I have not utilised hook fixings for slates during my time as a roofer but I have seen them being used on a number of projects. I worked with mostly double nailed Spanish slates or the likes of single nailed scotch slate. I am going to discuss the use of hook fixings, the relevant standards and the methods of repairing slated roofs that utilise hook fixings.
I am not overly keen on hook fixings mainly from an aesthetic stand point. The end of the hooks are visible and if the hooks are crimped, the gaps between slates could be up to 5mm which to me is quite unsightly. Maintenance could be an issue as well as a slate that is broken would be hard to detect as the hook could hold it in position.
Slate hooks can also potentially damage slates in three distinct ways; if they are overdriven they can cause a crack at the head of the slate, the hook could be too tight and snap the slates tail and if the hook is thicker than the slate it would cause the slate to snap in half. To combat these issues BS 5534 outlines guidance on the dimensions of slate hooks for example they should not be more than 2.7mm thick and should be less than the slate thickness.
According to the NFRC technical bulletin 03, crimped hooks should not be used below 25° and straight hooks should not be used below 30°. Slate hooks can however be used on a lower pitch, even down to 15° for the Marley Eternit Birkdale fibre cement roof slate, but this is achieved with a combination of nails, tail rivets and slate hooks.
Where hooks are used to secure slates, evidence should be obtained from the manufacturer of their suitability in terms of durability and strength for the purpose for which they are to be used. Amendment 2 of BS 5534 stated that drive-in slate hooks should be used as appose to wrap hooks which is following the guidance set out in the above NFRC bulletin.
In my previous blog post I discussed specifying adequate repairs for slate roof and the principals would be similar here too. Repairing a slate which is fixed with a hook system might in fact be easier than a double nailed alternative and a slate could be replaced using a slate hook or Hallhook slate fixing.
For those that have not read my previous blog, the Hallhook is a permanent, secret and fail safe device that can be used when replacing a broken slate. The Product was invented by Jimmy Hall who was a slater and tiler for the past 40 years. He found a better solution to using tingles or silicone to repair double nailed slates which had broken.
There are two main grades of steel used for slate hooks which are 304 and 316. Without getting too technical they have almost the same physical and mechanical properties but 316 stainless steel has a higher corrosion resistance so they should be used for industrial areas or severe coastal exposure. BS 5534 only allows for grade 316 stainless steel conforming to BS EN 10088‑3:2005, so it’s a worthwhile distinction if specifying.
There is certainly less wastage as the slates do not require to be pre-holed or holed on site and some say using slate hooks is a quicker method than nailing.
Hopefully through reading this blog post you now know what slate hooks are, how they are used, the standards in relation to them and their potential downfalls. for more on roofing and roofing defects check out some of my earlier blog posts where I delve into a variety of roofing topics.