According to Document ‘J’ of building regulations, a stove needs to sit on a hearth of non-combustible material which also needs to protrude 300mm in front of the appliance (sometimes only 225mm). The hearth thickness needs to be 125mm – this can include the constructional hearth beneath floor level.
The hearth thickness can be reduced to just 12mm if during the stoves approval testing the hearth temperature did not exceed 100 degrees centigrade. This will be advised in the manufacturers fitting instructions.
