Designing a slate roof is an exacting process where many factors need to be taken into account. These include the roof pitch, its shape and other architectural features.
The type and size of slate being used and the site’s exposure to wind and rain will also need to be considered when determining the best method of fixing. Designing a slate roof is an exacting process where many factors need to be taken into account. These include the roof pitch, its shape and other architectural features. The type and size of slate being used and the site’s exposure to wind and rain will also need to be considered when determining the best method of fixing.
Throughout Mainland Europe, North America, Canada and elsewhere around the world, hook fixing is widely used. The technique first started to appear in the mid 19th century and spread across most of continental Europe by the 1900’s. Since the 1950’s, with the growth in worldwide trade, a greater variety of slate types and sizes have become available to the UK market, and the hook fixing method is now beginning to enjoy a growth in popularity here.
Hook fixing is a system of holding double-lap slating to battens, without the need for nailing through perforations in the slates. Instead, slates are secured by attaching a hook to the batten, which extends below the end-lap to secure the slate by hooking the bottom edge at the position of the vertical joint of the lower course of slates. This method also creates a constant 2-3mm vertical gap between the slates. The technique reduces capillary action and helps to limit water damage and premature ageing of the slates.
There are two types of hook that can be used, one with a straight shaft and the other slightly crimped to further reduce capillary action. The NFRC recommends that, where hooks are used, they be manufactured from stainless steel to BS1449 Part 4 type 316.
The hook fixing system is simple and fast to install with only one hook to attach per slate, compared to hammering two nails for each slate with traditional nail fixing. Since the slates do not have to be holed, the risk of breakage or accidental damage is reduced, resulting in less waste material. Maintenance and repair are easy too, as the hook can simply be twisted to one side, the damaged slate replaced and the hook returned to position.
These benefits combine to offer significant time and cost savings for contractor and end customer alike. Anecdotal evidence places these savings at between 15 and 25% on installation time.
The use of hook fixings allows for greater flexibility in roof design, particularly when creating features such as valleys, cones and domes, which rely on being able to use narrower slates. It is generally accepted that the minimum slate width for nail fixing is 150mm in order to give a 25mm margin between the hole and the longitudinal edge, whereas those fixed by hook can be as narrow as 100mm. The system can be used on any roof with a pitch between 25° and 75°.
Interestingly, the hook fixing method is most popular within the UK in exposed coastal sites and areas such as Scotland, Cumbria, Cornwall and parts of Wales where the weather conditions can be the harshest. Hook fixing offers superior resistance to wind uplift and rattle as the hook secures the lower edge of the slate.
Visit www.spanishslateuk.com for further information and to view results of a filmed comparison of traditional and hook fixing methods, created with the help of roofing experts at a national construction college.