By Julian Patrick
By Julian Patrick
Okay, strap in - this is going to get technical. Got a coffee? Sandwiches?
A hearth is designed to protect a building and occupants from the solid fuel appliance. Basically we do not wish to set the floor surface on fire, either by heat from the stove or from fuel falling out when you open a door.
The rules are different if the stove is being inserted into a fireplace recess rather 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 or wall of the room). We shall come to the two different sets of rules soon.
Stove in a fireplace recess
A hearth, in the most stringent application of the rules, consists of two elements, one on top of the other:
Added together in thickness, the constructional hearth and VWA must have a combined depth of 125mm (5"). However, many modern stoves can have this combined depth reduced to 12mm. You will see this mentioned on our product pages under "Specifications & Dimensions".
In our image above the glass hearth is, surprisingly, acting as both the constructional hearth AND the Visual Warning Area at the same time as the single piece of glass fulfils the criteria for both. The glass covers a suitable geographic area (constructional-hearth box ticked) and follows the rules for Visual Warning Area. The slim thickness is possible because the stove is certified as "allowed to have a 12mm hearth depth" (if it did not have this certification then the floor under the glass would have to be reworked).
In the past, even if a room had a wooden floor, the area where the stove was to go would be concrete, 5" thick (you will often see this in fireplace areas where a wooden floor ends). On top of this concrete area would be placed a Visual Warning Area e.g. a slate slab, a tiled area, or similar.
If the stove is allowed to have a 12mm total thickness of hearth then you can lay down a sheet of non-combustible material of a suitable size (e.g. concrete/glass/tile). Just make sure the sheet is large enough to satisfy the rules for both the constructional hearth and the VWA (Visual Warning Area) as your "slab" of material will act as both types of hearth in one swoop.
Otherwise, the constructional hearth must be 5" (125cm) of concrete or brick and a suitable VWA must also be present. An example would be a solid concrete floor at least 5" thick with a slate-hearth laid on top. Another example would be a raised brick area 5" high.
The constructional hearth area for a freestanding stove must be a minimum size: 840mm x 840mm. If your room has a tiled or concrete floor you already have your constructional hearth! Lucky you ;-) That's assuming it is 5" thick (or 12mm thick if you have a certified stove). If you DO have this situation then you just need to present a suitable Visual Warning Area and we are coming to this.
If your floor is combustible (wood/carpet etc.) then you need to make sure your constructional hearth goes on top of the combustibles (or get rid of the combustible materials).
As we said earlier the constructional hearth must cover a minimum area of 84cm x 84 cm. If your whole floor is suitable then no need to even get the tape measure out as your whole floor complies anyway. But you cannot just plonk your stove down on a concrete floor, because then you would have no Visual Warning Area.
For a free-standing stove outside of a fireplace recess the rules for the Visual Warning Area are as follows:
In the image above the stove is allowed to be on a 12mm hearth so the thickness looks good. Assuming the tiles are 45cm x45cm we have our area of 84x84cm so that's good. I'm not sure we have the minimum 225mm in front of that stove though? And with a glass that large I would certainly want the full 300mm as recommended and would go for 400mm to feel really happy (big glass stoves throw a lot of heat forward)..
Looking at the blue stove on the metal plate. Assuming the stove in the image below has a compliant constructional hearth under the metal thin-sheet (min 84cmx84cm) then all is good. The metal provides a clearly-marked visual deterrent (VWA) and has at least 225mm of VWA in front of the stove glass. To the sides there is at least 150mm VWA between the edge of the stove and the edge of the hearth. There is at least 150mm gap between the rear of the stove and the rear edge of the VWA (or the wall is suitably heat resistant e.g. plastered brick).
Note that building regs do allow a difference of colour for a Visual Warning Area and a step up or down is not insisted on (so legally you could just spray your VWA onto a concrete floor). In my experience some experts feel there should be a definite step up or down and you may come across this viewpoint. If in any doubt step up or down and you will have everybody smiling.
Even if you think it not relevant you might wish to read the sections above as they thoroughly explain a few concepts.
Typical constructional hearth
Note that at first glance of Doc J of the building regs "inside a fireplace recess" is different to "freestanding out in the room" when it comes to the allowing of a 12mm thick hearth. The 12mm thick constructional hearth is not allowed in the fireplace recess but is allowed, in another part of the room, outside the recess. We'll get to this misnomer shortly.
So what’s a constructional hearth? If you have a fireplace and you take away any slate/tiles (Visual Warning Area) you should see a concrete area of floor (most noticeable if the rest of 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 - 5" thick minimum of non-combustible material).
Let’s take a look at a top down view of a constructional hearth:
Diagram 24 ADJ: Constructional hearth (the grey part)
The shaded area is the constructional hearth. Essentially the constructional hearth should project a minimum of 500mm into the room and be wider than the recess by 150mm within the room (again - if your whole floor is 5" thick concrete then no need to even get your tape measure out).
But what if your constructional hearth within your fireplace recess is a little shy of the required amount? What happens if somebody has taken half of it away? This happens! Well, it depends. It depends on the stove you are going to install. If your stove is allowed to have a 12mm hearth "and the manufacturer allows it" then you do not have to worry about the constructional hearth being up to scratch. In such a situation the manufacturer is saying "we are happy that the constructional hearth is not required as long as your Visual Warning area is of the correct size and thickness (12mm)". This is covered in Building Regs by Doc J Note 2.27 in that any hearth can be adjusted to suit the particular appliance. Using this as a basis it is fine to use a 12mm sheet within a fireplace recess if the constructional hearth is of inadequate materials or dimensions. It is also covered by HETAS in Bulletin 13. If you wish to buy one of our stoves and require this confirmation just ask us (we already have this confirmation for all "12mm hearth stoves" by Arrow, Arada, Hamlet, Ekol, Saltfire and Dik Geurts). If your stove is not allowed to have a 12mm thick hearth then you have no choice - you have to make your constructional hearth up to spec. I have known some installers lay down a Visual Warning Area big enough to cover all sins - and then nobody knows what's underneath!
Yes it is odd that the rules for "inside a fireplace recess" are more stringent than for stoves in other parts of the room. I can see why the rules are like this - somebody might later take the stove out and start using the fireplace for an open fire and then the regs make sense.
What about the 125mm depth? The next graphic shows all:
Diagram 25 of ADJ: Combustible hearth thickness
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 to be level with the floor 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.
Diagram 26 ADJ: Hearth dimensions with the grey part being the superimposed hearth.
As can be seen in the diagram above (ADJ diagram 26) the VWA should extend a certain distance in front of the closed doors of the stove (225mm minimum*, 300mm best practice and more if the stove manual says so e.g. Firewire Stoves 400mm). In the image the white box is your stove, the grey area your Visual Warning Area (VWA) and the dotted line is the constructional hearth.
Choose 400mm minimumVWA in front of the stove 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 front to back (e.g. 35cm) plus the 40cm in front of the stove doors (then check the flue will clear the lintel with or without an offset flue). In this case our Visual Warning Area 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 Visual Warning Area 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 Visual Warning Area 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.
Julian Patrick is the author of The Stovefitter's Manual and an experienced wood burning stove installer (including solid fuel heating systems).
Laid down tools in 2013 to write The Stove Fitter's Manual and open a small shop in North Wales (the Wood Stove Hut). Launched Stovefitter's Warehouse soon after due to fast growth of sales.
Own stove is a DG Ivar 5.
Stovefitter's Warehouse is owned and managed by Julian Patrick, blogger and author of The Stove Fitter's Manual. Julian was previously a full-time installer of wood-burning stoves (including solid fuel heating systems). He laid down the tools in 2013 to write his stove manual and open a small shop in North Wales (the Wood Stove Hut, soon to grow into The Stovefitter's Warehouse).