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DM Modular block pumice chimney specifications

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DM Modular block chimney system technical specifications

Structural Timber Clearances

A sliding joint must be made using mineral wool or similar noncombustible material where a DM system chimney passes through a structural floor. A minimum 38mm clearance must be maintained between the outer face of the chimney and any structural timber or loose combustible material. Floor boards, skirting boards and other non-structural components may, however, be in contact with the chimney.


Bends can be achieved using purpose-made chimney offset blocks. These blocks do not have a separate flue block. To maintain the correct matching of the joints the last flue block immediately below the first offset block should be trimmed to bring it to the same height as the top of the outer casing. Above the last offset block a starter flue block must be used.
All offset blocks must be fully supported.


An external chimney must be tied to the structure at maximum intervals of 1.5m and at the point where it departs from the roof line. Ties should be ISOKERN stainless-steel ties and bolts for use between the casings; or galvanised-steel straps and bolts for use around the casings.

Lead Flashings/External Finish

There are two alternative methods to fix a flashing to the outer casings of a rendered stack:

Fold the flashing in over the edge of the casing protruding through the roof by approximately 10mm and fix accordingly.

Scorch the protruding casings with a disc cutter or block saw to achieve a 10mm deep channel parallel with the roof. Fold the edge of the lead flashing into the channel and fix accordingly.

If the chimney is brick clad above the roof then traditional stepped flashing should be used. Proceed to normal building practice using a lead tray. The lead tray should be turned up on the outside of the flue blocks by approximately 50mm.

Weep holes should be provided below the chimney capping to allow for any water vapour to escape between the inner flue blocks and outer casings. Before the capping is lip glued into place a D.P.C should be fitted. Finally traditional flaunching is used to seal around the protruding flue block or chimney pot

Outer Surface

Casings should be dry lined or plastered internally. Externally they should be finished with waterproof render or brick cladding.

Heights Above Roof/Reinforcement

Rendered DM Chimneys can be installed without reinforcement up to 1.4m above the roof line. All casing and flue block joints to be sealed with lip glue.

ISOKERN DM outer casings include holes for the reinforcement rods and these rods should be grouted with 3:1 sand: cement mix.
It may in some cases, as with free-standing chimneys, be necessary to secure the reinforcement into the chimney foundation. In this way chimneys can be constructed up to 9m free-standing.
If the DM system is constructed inside a masonry chimney stack, (built from foundations not off a corbel unit) which is constructed so that the height of the chimney stack does not exceed 4.5 times its overall horizontal dimension in accordance with BS6461 Part 1 1984, reinforcement is not required. The DM casings should be tied to brickwork using wall ties.

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