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Building Regulations Document J (and other country equivalents) stipulates that for safety reasons all stoves require one of two types of non-comustible hearth depending on how hot it gets underneath the stove. This temperature is derived when the stove is assessed during the CE tests and it should be clearly stated in the stove's installation instructions or declaration of performance.
If a stove has the capability of raising the surface temperature of a hearth to more than 100ºC then it must always be placed on a 'constructional hearth', part of which is usually made from a thick layer of concrete. If you're planning on building a Class 1 brick or block constructed chimney with a traditional fireplace opening then it makes sense to create a constructional hearth at the same time, irrespective of your prospective stove's declared hearth temperature. Document J provides comprehensive guidance on how to build a constructional hearths and you can download it here.
If you intend to use a twin wall chimney system with a free-standing stove, then you must ensure that the stove is one that will not raise the surface temperature of the hearth above 100ºC and also comes with the manufacturer's declaration that it's suitable for use on a 12mm decorative hearth. Ideally it should be a convector stove because of the reduced safe distances to combustible materials associated with these types of stoves. This will allow you to specify a 12mm hearth (aka floor plate) from a big choice of labour-saving ready-made hearths in a variety of materials including toughened glass. These also come in a range of practical sizes including the Document J minimum 840 x 840mm for a free-standing stove. This size can usually accommodate the Document J requirement of providing a minimum area of 225mm from the front of the stove door to allow for potential live ash spillage as well as the 150mm requirement at each side.
Toughened glass is by far the most popular 12mm thick hearth choice because it allows the hearth to be placed on top of flooring such as combustible engineered wood and laminate boards or tiling etc to minimise any visual disruption to the living space aesthetics. Tiles, slate, stone, marble, granite, resin and steel are some of the other floor plate choices and these also come in a wide variety of shapes and sizes including shapes designed for corner installations including the popular tear drop-shape. A lot of these materials can also be custom cut.
Document J 2.26 (and its country equivalents) states that the boundary of any hearth should be visually apparent to warn the building's occupants of the proximity of an extremely hot stove and to discourage combustible floor finishes from being laid too close to it. It suggests that a change in level from the floor to the hearth is one way of doing this. For free-stnding stoves 12mm ready-made hearths are not only aesthetically pleasing but are also accepted by Building Control and Hetas as a means of satisfying this requirement without representing a particularly serious trip hazard. It is worth noting that even if a tiled or concrete floor is non-combustible the stove and its safety area will still require some form of visual demarcation. The gloss, reflective surface of toughened glass hearths is usually deemed sufficient for compliance to this part of the regulations.
You have a choice of two types. A non-combustible constructional hearth 250mm deep including any decorative surface (see Doc J diagram 27) or alternatively 125mm deep version which also provides a minimum air gap of 50mm beneath it to protect any combustible materials (see Doc J diagram 25). Consideration will also need to be given to a low energy home's air-tightness when constructing the 50mm air gap required for the125mm version.
Choosing a stove that has a hearth temperature of less than 100ºC will allow you to place it on 12mm non-combustible decorative hearth which is both highly cost-effective and labour saving compared to creating a constructional hearth. The minimum size for hearths for free-standing stoves is 840 x 840mm and they must not only accommodate the weight of the stove but also provide the space to meet Document J guidance on a minimum of 225mm in front of the stove and 150mm at each side. Space needed at the rear is determined by the manufacturer's instructions so it's worth checking this out before you commit to your hearth (or your stove).
All inset stoves are still required to have a hearth. It is highly unlikely that those insets designed to be placed within a traditional fire surround will have a hearth temperature of less than 100ºC and therefore they must always be placed on a constructional hearth. The overall depth of the constructional hearth in this case can include the hearth that is part of the fire surround and upon which the inset stove sits.
Where a contemporary inset stove is installed at height for a minimalist look then the hearth temperature will not usually be an issue and a 12mm non-combustible decorative hearth can be placed on the floor space below the stove. This should still follow the Document J requirement of providing a minimum 225mm deep space at the front of the stove and 150mm at either side. Since these are minimum distances we would suggest that a much greater depth would be safer to allow for any potential embers bouncing when falling from height. Glass hearths are generally the preferred choice for these type of installations as they allow for an uninterrupted view of the floor surface to enhance the minimalist styling.
Document J indicates the minimum sizes that are needed for a hearth to comply with building regulations but it is worth noting that they are not necessarily always the safest for your particular appliance. For example many wide body stoves and insets feature doors that, when open, would extend much further than the hearth's 225mm minimum requirement in front of the stove. As one of the purposes of the hearth is to protect the surrounding floor surface (which may be combustible) from any potential falling embers then it is obvious that the 225mm minimum would be insufficient. We suggest that when designing your fireplace opening and hearth that you also refer to the manufacturer's instructions as well as Document J and BS8303.
Download Document J and its country equivalents here
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