Wood Burner Hearth Size Calculator: Minimum 840 x 840mm

A hearth is not decoration, it is the bit of the installation Building Control actually measures. This calculator turns your stove's footprint into the minimum hearth width, depth and thickness for England and Wales, and tells you whether the slab you already have passes. The catch most people miss: even a tiny 4kW stove standing free in a room needs a hearth of at least 840 x 840mm, so the margins around the stove are only half the test.

How the calculator gets those numbers

Approved Document J, the part of the England and Wales Building Regulations that covers solid fuel appliances, sets the hearth by margins around the stove rather than by the stove's kW rating. A closed stove needs at least 150mm of hearth each side and behind it, and at least 225mm in front of the door, because that is where embers land when you refuel. If the appliance is an open fire, or a closed stove that can properly be used with its front open, the front margin goes to 300mm, and plenty of fitters use 300mm anyway as cheap insurance.

On top of the margins there is a floor on the overall size: a hearth for a free standing stove is never smaller than 840 x 840mm. That is why a slim 4kW stove measuring 400 x 300mm still cannot sit on a 700mm square slab, even though the margins alone would allow it.

Inside a fireplace recess the shape changes. The hearth covers the recess floor, then continues into the room: at least 500mm forward of the jambs, and at least 150mm past each side of the opening, so a 900mm opening needs a hearth 1200mm wide. If the stove pokes forward out of the opening, measure the 225mm or 300mm from the door and take whichever projection is larger.

12mm hearth or 125mm constructional hearth?

Modern stoves are usually tested so the surface under them stays below 100°C. Where the manual states that, and the stove is not standing in a recess, a 12mm thick non-combustible hearth is enough and it can be laid straight onto a timber floor. Glass, slate, granite and honed limestone hearths sold at 12mm and up are made for exactly this.

If the manual is silent, or the stove is old, or it heats the hearth past 100°C, or the stove stands in a recess, you are into constructional hearth territory: 125mm of non-combustible material, which can include the concrete floor slab and any decorative surface on top. Combustible material must not sit beneath it unless there is an air gap of at least 50mm under the hearth, or the combustible material is at least 250mm below the top of the hearth. Retrofitting that into a suspended timber floor is a real job, so check the manual before you buy the stove.

What this calculator does not decide

Hearth size is separate from clearance to combustible walls and furniture, which comes from the stove manufacturer's tested distances and varies enormously between models. The manual always wins: if it asks for 400mm in front or a wider hearth than the regulations, that figure is the one your installer and Building Control will work to.

These are the England and Wales figures. Scotland works to the Building Standards Technical Handbooks and Northern Ireland to Technical Booklet L, so confirm locally before you order stone. Any new installation must be signed off, either by a HETAS registered installer self certifying or by a Building Control application.

Once the hearth is settled, the other two numbers people get wrong are the flue and the air supply. Our flue liner size calculator gives the liner diameter and length to order, and the air vent calculator works out the free air area your room needs.