17/11/2025
20 mm slate paving refers to outdoor paving slabs made from natural slate that are roughly 20 millimetres thick. This thickness is common for garden paths, patios, and terraces, providing a good balance between strength, weight, and ease of installation.
Below is a clear, helpful overview
What 20 mm Slate Paving Is
Slate is a fine-grained, metamorphic stone that splits into smooth, flat layers, making it ideal for paving. A 20 mm thickness gives the slabs enough durability for residential outdoor use without being too heavy to handle.
Key Characteristics
1. Appearance
• Naturally riven (slightly textured) or smooth finishes.
• Rich, deep colours—commonly:
• Black/charcoal
• Dark grey
• Blue-grey
• Greenish tones
• Subtle variation between slabs gives a natural, premium look.
2. Texture
• Typically lightly riven for grip.
• Some suppliers offer calibrated 20 mm slate, meaning the underside is machine-cut for consistent thickness.
3. Durability
• Slate is dense and hard-wearing.
• Resistant to:
• Weathering
• Frost
• Algae (better than many sandstones)
• Low water absorption reduces staining and frost damage.
4. Size Options
Common slab formats:
• 600×600 mm
• 900×600 mm
• 600×400 mm
• Mixed-size patio packs
Installation Notes
• 20 mm slabs are usually installed on a full mortar bed.
• Joints are typically filled with an exterior grout or jointing compound.
• A solid sub-base (e.g., compacted MOT Type 1) is needed to prevent movement.
• Although 20 mm is strong, they are not normally recommended for driveway vehicle loads unless laid on a reinforced concrete base.
Maintenance
• Sweep regularly to avoid debris buildup.
• Power washing is fine if done sensibly.
• Optional sealing:
• Enhances colour
• Reduces staining
• Not always required, depending on the finish
Pros and Cons
Pros
• Beautiful natural appearance
• Strong and long-lasting
• More colour-stable than sandstone
• Non-slip surface
• Low maintenance
Cons
• Can be more expensive than concrete or sandstone
• Darker colours may show dust or salt marks.