Drive through any Cornish village and you're looking at slate. Delabole slate, Cornish scantle slate, mixed reclaimed slate - the county is roofed with it, especially in the older terraces, mining cottages and farmhouses. Slate is also the trickiest common UK roof for solar PV installation. Done well, it lasts 25+ years and looks tidy. Done badly, you get broken slates, leaks, and a £3,000 reroofing bill in year 4. Here's what proper slate solar install looks like, and how to make sure you get it.
Why slate is harder than tile
Concrete or clay tiles interlock and are loosely hung on battens - lift the tile, drop in a roof hook, replace the tile. Slate is different in three important ways:
- Nailed individually - each slate is nailed to a batten, often with copper or galvanised nails. You can't just lift a slate; you have to extract it without snapping it.
- Brittle - especially older or weathered slates. Walking on a Delabole slate roof carelessly will crack tiles you can't easily replace.
- Variable - slate thickness varies (scantle, rag, Cornish), so generic hooks don't always fit. The good installers carry stock for the typical local slate sizes.
Extra cost on a slate install
Expect 15-20% more than a tile install for the same system size:
| System | Standard tile | Slate roof | Extra |
|---|---|---|---|
| 4 kW (10 panels) | £6,500-£8,500 | £7,500-£9,800 | +£1,000-£1,300 |
| 6 kW (15 panels) | £8,500-£11,000 | £9,800-£12,700 | +£1,300-£1,700 |
| 8 kW (20 panels) | £10,500-£13,500 | £12,100-£15,500 | +£1,600-£2,000 |
The extra cost covers:
- Longer install time (2-4 days instead of 1-2)
- Slate-specific roof hooks (Sika SolarMount slate, K2 SS-Slate, Renusol slate)
- Lead flashing or aluminium soakers around each hook
- Stock of replacement slates (some breakage is normal)
- Specialist roofer or trained PV installer with slate experience
What good slate installers actually do
1. Slate hooks, not tile hooks
Slate roof hooks slide under an existing slate and fix to the rafter beneath without cutting or drilling the slate itself. The hook is shaped to follow the slate profile so water sheds correctly. Tile hooks (designed for interlocking concrete tile) damage slates and create channels for water ingress.
2. Lead flashing around penetrations
Where hooks come through, proper installers fit Code 4 or Code 5 lead flashing (or aluminium soakers) sized to extend at least 150mm above and to each side of the hook. Cheap installs use roofing tape or skimp the flashing - that's where leaks start.
3. Slate replacement, not glue
Some slates will break in extraction; this is normal. Good installers carry replacement slate stock and refit broken ones with copper rivets or slate hooks. Cheap installs apply silicone or mastic to broken slates - looks fine for a year, leaks by year 3.
4. Marine-grade stainless fixings
Within a mile of the Cornish coast (or anywhere prevailing wind carries salt spray inland), galvanised steel fixings corrode within 8-15 years. Specify A4 (316) stainless steel hooks, bolts, and clamps. Worth £100-£300 extra and saves a major re-fix mid-life.
5. Underlay inspection
A proper survey checks the underlay condition. Old bituminous felt (common under Cornish slate of 1950s-1980s vintage) tears easily and may be at end of life. If underlay is failing, the right time to address it is before solar goes on top - not in year 8 when leaks force the panels off again.
Red flags on slate quotes
- "We use the same hooks for all roof types" - no, slate needs slate-specific hooks
- No mention of lead flashing or soakers
- "We don't survey - just send postcode and roof photos" - you can't quote slate properly without seeing it
- Same per-kW price as tile - either they don't know it's slate, or they'll cut corners
- Install time quoted as "1 day" for a slate roof - rushing slate causes damage
- No replacement slate cost in the quote - they're hoping nothing breaks
- "We'll seal any broken slates with mastic" - that's not slate repair, that's covering a problem
The Delabole slate question
Authentic Delabole slate (still quarried at Delabole north Cornwall) is highly prized and increasingly hard to source. If you have an old Delabole roof, replacing broken slates means finding reclaimed or current stock at premium prices (£3-£15 per slate, sometimes more). Bring this up at survey - some installers carry Delabole stock, others don't.
For reclaimed or new Cornish scantle slate (graduated sizes from eave to ridge), make sure your installer understands the pattern. Done well, scantle solar looks remarkably tidy. Done badly, the pattern is broken at every hook.
Listed buildings and slate
Many Cornish slate-roofed properties are also listed buildings. The two issues stack: you need listed building consent AND you need a slate-competent installer. See our listed building guide for the consent process.
What if the roof needs work first?
If your slate roof is approaching end-of-life (50-100 years depending on quality and exposure), the right sequence is:
- Re-roof first (or strip and re-batten, replacing damaged slate)
- Renew the underlay with modern breathable membrane
- Install solar onto the renewed roof
The combined cost is significant - £8,000-£20,000 for the reroof plus £7,000-£9,000 for solar - but the alternative is removing panels in 8-12 years to do the reroof, costing £1,500-£3,000 in remove-and-refit labour. We've seen this story play out enough times to advise homeowners to be honest about roof condition at survey time.
The bird-proofing connection
Slate roofs in coastal Cornwall attract gulls and pigeons that love nesting under solar arrays. Bird mesh is worth specifying at install time - £400-£700 retrofit later. See bird-proofing solar panels Cornwall for details.
Want a Cornwall slate-competent installer?
We've vetted MCS-certified installers across Cornwall and prioritise those with documented slate roof experience. Submit your postcode for matched quotes - tell us if you've got slate so we route accordingly.
Frequently asked questions
Can solar panels go on any slate roof?
Most yes, but condition matters. A sound slate roof under 60 years old with good underlay accepts solar with no issues. Older or storm-damaged slate may need partial re-roofing first. Always have a structural and roof condition survey before installing.
How much extra do solar panels cost on slate?
15-20% more than on standard concrete or clay tile. On a 4kW system, that's £1,000-£1,300 extra. Worth paying for proper slate hooks, flashing, and longer install time.
Will installing solar damage my slate roof?
Done well: no - properly fitted slate hooks integrate seamlessly. Done badly: yes - broken slates, torn underlay, leaks. Choose an installer with documented slate roof experience and check references for older installs.
What's the difference between slate hooks and tile hooks?
Slate hooks slide under an existing slate and fix to the rafter beneath without cutting or drilling. Tile hooks raise tiles up and over the bracket, which damages slate. Using tile hooks on slate is the single most common botched-install mistake.
How long do solar panels last on a slate roof?
The panels themselves last 25-30+ years on any roof type. The fixings are the wear point - on slate near the coast, marine-grade stainless steel fixings last the full life; cheaper galvanised may need replacing at year 12-15.
Should I re-roof before installing solar?
If your slate is sound and underlay is in good condition (last 30-40 years of life), no. If the roof is end-of-life or underlay failing, yes - cheaper to do both at once than retrofit later.
Are there installers in Cornwall who specialise in slate?
Yes - most established Cornwall MCS installers know slate well because it's so common locally. Some national installers don't have the experience and either refuse slate jobs or do them badly. Ask installers how many slate roofs they did last year.
Can I do solar on a thatched roof in Cornwall?
Not directly on thatch (fire risk, unstable substrate). Possible on adjacent outbuildings or ground-mounted in the garden if conservation/listed status allows. Listed thatched cottages often have both planning and listed building consent issues.