Bosch Worcester Greenstyle Bewdley 8 Installation, Servicing And User Instructions Manual page 14

Hearths should be constructed of suitably robust materials and
to appropriate dimensions such that, in normal use, they prevent
combustion appliances setting fire to the building fabric and
furnishings, and they limit the risk of people being accidentally
burnt.
The hearth should be able to accommodate the weight of the
appliance and its chimney if the chimney is not independently
supported.
Appliances should stand wholly above hearths made of non-
combustible board/sheet material or tiles at least 12mm thick,
if the appliance is not to stand in an appliance recess, or on a
constructional hearth in accordance with the paragraph below.
Diagram A Constructional hearth suitable for a solid fuel appliance
Projection
at least
500mm
from jamb
Diagram B Constructional hearth suitable for a solid fuel appliance
14
At least
150mm
Plan (a) Fireplace recess
Top surface of hearth
At least
250mm
CONSTRUCTIONAL HEARTH
Constructional hearths should have plan dimensions as shown in
Diagram A below and be made of solid, non-combustible material,
such as concrete or masonry, at least 125mm thick, including the
thickness of any non-combustible floor and/or decorative surface.
Combustible material should not be placed beneath constructional
hearths unless there is an air-space of at least 50mm between
the underside of the hearth and the combustible material, or the
combustible material is at least 250mm below the top of the hearth.
(See diagram B below)
At least
840mm
(b) Free standing
Section through hearth
At least 125mm
Air space of
at least 50mm
Combustible material
INSTALLATION
At least
840mm
6720818040 B 04/2017