Advanced

Slate Roof Installation: Method, Gauge, Hooks

15 min read

Slate remains the most noble and durable roofing material in the traditional European repertoire — a natural slate roof well laid will last a century without complaint. But it is also the most technically demanding to install: rigorous setting out, minimum pitch of 70%, working flat on a steeply inclined roof, stainless hook fixing and millimetre-precise gauge. The self-builder who takes it on must accept the learning curve — and know that a fixing error costs dearly at the first driving rain. This guide details the method according to DTU 40.11 (French norm; the UK equivalent is BS 5534 — Slating and Tiling for Pitched Roofs), from choosing between natural and synthetic slate through to setting out the gauge, hooks, verge gauge and flashings.

SLATE ROOF SECTION — STAINLESS HOOK FIXING Section along the slope — eaves at left, ridge at right EAVES (bottom) RIDGE (top) water flow Interior — rafters and loft RAFTER (structural timber) BREATHABLE UNDERLAY (Sd < 0.2 m) AIR — ventilation under slate GAUGE (120-140 mm) 1 Rafter 2 Breathable HPV 3 Spruce batten 4 Stainless hook A2 5 Slate

Natural slate vs fibre-cement slate

The word “slate” covers two product families with very different performance and prices. Choosing between them determines the budget, installation method, durability and even the admissible roof pitch.

Natural slate

A schist rock quarried, split into thin slabs (3 to 5 mm) and cut to size. Three origins dominate the European market:

Origin Source Lifespan Indicative price Notes
Anjou / Trélazé Maine-et-Loire, France (new extraction ceased 2014) 100–150 years 120–180 €/m² Heritage quality, deep blue-grey, rare
Spain (Galicia) Iberian Peninsula, world’s leading producers 80–100 years 70–110 €/m² Standard across Europe today
Corrèze / Other French Small active French quarries 100+ years 100–150 €/m² French supply chain, CE certified

Quality grades are classified under NF EN 12326-1: Class A (best) resists water, freeze-thaw cycles and atmospheric pollution without alteration. Classes B and C age less well.

Fibre-cement slate

Manufactured slabs made from cement + fibres (cellulose, PVA, sometimes mica), pressed and tinted through the body. Historical brands: Eternit (Etex group), Cembrit, Edilians. Earlier generations contained asbestos — this was phased out by 1997 in France; current fibre-cement slates are entirely asbestos-free.

Criterion Modern fibre-cement
Lifespan 30 to 50 years
Weight 18–22 kg/m² (vs 25–30 for natural)
Installed price 60–95 €/m²
Minimum pitch 50% (vs 70% for natural)
Appearance Uniform, varied tones
Carbon footprint Lower (no quarrying)

Tip — For a first self-build, fibre-cement slate is more forgiving: lighter, factory-dimensioned (calibrated to a tenth of a millimetre), less fragile if dropped, and compatible with a shallower pitch. Natural slate requires a roofer’s eye to sort damaged slabs at the workshop, adjust thicknesses and align angles. If this is your first roof, fibre-cement will save you considerable time.

Question

When can slate be used?

Slate is not suitable for every configuration. Three conditions must be met before even placing an order.

Minimum pitch

The minimum pitch depends on the climatic zone and the exposure (sheltered, normal or exposed site). DTU 40.11 distinguishes three zones: Zone 1 (plains, southern Atlantic coast), Zone 2 (centre and north), Zone 3 (mountains and exposed coast).

Slate type Sheltered site Normal site Exposed site
Natural slate Zone 1 65% (33°) 80% (39°) 100% (45°)
Natural slate Zone 2 80% 100% 120%
Natural slate Zone 3 100% 120% 150%
Fibre-cement (all sites) 50% (27°) 70% (35°) 100% (45°)

An insufficient pitch exposes the roof to capillary rise beneath the gauge: water enters at the head of each slate despite the lap. If your roof structure is shallower than 70%, abandon natural slate and look at steel roofing or a waterproofing membrane instead.

The substrate

Slate is laid exclusively on:

  • Sarking boards (close-boarded timber) — the traditional solution, compatible with nail fixing
  • Spaced battens (slating laths) — the modern solution, ensures under-slate ventilation; hook fixing only

No direct fixing to OSB or plywood panels (they do not hold fixings). The substrate must be dry, sound, free of protruding knots, and must be overlaid by a breathable roofing underlay (HPV) to secure the watertight barrier.

The roof structure

The weight of natural slate (25 to 30 kg/m², plus 10 kg for battens and 5 kg for fixings = ~45 kg/m²) requires a roof structure dimensioned accordingly. If you are reusing an existing structure, have it checked by a structural engineer.

Warning — Replacing a clay tile covering with slate is not a neutral operation: even though slate is lighter, the rafter sections, load distribution and bracing differ. Never lay slate on a roof structure originally designed for clay tiles without prior verification. In the reverse direction (slate → heavy tile), the problem is even more serious.

Anatomy of a slate installation

Each course of slates follows a set of strict geometric rules, the heritage of centuries of craft. Understanding them is the foundation for everything that follows.

The 5 key dimensions

  1. Length (L) — dimension of the slate along the slope (e.g. 32 cm)
  2. Width (l) — perpendicular dimension (e.g. 22 cm)
  3. Head lap (HL) — the length overlapped between three successive courses, varies with pitch (80–130 mm)
  4. Gauge (G) — the exposed portion of each slate = (L − HL) / 2
  5. Batten spacing — the vertical spacing between two successive battens = G
SLATE SETTING OUT — GAUGE, LAP, BONDED JOINTS Roof face view — ridge at top, eaves at bottom ridge eaves L = 32 cm (length) l = 22 cm (width) G gauge 12 cm HL head lap 8 cm min G = (L - HL) / 2 = (32 - 8) / 2 = 12 cm

Basic formula

Gauge G = (Length − Head lap) / 2

For a 32 × 22 cm (12.6 × 8.7 in) slate with a head lap of 80 mm: G = (320 − 80) / 2 = 120 mm.

The gauge is the master dimension: it sets the batten spacing, the number of courses over the roof height, and therefore the entire setting out.

Common sizes

Format (cm) Typical use Gauge at HL=80 mm
22 × 32 Small roof, outbuildings 120 mm
24 × 36 Standard residential 140 mm
27 × 36 Large roofs 140 mm
32 × 45 Exposed buildings 185 mm
40 × 40 “square” Contemporary diagonal laying Specific calculation

Best practice — For a self-build project, choose the 32 × 22 cm format in natural slate or 40 × 24 cm in fibre-cement. They offer a comfortable gauge (120–140 mm), straightforward setting out, and enough courses to absorb small discrepancies without on-site cutting. Smaller formats (18 × 25 cm) look attractive but require 35% more handling.

Fixings: hooks or nails?

Two techniques coexist. The choice depends on the substrate and the climatic zone.

Stainless slate hook (modern)

This is the dominant solution today in new construction. A stainless steel hook (A2 for normal exposure, A4 for coastal locations) is nailed into the batten, rises up the face of the slate and holds it by its hooked tail.

  • Speed: 2 to 3× faster than nail fixing
  • Compatibility: fixing to battens, under-slate ventilation assured
  • Durability: stainless hooks are rot-proof, lifespan equivalent to the slate itself
  • Cost: approximately 0.10 to 0.15 €/hook, i.e. 3 to 5 €/m²

Standard lengths: 90, 100, 110 mm depending on slate thickness and gauge. The tail must protrude beyond the slate by 1 to 2 cm maximum to avoid snagging snow or causing hand injuries.

Nail fixing (traditional)

Two copper or stainless nails per slate, through a pre-drilled hole near the head. Used on close-boarded sarking in heritage renovation. Slower, but invisible from outside and considered more durable on historic buildings.

  • Nails: copper (traditional, develops a patina) or stainless A2/A4
  • Length: 30 to 35 mm, flat head
  • Drilling: with an awl or drill, staggered holes to avoid splitting

Warning — Standard galvanised steel nails rust within 5 to 10 years in contact with moisture and tannins present in the timber (especially oak). The result: slates unclip course by course, and the roof “moults” at the first storm. On a slate roof, fixings are always copper or stainless, never galvanised steel — the additional cost of 200 to 400 € on a 100 m² roof bears no comparison with the cost of a complete re-roofing.

The installation method, course by course

Tip

Step 1 — Laying the breathable underlay

Before any slate, roll out a breathable HPV underlay over the rafters, working from eaves to ridge, with a 10 cm overlap. Staple to the rafters, joints sealed with specialist adhesive tape. This underlay is your second line of defence against water and condensation — invisible once the roof is finished, but it saves the structure in the event of a leak.

Step 2 — Battening to gauge

Mark the batten lines with a chalk line. The first (eaves) batten sits approximately half a gauge from the edge (60–70 mm for 22 × 32 cm slate). Subsequent battens are then marked at intervals of G = 120–140 mm depending on the chosen format. Millimetre precision is required: half a gauge out of line and the entire roof is thrown.

Standard batten section: 24 × 38 mm to 27 × 50 mm, spruce/fir, preservative-treated to Class 3 if exposed.

Step 3 — Laying the first course (eaves course)

The first course (or “eaves course”) is doubled: two slates superimposed for reinforced watertightness at the eaves. The first slate is a short (starter) slate, then a full slate on top. Fixed with two stainless hooks (90–100 mm) driven into the eaves batten.

Step 4 — Working up the roof, in broken bond

Slates are laid in broken bond (the vertical joint of one course falls at the mid-point of the slate in the course below) — never in straight lines, which would create a continuous water path. Lateral offset = half the slate width.

  • Each course offsets by a half-width relative to the previous one
  • The verge course (left and right edges of the roof) uses three-quarter width slates to start the offset pattern
  • Each slate is held by its hook at the tail, driven into the underlying batten

Step 5 — The verges (lateral edges)

The two lateral edges of the roof (verges or barges) present a finishing problem. Three solutions:

  • Bedded verge: a strip of zinc or lead dressed against the gable masonry
  • Torching verge: a slate laid diagonally over the last course, bedded in mortar
  • Preformed zinc profile: the modern solution, fast and neat

Step 6 — The ridge

The top of the roof (ridge) is closed with a zinc or lead strip, or a slate ridge laid in counter-fall. This is a vulnerable point for watertightness: ensure good overlaps and fixings, and ventilate beneath the ridge to allow air to exit the roof void.

Step 7 — Flashings and special details

  • Valley (internal angle between two roof planes): zinc flashing minimum 250 mm wide, folded into a V, with valley slates cut on the bias over the top
  • Eaves: zinc apron flashing fixed to the fascia board, projecting 50 mm into the gutter
  • Penetrations (chimney, extract vent, roof window): lead or zinc flashings dressed on site, integrated into the slate course

Tip — Allow a 10 to 15% wastage allowance on the volume of slate ordered to cover: handling breakages, verge cuts, valley cuts and a reserve stock for future repairs. On a 100 m² roof with 18 slates/m², this represents 200 to 270 slates of margin. Nothing is more frustrating than having to call the supplier for 15 slates in an unobtainable shade three years later.

Tools specific to slate work

Slate is worked by hand using dedicated tools. Budget: £130 to £210 (150–250 €) for basic equipment.

Tool Use Indicative price
Slater’s hammer Conical head (nailing) + pointed peen (trimming) 40–70 €
Roofer’s stake (zax) Small metal stake fixed to the roof for trimming 30–50 €
Awl / slate ripper Drilling the nail hole in natural slate 10–15 €
Slate cutter (chisel) Straight and angled cuts 25–40 €
Roofer’s ladder (cat ladder) Moving about without walking on laid slates 120–250 €
Harness + lifeline MANDATORY from 3 m height 150–300 €
Chalk line Marking battens and courses 10–15 €
Slate hook set Assorted kit 90/100/110 mm 50–80 €

Warning — Fall risk is the primary danger on a roofing site. A roof at 70% pitch can cause a fatal impact from just 2.5 m. Harness, lifeline, validated anchor points, perimeter scaffolding and a roofer’s cat ladder are the non-negotiable minimum. Consult our working at height safety guide before going up on the roof.

Decision tree: which slate to use?

flowchart TD A{Building type?} -->|Heritage / character house| B{Budget?} A -->|Standard new build| C{Roof pitch?} A -->|Outbuilding / garage / shelter| D[FIBRE-CEMENT SLATE
min pitch 50 percent] B -->|Premium 150+ eur m2| E[NATURAL SLATE
ANJOU or CORREZE] B -->|Standard 80-130 eur m2| F[NATURAL SLATE
SPAIN class A] C -->|Over 70 percent| G{Wind exposure?} C -->|Between 50 and 70 percent| H[FIBRE-CEMENT SLATE
40x24 format] C -->|Under 50 percent| I[ALTERNATIVE
Steel sheet or flat tile] G -->|Coastal| J[NATURAL SLATE
stainless A4 fixing] G -->|Mountain| K[THICK SLATE 4-5 mm
increased head lap] G -->|Standard inland| L[SPAIN SLATE
or FIBRE-CEMENT] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#0F4C81,stroke:#0F4C81,color:#fff style C fill:#0F4C81,stroke:#0F4C81,color:#fff style G fill:#0F4C81,stroke:#0F4C81,color:#fff style D fill:#56C6A9,stroke:#56C6A9,color:#fff style E fill:#6B5876,stroke:#6B5876,color:#fff style F fill:#56C6A9,stroke:#56C6A9,color:#fff style H fill:#56C6A9,stroke:#56C6A9,color:#fff style I fill:#CD212A,stroke:#CD212A,color:#fff style J fill:#6B5876,stroke:#6B5876,color:#fff style K fill:#6B5876,stroke:#6B5876,color:#fff style L fill:#56C6A9,stroke:#56C6A9,color:#fff

Cost per m² installed (2026 figures)

(prices in €/m², ex VAT)

Natural slate

Item Price
Spain Class A slate, 32 × 22 cm format 45–60 €/m²
Corrèze or Anjou slate 75–120 €/m²
Battens + breathable underlay + stainless hooks 18–25 €/m²
Roofer / slater labour (installation only) 50–80 €/m²
Total installed by contractor 120–210 €/m²
Total self-build (materials only) 65–85 €/m²

Fibre-cement slate

Item Price
Fibre-cement slate, 40 × 24 cm format 28–40 €/m²
Battens + breathable underlay + stainless hooks 18–25 €/m²
Roofer labour 40–60 €/m²
Total installed by contractor 85–125 €/m²
Total self-build (materials only) 46–65 €/m²

For a typical 120 m² roof (8 × 15 m, two slopes)

  • Contractor installation in Spanish natural slate: €14,400 to €25,200
  • Contractor installation in fibre-cement: €10,200 to €15,000
  • Self-build in Spanish natural slate: €7,800 to €10,200
  • Self-build in fibre-cement: €5,500 to €7,800
  • Self-build saving: €6,000 to €15,000 depending on choice

Maintenance and lifespan

Natural slate

Almost maintenance-free over 80–100 years. Check every 5–10 years:

  • Inspect hooks (no broken tails, no slipped slates)
  • Clean moss on north-facing elevations (soft brushing; never a pressure washer, which dislodges slates)
  • Repair ageing flashings and zinc strips (30–50 year lifespan)
  • Replace individual broken slates as required

Fibre-cement slate

Every 15–20 years, plan a gentle chemical moss treatment and check the fixings. Some batches (notably Eternit slates from 1970–1990) show surface chalking caused by cement carbonation: an aesthetic phenomenon that does not affect watertightness, but signals end of life at 30–40 years.

Common mistakes to avoid

  1. Insufficient pitch (< 70% for natural slate) → water ingress through capillary rise
  2. Galvanised steel fixings → corrosion, slates unclipping
  3. No breathable underlay → condensation in the roof structure, mould
  4. Aligned joints (not in broken bond) → continuous water path between two courses
  5. Poorly marked battens (wrong gauge) → uneven courses, visible misalignment
  6. Poor cuts on verge slates → water ingress at the gables
  7. Forgetting under-roof ventilation → condensation, rafter rot
  8. Walking on laid slates → invisible cracks, leaks two years later
  9. Single eaves course (not doubled) → systematic leak at the gutter head
  10. Hooks too long → protruding points that snag snow, causing tearing

Standards and references

  • DTU 40.11 — Natural slate roofing: reference standard (French norm). The UK equivalent is BS 5534 — Slating and Tiling for Pitched Roofs and Vertical Cladding
  • DTU 40.13 — Fibre-cement slate roofing
  • NF EN 12326-1 — Quality classification of natural slates (Classes A, B, C)
  • CSTB — French Centre for Building Science and Technology, technical data sheets
  • GCCP Couverture — French Roofers’ Federation, professional resources

Pre-installation checklist

Checklist: preparing your slate roofing project

  • Roof pitch validated for climatic zone (≥ 70% for natural slate on a normal site)
  • Roof structure designed for slate loading (30–45 kg/m² total)
  • Slate chosen: natural Class A or fibre-cement certified NF EN
  • Volume ordered with 10–15% wastage allowance (breakages + reserve stock)
  • Breathable HPV underlay (Sd < 0.2 m) laid and sealed
  • Spruce/fir battens Class 3, section 27 × 50 mm, treated
  • Gauge calculated and marked with chalk line to millimetre accuracy
  • Stainless hooks A2 (normal zone) or A4 (coastal) in sufficient quantity
  • Copper or stainless nails if using traditional nail fixing
  • Zinc/lead strips for verges, ridge, valleys and flashings
  • Slating tools: slater’s hammer, zax/stake, awl, cutter
  • Harness, lifeline, cat ladder, perimeter scaffolding
  • Stable weather forecast for 3–5 days minimum, no wind > 40 km/h
  • DTU 40.11 (or 40.13) / BS 5534 consulted, manufacturer’s data sheet printed
  • Emergency contact available on site (phone, emergency contact)