Copper Nails vs Stainless Steel Nails for Slate Roofing: Which One Lasts Longer?
Somewhere in Wales there is a roof built in 1921 that I walked on last year. The slates had been replaced twice in places, but the copper nails that originally fixed them were still doing their job — green, crusty, and absolutely sound. Meanwhile, a 2009 house I inspected had used cheap galvanised nails: eleven months after the first winter, one nail had rusted through and let a slate slip, and a thin red line ran down the white render like a confession.
That contrast is the whole argument for this article. On a slate roof the slates are usually the honest part — a good slate will sit there for 75, 100, 150 years without asking for anything. The nails are the part where people quietly save money and regret it for decades. Copper and stainless steel are the only two materials that come close to matching a slate's life. This page compares them properly: corrosion mechanism, cost, coastal performance, and the order details that actually matter. By the end you should be able to write the nail line of your specification without hesitating.
First, Match the Fixing to the Life of the Slate
Before we argue copper against steel, we need the rule that frames everything: the fixing must be expected to last at least as long as the slate it holds. A good natural slate, graded under EN 12326 or ASTM C406, is not a 30-year product. It is a 75–150 year product if the roof system around it does its job.
Look at the arithmetic instead of the marketing. A galvanised nail carries a zinc coating that, in a clean inland roof, may survive 10–20 years; in coastal or industrial air, 5–10 years. Once the zinc is gone, the steel core rusts fast. A slate roof is a 75+ year asset, so the galvanised nail is not a cheap option — it is a premature-maintenance plan with a guarantee date before the first re-roof. This is the single most common fixing mistake I see on older roofs, and it is almost always the nails, not the slate, that fails first.
That is why, of the 1,800 or so nails in an average two-storey slate roof, you want every single one to be copper or stainless. The 2–3% difference in total material cost between good nails and galvanised nails is nothing compared with the labour of re-nailing a whole roof in year 40.
Copper Nails: the Slow Death That Isn't
Copper nails have fixed slate for about 400 years of continuous use, and the reason they still do it is that copper corrodes in a controlled, self-limiting way. When copper is exposed to air and rain it does not just rust away like steel. It grows a thin, dense layer — copper oxide that slowly turns into the green patina you see on church roofs and old nails. That patina is a solid skin that slows further corrosion to almost nothing. In clean rural air a copper nail loses something like 0.005 mm per year of its thickness in the early years, then almost nothing after the patina is complete. A 3.35 mm copper nail has a working margin of centuries, not decades.
Copper in the field behaves differently from what people expect. Driving in a copper nail at the wrong angle, it bends instead of snapping, which some installers hate and others love — you can catch a bad blow before it damages the slate. On old or re-used slates, the softer metal is also more forgiving, and this is one reason copper remained the traditional answer for repairs on heritage roofs. On the slate's surface nothing else is going on: copper has no iron, so no rust stain runs down your face to the wall. The worst that happens is the patina — darkening from pink to brown to green — which nobody sees because it is under the headlap anyway.
The price is the real pain point. Copper is a commodity that follows the London Metal Exchange, and in some years the box of nails costs double what it did the year before. Contractors on fixed quotes do not love that. But the alternative is to stop at one key: in coastal and industrial air, copper keeps working where a 304 stainless nail would start to pit. That is a sentence you will see many supplier catalogues skip.
Stainless Steel Nails: Fixed Until They Are Not
Here is the part that surprises a lot of buyers: stainless steel does corrode, and it corrodes in a less visible way. Stainless is "stainless" because a chromium-rich oxide film forms instantly on the surface and seals it from the environment. That film is only a few atomic layers thick, but it is enough — as long as nothing punches through it.
The hole-puncher is the chloride ion. Sea air, road salt, industrial acids — any chloride-rich environment attacks the passive film locally, the metal underneath is a little pinhole and starts a pitting cell. The classic result is the small, dark pit, like a pin prick, that grows inward while the surface above it still looks perfect. You cannot see the nail is failing until it snaps or a slate slips. This is precisely why 304 stainless steel (18% chromium, 8% nickel, no molybdenum) is the wrong choice in a coastal band of roughly 1 km, and 316 stainless steel — with 2–3% molybdenum added — is the right one. The molybdenum raises the steel's resistance to chloride pitting by a factor that matters. If you live near the sea, read the label twice. If the box simply says "stainless steel", it is probably 304, and the salesperson has saved you nothing on a 60-year window.
What stainless gets right is strength and economics. A 316 nail in 2.8 mm diameter carries more pull-out resistance than a 3.35 mm copper nail, and stainless prices are stable compared with copper. In inland, sheltered climates — most of central and northern Europe — either will do the job; stainless is often chosen simply because it is cheaper and easier to source in bulk. On exposed coastal elevations, double-nailing with 316 nails is the modern specification and performs beautifully.
The second corrosive risk people rarely think about: galvanic corrosion, the difference between two dissimilar metals in contact with an electrolyte (wet battens, salty rain). Copper and stainless steel are close enough in the electrochemical series that the couple is mild; the risk is the moment someone mixes copper nails with a galvanised or aluminium accessory — then the less noble metal sacrifices itself to feed the copper. The rule: on a copper-or-stainless fix, keep every fix of the same metal family, or if you mix for a repair, check the chemistry and not the price.
Copper vs Stainless: The Side-by-Side Table
| Dimension | Copper nails | Stainless steel nails |
|---|---|---|
| Corrosion mechanism | Grows a dense, self-sealing patina; gets more stable with age | Passive chromium-oxide film; pitting once chlorides break through |
| Coastal (<1 km from salt) | Excellent, proven over centuries | Must be 316/316L; 304 will pit |
| Inland / sheltered | No problem at all | 304 is enough and will outlast the roof |
| Typical diameter | 3.0–3.35 mm (thicker, relies on cross-section) | 2.2–2.8 mm (harder, can be thinner) |
| Tensile strength | ~200–300 MPa, soft | ~500–700 MPa, hard |
| Driving behaviour | Bends before snapping; forgiving on old or uneven slates | Straight and stiff; over-driving can crack a slate edge — pre-drill |
| Price & volatility | Expensive, swings with the LME copper price | Cheaper, stable pricing |
| Staining | No rust runs; rare pyrite slates may need compatibility check | No staining; pit corrosion is easy to miss |
| Tradition & fit | Heritage roofs and re-covering the classic answer | New-build and large projects the value answer |
Typical commercial-grade reference values; always confirm against the supplier's test certificate and local codes.
Three Questions That Decide It for Your Roof
Q1. What is your climate exposure? Coastal (within roughly 1 km of salt water, or any headwind salt), or industrial air with acidity? — You are in the zone where 316 stainless or copper is mandatory. Inland and moderate? — 304 stainless is plenty, and copper is still a fine traditional choice. There is no "standard stainless steel nail" that works everywhere; the grade is the spec.
Q2. What are you nailing into? New battens, fresh timber, uniform gauge — stainless steel is efficient and cheap. Re-roofing an old slate roof where every slate is a different thickness, or a listed building where authenticity matters — copper's softness and its centuries of proven behaviour make it the gentle answer. If you buy copper nails for a re-cover, get them a size or two longer than you think; the old battens are often softer than they look.
Q3. What is the slate itself? Here is a fact few suppliers tell you: a small number of slates contain pyrite — iron sulphide. Under damp conditions, pyrite can slowly oxidise and produce acidic conditions; in combination with copper this can in very rare cases cause copper salts to stain the slate edge or speed the nail's corrosion. The fix is not to avoid copper everywhere, but to ask your slate supplier whether the specific colour and source are pyrite-tested. Our Jiujiang production, for example, screens for pyrite spots at inspection, and our test reports include it — because the nail and the slate have to live together for a century, not just arrive in the same box.
When in doubt, the robust default for a modern roof is: 316 stainless, 2.8 mm, double-nailed at the headlap. The traditional default is: copper, 3.35 mm, centre-nailed. Both will take care of your grandchildren's roof if the rest of the system is right.
Facts Most People Still Get Wrong About Slate Nails
- "Stainless steel never rusts." — It pits when chlorides break the film. Stainless is only as good as its grade in your climate. This is the most expensive misunderstanding in roofing.
- "Copper nails rust faster than steel in the rain." — The opposite: copper corrodes into a self-sealing skin; plain steel rusts until it disappears.
- "Galvanised nails are fine, they're just not premium." — Not "not premium"; they are not fit for purpose against a 75-year slate. The difference is a nickel per nail, the downside is a re-roofing bill.
- "A thicker nail is always stronger." — For a slate, the holding point is the battens and the slate nib, not the nail's flex. An over-thick nail can split a nib or the batten edge. Use the specified diameter.
- "Mixed fixings on one roof save money." — Two metals at different rates, plus galvanic risk at contact points. Choose one system for the whole roof.
- "The nail head must be big to keep the slate on." — The head matters only where it seats in the nib/slot. A 6–8 mm head with a sharp straight-shank (and sometimes a slight ring) is enough; huge heads slide on a smooth slate surface and mask a bad nail gauge.
What to Write on the Order: A Nail Spec in Six Lines
Whether you buy from a building merchant or import with your slate, the order must say more than "copper nails". Here is the six-line check that catches the difference between a roof that's done and a roof that's guaranteed:
| Line | What to write | Why it matters |
|---|---|---|
| 1 Material | Copper 99.9% soft, or Stainless 304 / 316 (L) | Material decides the corrosion mechanism; coastal must state 316 |
| 2 Diameter | Copper 3.35 mm / stainless 2.5–2.8 mm | Match slate weight and batten; don't just go bigger |
| 3 Length | Penetration 12–20 mm into the batten | Slate thickness + batten + penetration, then round up |
| 4 Head | Round head 6–8 mm, domed or flat | Holds the nib without cracking the slate edge |
| 5 Surface | Plain or ring-shank, stated on the order | Ring-shank grips the batten better in high-wind areas |
| 6 Certification | Batch certificate (material / dimension) | Stops a 201/430 label hiding inside a "stainless" box |
One more piece of advice from the quarry side: ask your slate supplier what they ship in the same box, and ask them to pack enough nails for the re-roof allowance. A slate roof is a 75-year project; the nails are the cheapest insurance policy you will ever buy at about 0.5% of the total roof cost — if you buy the right ones.
If you are specifying a new roof, our roofing slate range includes fixing guidance with every quotation, and the nailing and hooks guide pairs with this page — read them together and the nail decision writes itself.
Copper vs Stainless Slate Nails: Frequently Asked Questions
Do stainless steel nails rust on a slate roof?
They can, but not in the way steel does. In salt-rich coastal air, chlorides can break through the passive oxide film and cause small deep pits (pitting corrosion) that look fine on the surface and fail inwards. This is why coastal roofs should specify 316 stainless, not generic 304. Inland, 304 typically lasts the life of the roof.
Copper or stainless steel nails on the coast — which is better?
Both work if chosen correctly: copper performs well in marine air thanks to its self-sealing patina; 316 stainless is the modern coastal specification. Avoid 304 stainless near the sea. For a heritage re-cover, copper remains the gentler, traditional choice.
Are copper nails more expensive than stainless steel nails?
Yes, and they fluctuate. Copper follows the London Metal Exchange and can cost roughly 1.5–3 times more than 304 stainless of the same size per box, and a box price that moves. Stainless is stable in price and easy to buy in bulk. Over a whole roof the cost difference is a very small fraction of the project, so choose by environment first, price second.
Can I mix copper and stainless steel nails on the same roof?
Generally no. Two different metals corrode at different rates and can set up galvanic cells where they touch wet timber or underlayment, and when one system fails early it takes the fix with it. Keep one metal for the whole roof; if a repair is needed, re-use the same nail type.
Why do copper nails turn the slate edge green sometimes?
That green is a copper salt leaching on the surface, usually where the slate chemistry or the local acid environment reacts with the copper. It is rare and normally cosmetic — but if your slate is known to contain pyrite (iron sulphide) spots, check compatibility before ordering: some slates and copper react more. Any serious supplier will discuss this honestly.
What size slate nails should I use?
Copper nails are commonly 3.0–3.35 mm diameter; stainless steel 2.5–2.8 mm. Length depends on slate thickness plus batten plus 12–20 mm penetration. For a 500×250 mm slate at ~6–7 mm thick on a 25 mm batten, a 38–45 mm nail is typical. Check the roof system, not a blanket rule.
Related Reading
Spec the Right Slate — and the Right Nail
We produce EN 12326 / ASTM C406 tested roofing slate with 100% individual inspection, and our team gives straight answers on fixing materials, coating and coastal performance. Request samples and a technical spec for your next roof.