A hearth is designed to protect the building from the solid fuel appliance. The rules are stricter if the stove is being inserted into a fireplace recess than if the stove is to be “freestanding” outside of the usual stove recess (e.g. a modern stove connected to a twin wall flue that exits the building via the ceiling of the room).
If your stove is freestanding in a room and therefore NOT going into a recess under a chimney, AND the stove is independently certified not to heat the hearth underneath it to more than 100 degrees centigrade (many stoves available), then hearth regulations are very simple as no constructional hearth is required: allowing a minimum 12mm thick non-combustible superimposed hearth straight onto the floor (even onto carpet/wood floor etc.).
In this case the hearth must be a minimum size: 840mm x 840mm. There must also be a minimum 150mm of hearth at each side and rear of the stove and a minimum 225mm in front of the stove door (best practice is 300mm and if the stove is designed to burn with doors open then 300mm is a MUST).
If the stove has no such certification then the rules for a “stove inserted into a fireplace recess” apply and the usual constructional hearth IS required (see below).
Diagram 27(c) of ADJ (Document J of the building regulations) states that any solid fuel appliance situated within a fireplace recess must have a constructional hearth.
Update: Hetas confirmed to me that a constructional hearth is not required if the stove is certified to not heat the surface under the stove to more than 100 degrees centigrade.
So what’s a constructional hearth? If you have a fireplace and you take away any raised superimposed hearth away from that fireplace you should see a concrete area of floor (most noticeable if the floor of the room is wood). If the whole of the floor is concrete then the whole floor forms your constructional hearth (assuming the thickness and quality adhere to the regs as required for a constructional hearth).
Let’s take a look at a top down view:
The shaded area is the constructional hearth. Essentially the hearth should project a minimum of 500mm into the room and be wider than the recess by 150mm within the room.
I can see why the rules are like this. It can however seem a little strange when you have a six-foot wide, 3 ft. deep inglenook with a tiny little stove and you still have to have that 6″ bit!
What about the depth of the concrete? The next graphic shows all:
Put simply, the hearth should be 125mm deep and combustibles should NOT be underneath the hearth unless they are a minimum 250mm deep (unless there is an air space underneath the hearth of at least 50mm seperating the hearth from the combustible material). Basically air is a good insulator.
Note that the 125mm thick concrete can be sunk into the floor or raised part or fully above the level of the floor. It is generally the case that the top of the constructional hearth is level with the floor level.
Creating a constructional hearth under floor level is beyond the scope of this manual. This is a builder’s job. One can create an above floor constructional hearth using non combustible materials.
The regs for a constructional hearth are particularly “belt and braces” unless the stove is an inset stove (does a stove with legs really reguire 5″ of concrete underneath of it?). But rules are rules and we must follow them.
Note: On top of your constructional hearth will be your superimposed hearth (sometimes referred to as a decorative hearth) and we are coming to that next. The thickness of the superimposed hearth can be included in the 125mm thickness mentioned above (e.g. if constructional is 100mm and superimposed is 25mm then all is well).
Note: You will see stoves with log stores underneath. These stoves must not, if building regulations are to be adhered to, be lit whilst logs are in the log store (’tis true).
The superimposed hearth sits on top of the constructional hearth (e.g. slate or tiles or granite etc.). The job of the superimposed hearth is to act as a warning to building occupants and to discourage carpet or other such combustible floor coverings from being laid close to the stove.
As can be seen in the diagram above (ADJ diagram 26) the superimposed hearth should be a certain distance in front of the closed doors of the stove (the white box is your stove). Choose 30cm minimum for all stoves and you cannot go wrong – it does mean that you have to work out, ahead of time, where your stove is going to sit. I usually work out what the air gap requirements behind the stove are (e.g. 8cm) and add this to the depth of the stove (e.g. 35cm) plus the 30cm in front of the stove doors (then check the flue will clear the lintel with or without an offset flue). In this case the superimposed hearth would need to project 8+35+30cm = 73cm from the back wall of the recess. Add a little extra just to be sure.
Note also that the superimposed hearth should be wider than the stove (left and right) by a minimum of 150cm and that the perimeter should be “visually apparent” (see 2.26 below).
So how thick should the superimposed hearth of a fireplace recess be? Well according to the above you just have to have a boundary that is visually apparent AND discourage the laying of carpet etc. Easiest way is to lay tiles or slate or granite to provide a raised surface. As can be seen though you just have to fulfill what is requested (e.g. a small raised piece of metal, like a fence, would do the job).