In the latest instalment of his excellent consultant case study series of articles, Technical Roofing Consultant John Mercer outlines the key points to consider when securing lead flashings, and why fixing clips should never be overlooked…
I was recently consulted about a roof where the local building inspector had found that the lead flashings had not been mechanically fixed. Fixing clips are often overlooked when installing lead flashings and whilst lead is a heavy material, when used as cover flashings it is still vulnerable to wind uplift, particularly at the exposed edges.
Top abutment flashings can be particularly vulnerable as they usually cover the top course of tiles by at least 150mm, 200mm for low pitched roofs, and turn up the wall 75mm, with a 25mm turn into the masonry. If the lead is not clipped, this means that most of the lead – a 225mm width – is only held in place by its own weight and a 25mm turn into the wall.
The metal matters
The very properties that make lead ideal for forming roof flashings, i.e being soft and malleable, make it totally unsuitable for most fixing clips. This is particularly true for top abutment flashings, where the clip fixing point will be relatively high and the clip may need to have quite a long reach. Bearing this in mind, clips to secure lead must be produced from other metals, metals that will not react with the lead where exposure to the atmosphere could otherwise cause ‘bi-metal corrosion’.
In normal building usage, clips formed from tinned copper or stainless-steel can be used without any adverse reactions in the long term. Similarly, nails used to secure the clips should be large headed copper or stainless steel and be at least 3.35mm in diameter. Screws should be brass or stainless steel, with a minimum 3.35mm diameter.
For general usage, clips used to secure the free edges of lead flashings can be produced from 0.38mm thick stainless-steel or 0.6mm thick copper strip. For exposed situations, the thickness can be increased to 0.46mm stainless steel or 0.7mm copper. Whichever material is used, the clips should be at least 50mm wide and be long enough to extend sufficiently to turn over the lead flashing by at least 25mm.
From the top – installation
For top abutment flashings, each lead flashing piece must be no longer than 1.5 metres and laps should be at least 100mm, or 150mm in exposed locations. Clips should be installed at maximum 500mm centres and should also coincide with the flashing laps. For exposed locations, the spacing of the clips should be reduced to no greater than 300mm centres. The clips can normally be secured using nails or screws through the tile nail holes which are the same length as the standard tile nails.
Once the lead flashing is installed, the ends of the clips can be turned over the free edge of the flashing apron by at least 25mm to secure it.
Wedges should be used to secure the lead flashing turn-in into the masonry joint. These can be formed from strips of lead, 20mm wide, rolled several times to form a tight fit when driven into the masonry joint. Alternatively, proprietary stainless steel wedges can be used. For top abutment flashings, the wedges should be set at maximum 450mm centres.
The wedges can be driven into the masonry joint using a hammer and plugging chisel and should be driven in far enough to be completely covered once the joint is filled and pointed. Once the flashing and wedges are secured in place, the joint can be sealed using mortar or a proprietary sealant.
To secure the lead cover flashings at side abutments, a clip should be installed every third tile course for single lap large format tiles, increasing to every tile course in exposed locations.
The clips can be inserted between the tile laps and extend down each tile far enough to allow the lead to be dressed over the tail of the tile above. Depending upon the width of the lead cover flashing, it may be necessary to secure the clips by turning each one over the head of a tile if it is not possible to use the tile nail holes.
Lead or stainless steel wedges should be used to secure the lead into the masonry joint at each step.
- Always securely clip the free edges, or aprons, of cover flashings to prevent wind uplift.
- To avoid the risk of bi-metal corrosion – use tinned copper or stainless steel for the clips and brass, copper or stainless steel for the fixing nails and screws.
- For general use, set the clips at maximum 500mm centres and coincide clips with the lead laps. Reduce the clip spacings to 300mm for exposed locations. At side abutments, set the clips at every third tile course, or every tile course in exposed locations.
- Use lead or stainless steel wedges to secure the lead flashing into the masonry joints. Set the wedges at maximum 450mm centres for continuous joints and at each step for stepped flashings at side abutments.