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Natural slate remains widely used in many areas for historical and technical reasons. For cost reasons, it is sometimes replaced by equivalents based on fiber and cement.

Slate roof

Slate is defined primarily by the material from which it is derived, shale. This gives it properties that have been successful for centuries. However, an alternative is developing from the first half of the 20th century: asbestos cement. It quickly gave rise to various roofing products, before giving way (after the ban on asbestos in 1997) to fiber-cement materials.

The history of natural slate

With their shades usually gray-bluish more or less dark that change according to the light, the slate roofs are recognized at first glance. They are also inseparable from the old mining areas: Anjou, Aisne, Oise, Ardennes, Corrèze... But nowadays, slate is almost no longer produced in France; it comes mainly from Spain and to a small extent from Canada and Brazil.
With its lightness and ability to cover any slope from almost horizontal (11°) to vertical (90°), its impermeability makes slate an inherently ingenious product. Slate also offers a relative elasticity. Resistant to breakage, it supports crushing and exposure to acids from atmospheric pollution. Usually guaranteed 10 or even 30 years, its longevity is actually much more important since it reaches 60 to 70 years, sometimes even more. But everything depends on the products, their quality indices A, T and S, as well as their choice (or sorting).

Slate: a material of variable composition

Slate is a schist and comes from the high-temperature compression of thin argillaceous sedimentary layers over 450 million years ago. Whatever its origin, slate shale is still mainly based on quartz and alumina silicates. It also contains some calcium carbonate (CaCO3) and iron.
Under the effect of carbonate, slate can tend to whiten and disintegrate with time. While iron can cause disorders with various consequences. The iron pyrite can appear as large cubic inlays of 5 mm side. This crystallized form of iron sulphide is both the most stable and the least troublesome. To distinguish forms more irregular or more underhand, because not crystallized.
The grainy pyrites (mixture of iron, sandstone and other minerals) thus generate reddish streaks when they are on the slate. The disadvantage is only aesthetic unlike micropyrite. Its incrustations, initially invisible, are also manifest in the form of streaks of rust but can go as far as perforation.

Natural slates: the terms to know

Roof in natural slate

  • spall: Beveled waist of the edges of a slate.
  • shoulder: upper angle sliced.
  • Quartz wire: quartz vein without affecting the strength of the slate.
  • Fine: fracture perpendicular to the longrain.
  • Flexuration with marked cleavage: deformation of the surface of the slate produced by sudden changes in the orientation of the planes of foliation.
  • Longrain: general orientation and direction of better flexural strength (corresponds in principle to the length of the slate).
  • Node: bulge that can incorporate foreign bodies.
  • roundup: angled fracture without impacting the slate's strength.
  • Bolster or lathing: sedimentary band.

Appearance quality of natural slate

The notion of sorting or choosing only concerns natural slates and notes the quality of appearance, the flatness or its opposite, the "veil". Second- or third-choice slates, which are less flat and of less regular thickness, require a more precise installation to achieve perfect sealing of the roof. These characteristics affect the price of products, but do not affect their durability. Reason why they are not taken into account by the standard (NF EN 12326-1 and -2) and partly by the mark NF Slates.

Place natural slate on the hook

More traditional than the hooking, the pose with the nail remains the favorite of the purists. But it is more expensive because it involves the implementation of a decking in pieces while the hooks are satisfied with batten, faster to ask.

Fiber cement: an industrial origin

Pure cement used in 4 to 5 mm thick would break easily. This is why we add fiber, today synthetic PVA (Polyvinyl alcohol or polyvinyl alcohol in French) and vegetable (cellulose). Amorphous silica is also added to improve its shock and frost resistance and various additives (fillers); for example, to obtain a particular shade: gray-green, brown, dark red, etc.
As fine and rot-proof as natural slate, slate fiber-cement even manages to reproduce its appearance: structured faces, fringed edges, round shapes (shell), shouldered versions. With all the advantages of an industrial product of calibrated thickness, without flatness or rust, and requiring no sorting on site... The product is also treated against mosses and available in half sizes, or even with matching accessories like those tiles (ridge and abouts).

Dark red fiber cement slate

Fiber cement slate is 15 years warranty, including the holding of hues. Their palette is not limited to anthracite gray, but goes from light gray to brown, through gray-green or dark red. In addition to the hue, a treatment slows down the formation of the mosses.

What format choice?

Various slate formats depend on aesthetic preferences but above all, technical criteria as they appear in professional documents and recommendations (DTU 40.11 and 40.13). As with the tiles, overlapping values ​​are to be applied depending on the wind exposure, the geographical area, the slope of the roof and the length of the ramp. This cover is intended to prevent infiltration by the upper edge of slates following upwelling rain, either by capillarity or under the action of wind.

What recovery values?

Recovery for the zones of France

Metropolitan France is divided into three zones.

  1. An area I less than 200 m altitude and mainly located more than 20 km from the coast (North Sea, Channel and Atlantic).
  2. Zone II located inland, from north to southwest between 200 and 500 m altitude. In addition, most of the Atlantic coastline is up to 20 km inland and portions of territory between zones I and III along the Channel and North Sea coasts.
  3. Zone III is made up of the Channel and North Sea coastal strip with also the Pyrenees, the Mediterranean coast as well as a large south-east quarter and, more generally, any zone from 500 m above sea level.
Thus, for example, a 27 x 18 cm natural slate in zone I for a 35° sloped roof hook installation with a horizontal projection of <5.5 m crawl must be laid with a 86 mm overlap. At least 3 times the height overlap (3 x 86 = 258 mm) and about 2 times in width (2 x 86 = 172 mm). In the same context, fiber cement slates (DTU 40.13) would require a 130 mm overlap, with dimensions of 45 x 30 or 60 x 30 cm.

Quality indices: "A", "T" or "S"

In 2006, the European standard EN 12326 has replaced the old A, B, C classification by the quality indices A, T, S. They must be included in the criteria to be taken into account as the source (career details), and remain on the labeling of a palette of natural slates to attest to their main performances:

  • Water absorption rate A (A1 0.60%)
  • Resistance to oxidation and thermal shock T: little or no change in appearance (T1), change of appearance of metal inclusions without structural change (T2), change of appearance of metallic inclusions with risk of perforation (T3)
  • Conditions of exposure to acid rain S (S1: unrestricted, S2 and S3: according to thickness and calcium carbonate content).
It is important to note that these indices do not reflect all the characteristics of a slate. In particular, the criteria of choice (or sorting) which, themselves, are found more among the criteria of attribution of the mark NF Slates.

Certification of slates

It is always a voluntary process on the part of the producer or the manufacturer. Compliance with the requirements of the standard (NF EN 12326-1 and -2) in terms of thickness (≥ 2 mm), flexural strength, water absorption (≤ 0.4 \%), is one of the criteria for obtaining the "NF Ardoises" quality mark. Its specifications also impose the absence or limitation of certain aesthetic or structural defects.

Six natural and fiber cement slates: summary table

Summary table of natural and fiber cement slates

Click on picture to enlarge

Fiber cement slate: Eternit

Fiber cement slate Eternit

  • Brand (Distributor): Eternit
  • Model: Kergoat
  • Range: Relief
  • Unit price: 1,08 € in 40 x 24 cm *
  • Available sizes (in cm): From 33 x 11.3 to 60 x 30
  • Thickness: 3.8 mm
  • Aspect: Structured
  • Wrinkled edges: Yes
  • Warranty: 15 years
  • Certification: NF EN 492
  • Provenance: France
The strong points
Available in half sizes and other colors (smooth version)

Fiber cement slate: Cembrit

Cembrit fiber cement slate

  • Brand (distributor): Cembrit
  • Model: Dolmen
  • Range: -
  • Unit price: 0,93 € in 40 x 24 cm *
  • Available sizes (in cm): From 33 x 24 to 60 x 30 and 40 x 40
  • Thickness: 4 mm
  • Appearance: Smooth
  • Wrinkled edges: No
  • Warranty: 15 years
  • Certification: NF EN 492
  • Provenance: Czech Republic

The strong points
Available in square format

Natural slate: Castorama

Natural slate Castorama

  • Brand (distributor): Castorama
  • Model: Standard
  • Range: Surchoix Pro
  • Unit price: 0,95 € in 32 x 22 cm *
  • Sorting: 1st
  • Available formats (in cm): 32 x 22
  • Thickness: 4 mm
  • Aspect: Structured
  • Wrinkled edges: Yes
  • Warranty: -
  • Performance: A1T2S1
  • Certification: NF EN 12326
  • Provenance: Spain

Natural slate: Min'Ardoises

Min'Ardoises natural slate

  • Brand (Distributor): Min'Ardoises
  • Model: K-08
  • Range: Advantage
  • Unit price: 0,59 € in 32 x 22 cm *
  • Sorting: 2nd
  • Available sizes (in cm): 27 x 18 to 46 x 30
  • Thickness: 4 mm
  • Aspect: Structured
  • Wrinkled edges: Yes
  • Warranty: 30 years
  • Performance: A1T1S1
  • Certification: NF Slate
  • Provenance: Spain
The strong points
Custom made

Natural slate: Cufica

Natural Slate Cufica

  • Brand (Distributor): Cufica (Point P)
  • Model: 1AS
  • Range: -
  • Unit price: 0,90 € in 30 x 18 cm *
  • Sorting: 1st
  • Available sizes (in cm): From 25 x 18 to 46 x 30
  • Thickness: 3.2 mm
  • Aspect: Structured
  • Wrinkled edges: Yes
  • Warranty: 10 years
  • Performance: A1T1S1
  • Certification: NF Slate
  • Provenance: Spain

Natural slate: Cupa Pizarras

Natural slate Cupa Pizarras

  • Brand (distributor): Cupa Pizarras
  • Model: Cupa 3
  • Range: Excellence
  • Unit price: 0,90 € in 32 x 22 cm *
  • Sorting: NC
  • Available sizes (in cm): From 25 x 15 to 50 x 45
  • Thickness: 3.5 mm
  • Appearance: Smooth
  • Wrinkled edges: Yes
  • Warranty: 30 years
  • Performance: A1T1S1
  • Certification: NF Slate
  • Provenance: Spain

Video Instruction: How to replace a broken roof slate. Cracked fibre cement slate repair.