What Thickness Hearth Do I Need for a Wood Burner?
What Thickness Hearth Do I Need for a Wood Burner?
Choosing the right hearth thickness is one of the most common questions we get — and it matters both for safety and aesthetics. Here's everything you need to know.
The Short Answer
For most wood burners and multi-fuel stoves, a 20mm thick hearth is the standard choice. If you have a larger, higher-output stove or want extra durability underfoot, a 30mm hearth offers additional strength and a more substantial look.
UK Building Regulations: What's Required?
Under UK Building Regulations (Approved Document J), a constructive hearth must be:
- At least 125mm thick in total (including the sub-hearth beneath)
- Made from non-combustible material
- Extending at least 300mm in front of the stove door and 150mm either side
The slate or limestone slab you see on top is the decorative hearth — it sits on top of the constructive hearth (usually a concrete base). The decorative slab itself does not need to be 125mm thick; 20mm is perfectly compliant when laid on a suitable sub-hearth.
Always confirm your installation with a HETAS-registered installer or your local building control.
20mm vs 30mm — Which Should You Choose?
| 20mm | 30mm | |
|---|---|---|
| Best for | Standard wood burners, most fireplaces | Large/high-output stoves, heavy use |
| Weight | Lighter, easier to handle | Heavier, more substantial feel |
| Look | Sleek, contemporary | Chunky, traditional |
| Price | More affordable | Premium |
| Compliance | ✅ Compliant on suitable sub-hearth | ✅ Compliant |
Does the Finish Affect Heat Resistance?
No — whether you choose riven (natural split), honed (smooth matte), or brushed (textured), the finish is purely aesthetic. All finishes perform equally well under normal stove operating temperatures.
Our Recommendation
For the vast majority of installations, a 20mm black or grey slate hearth is the ideal choice — durable, heat-resistant, and competitively priced. If you have a large inglenook or a high-output stove (above 10kW), consider stepping up to 30mm for peace of mind.