The short answer
Slate hooks are an accepted fixing in modern standards, but they are not the traditional way of holding slate and they change how a period roof is built. They are not used below 25 degrees of pitch, and a listed house, or any roof that sets the tone of a conservation area street, may need the council's agreement first. Whether hooks are acceptable depends on the building, the pitch and the officer, so ask before ordering.
What a slate hook is
A conventional slate is nailed through two holes near its head to a batten. A hook-fixed slate is held differently. A metal hook is nailed to the batten, and the bottom edge of the slate sits in it. Hook fixing is also called tail fixing or bottom fixing, because the slate is held at its lower edge rather than near the top.
The idea is to hold the slate without piercing it, so the slate is not weakened by a nail hole and cannot split from one. It also means a slate can be lifted out of the hook and slid back without the nail work a normal repair needs.
Hooks are typically stainless steel or copper. They are used with slates cut with a notch or with plain, unholed slates, and the roof needs the right gauge and headlap for the slate size. Hooks are often found on roofs where the slate is thin and fragile, or where wind uplift is a concern. Roofers who favour them say the unpierced slate is stronger, while critics note that a hook leaves the slate loose at the head if it is ever knocked sideways.
Where hooks are used, and where they are not
Hooks suit some roofs well and others badly. Cupa Pizarras advises against hook fixings below 25 degrees of pitch, and that limit matters on shallow roofs, where wind-driven rain can get past a slate held only at the foot.
On a modern steep roof, or on a re-roof where the original slates came loose because their holes had broken out, a hook can be a reasonable repair. It is also used by some roofers on mansard slopes, where the pitch is steep and the wind exposure high.
It is less often the answer on a period roof that was laid with nails from the start. A slate designed to be nailed behaves differently from one designed to be hooked, and mixing the two can leave a roof that looks right from the street and performs wrongly in a storm. The right choice is the one that suits the slate you have, not the one that is quickest to fit.
Listed buildings and conservation areas
Appearance and construction both matter to a conservation officer. A hook is visible at the bottom edge of a slate if it is not set correctly, and a hooked roof can read differently in raking light than a nailed one. Where the building is listed, the method of fixing may form part of what the consent covers.
B&NES is blunt that unauthorised work on a listed house is an offence, and that the two permissions, listed building consent and planning, are separate. Its advice is to talk to the council before anything starts. The Bath Preservation Trust reads roof coverings closely in its casework, and has supported a change away from imitation slate on one Bath terrace in favour of natural Welsh, so expect both the covering and its fixing to be examined.
We have found no published B&NES guidance that deals specifically with hook fixing, so we cannot tell you it is always allowed or always refused. The sensible course is to ask the conservation team, with a sample detail, before you agree a method with any roofer. Outside Bath, ask the planning department of your own council.
Weight, structure and the sums that follow
Hook fixing does not make a roof heavier in itself. The weight of the roof depends on the slate and how it is laid, and our answer on how heavy slate is sets out why that matters. A thicker slate, a larger one or a double layer changes the load on the rafters.
Should the covering you choose be 15 per cent heavier or more than what it replaces, Planning Portal says Building Regulations approval follows. A re-roof on a bigger scale can also bring in the thermal upgrade rules. These are separate from the question of nails or hooks, but they often turn up in the same conversation, and it is easier to settle them together at the start.
Fixing method also affects the price. Hooks are an extra component for every slate, and the batten set-out has to be exact. Treat any quote that does not state the fixing as incomplete. Look for the hook type, its metal and the batten gauge in writing, because a job that names them has usually been thought through.
Our approach on period slate roofs
On a period roof we start from what was built. Natural slate stays natural slate, and in most cases a nailed fixing in copper or stainless steel, as covered on our page about slate nails, is the closest to the original. Hooks come into the discussion where a particular slate or exposure calls for them, and we explain why.
If you are weighing Welsh against Spanish slate for the same roof, our note on Welsh against Spanish slate covers how the stone and the fixing interact.
We start with a free survey, take photographs of the slopes and send a written, fixed price. Ask which fixing the quote allows for, and in what metal, since the choice of hook or nail decides how the finished slopes look from the street. If consent is needed we explain what to request and from whom, and for notifiable work the Building Regulations application is part of our job. Details of the wider service sit on our slate roofing page.
