Synthetic Slate

Synthetic slate is a manufactured roofing product moulded or extruded to imitate natural slate. Most products are made from engineered plastics such as polypropylene, polyethylene or TPO, or from EPDM and recycled rubber, combined with mineral fillers, colour pigments and ultraviolet stabilisers. They are also marketed as composite or polymer slate.

Properties

• Weight: about 200-350 lb per square (roughly 10-17 kg/m²), compared with about 700-900 lb per square for standard natural slate. Most can be installed on ordinary framing without reinforcement.
• Impact resistance: many products carry UL 2218 Class 4 ratings for hail.
• Wind: products are tested to standards such as ASTM D3161; Class F is quoted for some.
• Fire: ratings vary by product and assembly, so the listed assembly should be checked.
• Appearance: products vary from single-colour units to blended, textured moulded surfaces. Synthetic slates are lighter and more flexible than stone, and do not have the natural cleft surface.

Installation

Synthetic slates are usually nailed to a solid deck, for example 7/16 in (11 mm) OSB or 1/2 in (12.7 mm) plywood, with corrosion-resistant ring-shank nails, often from a pneumatic coil nailer. Because plastics expand and contract more with temperature than stone, manufacturers specify side gaps between units, and these must be followed. Many products can be cut with a utility knife or saw rather than a slate cutter.

Durability

Manufacturers offer warranties that are often long, but real-world service lives are still being established for many products. Concerns include colour fading, warping or curling in high heat, and brittleness with age as stabilisers are used up.

Use in conservation

Heritage bodies generally do not accept synthetic slates as replacements on historic buildings, because they do not match the colour, texture, weathering and thickness of natural stone.