The short answer
Either can be right. Copper and stainless steel nails both resist the corrosion that destroys plain steel nails, and the fixings decide whether a slate roof lasts. Slates endure so long that the nail is usually what fails first. Choose the metal and length that the slate supplier's data sheet and the roof's exposure call for.
A slate can outlive the house, but only if its nail does too. Nail choice is a small cost at the start and a large one if it goes wrong.
Why the nail matters more than the slate
Natural slate is one of the longest-lasting roof coverings. CORC, the roofing contractors' confederation, puts a well-kept slate roof at 100+ years. The slate itself rarely gives out. What holds it on is a pair of nails driven through the top edge into a batten, the timber lath each course hangs from, and those nails are under attack from the first wet day.
What goes wrong with a poor nail?
Plain steel and many coated steel nails rust. As the metal corrodes it swells, weakens and finally lets go. The slate then slips because nothing is holding it, even though the slate is sound. Roofers call this nail sickness, and it leaves a roof of good slates that keep sliding out of place one after another.
By the time slipped slates show, many nails on the same slope are likely to be in the same state, since they share an age and a weather history. A lone failure can be a one-off, but a second and third on the same slope point to the fixings. Our answer on nail sickness in slate roofs lists what you can see from the garden.
Copper nails: the traditional answer
Copper has been used for slate fixings for a long time, and it earns its place. It does not rust, so it does not swell and split the slate or snap through the head. It is soft enough to drive easily and suits the thin edge of a slate. It also sits well beside lead and copper flashings and the little lead or copper tabs that hold a replacement slate at its tail.
Are copper nails strong enough?
For the job they do, yes. A slate hangs from the nail and is held in place by the course above, so the load is modest. Copper's softness means it can bend if the batten is hard or the nail is driven carelessly, so the roofer's skill matters as much as the metal. Copper also costs more than steel, which is why a corner-cutting job may use something else.
That choice shows up years later. Where copper and stainless are both on offer, the extra cost on a whole roof is small beside the scaffold and labour that would be needed to strip and re-slate it. On a repair the same thinking applies to every metal part of the slope, from nail to tingle to flashing.
Stainless steel nails: the other good choice
Stainless steel also resists corrosion and is stiffer than copper. It holds its shape when driven into a firm batten and keeps its grip. On exposed sites, such as the Quantock and Blackdown Hills, where wind and driving rain test every fixing, that stiffness is useful. The metal grade matters, so the supplier should state it on the product data.
Which should I choose for my roof?
Follow the maker's data sheet and the fixing code for the covering. The right length depends on the thickness of the slate, the headlap, meaning the lap of one course over the second row below, and the batten. Pitch counts too: BS 5534 indicates a minimum pitch of 20 degrees for natural slate, and Cupa sets its limit for hook fixings at 25 degrees. Our answer on slate hooks versus nails covers the choice between the two.
Either metal, correctly sized and driven, will see a slate roof through a lifetime. What fails is a nail made of the wrong material or put in badly. Mixed batches on one roof are best avoided, and we ask what is already there before choosing.
What this means for a repair or a re-slate
Before any slate work, we look at the nails. If a slate has slipped, we lift the course above and look at the old fixing. Rust, a thinned shank or a nail pulled out of a soft batten tells us whether the problem is one slate or the whole slope.
Is it worth re-nailing an old roof?
Sometimes the slates are good and the nails are not. A roof of sound slates on failing fixings can be stripped, the slates sorted and re-fixed with new copper or stainless nails on sound battens. That keeps the original slate and the look of the roof, which matters on older and listed homes. If the battens are also tired, they are renewed in the same visit. Our page on re-slating a roof explains the process.
We survey, send you photos of the nails and battens we find, and write one total price for the work. If you are comparing quotes, ask each roofer what nail they will use and why. The answer tells you a lot about the job. See also our main slate roofing page.
More on fixing slates
Can I mix old steel nails with new copper ones?
It is better not to. The old steel fixings remain the weak point on the slope, and a repair mixes metals of different ages. If most of the slope is on rusting steel, replacing the nails across the course gives a longer-lasting result than patching one slate at a time.
Do I need to worry about nails on a new slate roof?
Ask what nail is specified and check it against the slate supplier's data sheet. A new roof should use a corrosion-resistant nail, and the written specification should say which. It costs nothing to ask before work starts, and it is hard to change once the slates are on.
The full picture: Slate roofing in Taunton ยท More short answers