters? Traditionally gutters of this type incorporate steps or ‘drips’ in the design. These are intended to break up the lengths of lead sheet used to ensure that thermal expansion is within guidelines for individual pieces of lead. In many instances, this works wonderfully well but in others there simply isn’t the height to allow for them. Our advice is simple really – wherever possible stick to traditional tried and tested methods and use neoprene expansion joints as a last resort. New build design should allow for drips only.
3. Lead Sizing
Getting the right lead specification for the job is essential yet so many times we see the wrong codes of lead used in particular situations or the individual pieces of lead sheet installed in too long a piece. Lead sheet is a soft and malleable metal which means it’s an excellent roofing material because it can be formed and shaped to suit any detail. However, it is important to follow the basic principles of sizing and fixings as indicated in the Rolled Lead Sheet Manual to reduce the effects of fatigue cracking or creep.
When asked what underlay if any is required the answer is quite simple. If exterior grade plywood or blockboard is used a building paper to BS1521 Class A should be used. However, if the substrate consists of traditional boarding with penny air gaps you can use a geotextile fleece as an alternative.
5. Flashing Lengths
Lead flashings should always be installed in 1.5m lengths, however there are exceptions. Lead flashings to asphalt, bituminous felt, or single ply roof coverings are continuously bonded along one side therefore it is essential to limit the lengths to 1m so that the thermal movement of the lead sheet is not affected when exposed to long periods of summer sun.
All lead sheet must be laid on a minimum fall of 1:80 or just over 1 degree. Failure to do this can result in ponding water which may find its way into the fabric of a building.
There are so many instances where we see lead flashings or other edges of roofing, cladding and weathering’s missing any form of restraint to their free edges. All such edges must be adequately clipped to prevent lifting and distortion in high wind conditions and it is amazing to see how often these are missed out or not considered at all. Clipping of lead sheet varies considerably in both material used and the distances that they are set apart – this is covered in the LSA Manual.
A lot of enquiries assume that a flashing should lap over a pitched roof covering by 150mm. This is true for a 30 degree slope but what a lot of people don’t realise is that the lap length is wholly dependent on pitch and can vary considerably. We usually end up referring clients to the 75mm lap diagram that we show in the manual. Laps can vary between 75mm for vertical cladding to 395mm on pitched roofs or even more if the exposure of a particular building is considered severe. It is also worth noting that the lap should always be taken from the lowest row of fixings in whatever it is covering.
The fixing of lead sheet is carried out in many different ways, all of which are dependent on the particular application. For instance, a lead bay on a roof with a pitch of up to 3 degrees differs in the arrangement and position of its fixings when compared to the same bay on a higher pitch. These differences are very important to ensure correct installation and future longevity.
10. Damp Proof Courses
Lead DPC’s and trays are designed to prevent moisture that penetrates brick or stonework finding its way into the building. Many of the enquiries we receive relate to mysterious staining appearing. In nearly all cases, this is caused by failure to treat the lead that is built into the walls with bituminous black paint on both sides prior to fitting. This coating prevents free alkali in fresh Portland cement mortar and sulphuric acid vapours that are contained in the moisture within walls and chimneys from corroding the lead sheet.
All the details covered here can be found in the LSA’s newly updated Manual. The LSA's Technical Team help thousands of people each year through our enquiry service so if you need any support please do contact us.