11
Typical DM Installation Detail
CHIMNEY CONSTRUCTION
Bed the first outer casing using lip glue making sure the
rebate (raised lip) is uppermost. The starter flue block is
then put inside the casing bedded with lip glue. The socket
on the starter flue block must be uppermost and the air
gap between the starter flue and the casing should be kept
clear. The finished lip glue joints should be 2-3mm thick, a
special bag is provided for ease of application and the lip
glue should be applied in 12-15 mm beads.
Offset blocks ‘if required’ must be used immediately
above the support block or firechest gather. They must
be glued together with lip glue and be fully supported.
Please note a chimney should be built straight wherever
possible. A starter flue block will be required above the
offset blocks to stagger the joints between the outer
casing and inner flue blocks.
Casings and flue blocks are added using lip glue for all
joints. Ensure the air gap between the inner flue and outer
casing remains clear. The outer surface of the casings to be
finished with plasterboard on dabs or plaster. The side that
faces the wall does not need an external finish.
On a rendered stack with a top ring fitted (see p.12 for
example), where a straight chimney passes through a floor
or roof, zero mm distance to combustibles can be applied.
A sliding joint is made using mineral wool or similar non-
combustible material. In all other cases, 38mm clearance
must be maintained between the outer face of the
chimney and any structural timber or loose combustible
material. Floor boards, skirting boards, dado rails and other
non-structural components may, however, be in contact
with the chimney.
External chimneys must be tied to the structure at
maximum intervals of 1.5m and at the point where it
departs from the roof using 2 x suitable stainless steel wall
ties. These are fitted into the outer casing joints. They are
not always required for internal chimneys. Please consult
the Isokern technical office.
High tensile steel reinforcement rods will be required for all
chimneys with a height that exceeds 1.4m above the roof
line, 1.1m if the wind speed exceeds 44ms. These rods are
available from Isokern and must be grouted (1:3 cement/
sand) into the holes provided in the outer casings. You must
start the rods at least the same height below the roof as
what’s above, please check with our technical department.
HOW TO USE LIP GLUE
Reinforcement rods
Load bearing
support
Starter flue block
Offset blocks must
be fully supported
and bedded with
lip glue
Plasterboard
DM outer casing
Stainless steel ties
DM outer casings
Inner flue blocks
Void to be
kept clear
38mm clearance from
structural timber
Non combustible
material
Lip glue
Lip glue
