The short answer
Rust stains on slate usually come from iron minerals, often pyrite, in the stone itself. Where they oxidise, they leave brown marks or streaks that can run down the slope. In a good grade of slate this is rare. Stains can also come from corroding nails or metalwork, which is a different problem, so the source needs identifying before you decide whether it matters.
Pyrite and iron in the stone
Slate is a natural stone, and what is in the rock decides how it weathers. Some slates contain small crystals of iron minerals, of which pyrite is the best known. Pyrite is an iron sulphide that shows as pale brassy flecks, sometimes called fool's gold. When it is exposed to rain and air, it can oxidise, and the products are rusty brown.
The effect varies. A slate with a few flecks of pyrite may show a faint orange spot where each one sits. A slate with a lot of it can develop larger brown patches and streaks, and the staining can run down onto the slates below, or the wall or gutter beneath.
Origin by country tells you less than the quarry and the grade. Spanish slate is quarried in numerous places and sold in grades, the denser ones weathering steadily and the poorer ones carrying more iron mineral. Our page on Welsh versus Spanish slate covers the differences, and the fuller picture of slate against other coverings is on the slate versus tiles page.
Telling a stone stain from a nail stain
Not every brown mark comes from the slate. Start by checking where the staining begins. A close-up photograph of the stain and the nail position, taken before anyone climbs up, lets a roofer compare it with what they find on site.
If the marks sit in the body of the slate, as spots, blotches or streaks that begin in the middle, the cause is likely the stone. If they start at the nail position near the top of the slate, or at a metal strap, clip or flashing fixing, they are more likely to be from corroding metal.
Where nails are iron or badly galvanised, rain washes the rust down the face of the slate. That staining is a signal about the fixings. The Stirling City Heritage Trust notes that iron or poor galvanised nails corrode over time and allow the slate to slip, which is the process called nail sickness. Our page on nail sickness explains it.
A third source is metal elsewhere: an old iron rainwater pipe, a steel aerial bracket or a rusting ridge fitting. Water runs over it and stains the slate below.
Does it matter?
The answer depends on the cause. Staining from the stone is largely cosmetic at first. It can spoil the look of a roof, particularly on a terrace where it is visible, but a single stain is not a leak.
There is a longer-term concern. Lower grades may carry iron minerals that oxidise and streak, and they may also delaminate earlier than a dense stone. That is one reason to ask about grade and source when you buy. If the stained slates are going soft or flaking as well, treat the stain as a warning that the slate may not last as long as a good one.
Staining from rusted nails matters more, because the fixing is failing. A row of rust marks near the head of each slate says the nails are corroding, and slipped slates are likely to follow. Our nail sickness page explains what that means for the roof as a whole.
What can be done about the marks
There is no simple fix for staining that comes from the stone, because the source is inside the slate. Cleaning may lighten it for a while, but the minerals keep reacting. Be careful about method, because forceful cleaning can harm a slate. MyBuilder's cleaning guide cautions that forceful washing can damage delicate tiles, and slate is not exempt.
If only a handful of slates are marked, the practical fix is to replace those slates with sound ones. Matching for colour and size is possible, and reclaimed slate often suits an older roof. If the staining is widespread, it is better to think of it as a sign the slate was a poor grade, and to consider what that means for the roof's life.
If the cause is nails or metalwork, the fix is to deal with the metal. That could mean replacing a rusting fitting, re-fixing slates with copper or stainless nails, or renewing a section of roof. A roofer can tell which by lifting a slate and looking at the nail, and a photograph of the nail and the batten beneath makes the answer plain to you as well.
Choosing slate to avoid it
If you are buying new slate, the best protection is to ask about the grade and the quarry, and to see a sample. Look for evidence of mineral flecks. Ask whether the supplier gives a test certificate and what the quarry's track record is on weathering. Cheaper slates can be good, but price alone is not the guide.
The cost range is wide. MyBuilder's 2026 slate guide quotes installed prices of £100 to £180 a square metre, Welsh at £140 to £180 and Spanish at £100 to £130, and those are national averages rather than an Empire quote. Your own price follows our survey.
Winter conditions do not stop slating, but they can make it harder to judge colour on a wet day. If you are choosing slate, view samples dry as well as wet. Colour is easier to judge in good light. For work in the cold, dull months, slating in winter is the page to read. For the wider picture see our slate roofing service.
