HETAS registered twin wall flue installation for properties without an existing chimney. External and internal systems. Approved Document J compliant. Compliance certificate on every installation.
Twin wall is an insulated factory-made flue system used where no existing chimney is present or where an existing chimney cannot be used. The most common applications in Romford and Havering are extensions where no original chimney was built, outbuildings and garden rooms where a stove is required, and older properties where the chimney has been removed or sealed and the reinstatement of a liner is not possible.
The system consists of double-wall insulated stainless steel sections that rise from the appliance to a terminal above the roofline. The insulated construction maintains flue gas temperature and draw across the full length of the run, which is particularly important in external installations exposed to cold.
The system exits through the external wall via a thimble or wall plate, rises on the outside of the building, and terminates above the roofline. Wall brackets are fitted at minimum 2m centres to support the rising sections. The roof penetration — where the system passes through the eaves or rises beside the building — is weathertight with lead flashing or a firestop assembly. Terminal height is 600mm above the ridge if within 600mm horizontally, or requires 2.3m horizontal clearance at the same height.
External twin wall is generally simpler in terms of structural impact — no ceiling or floor penetrations are required. It is typically the preferred route for extensions and outbuildings.
The system rises through the interior of the building, passing through each ceiling and floor. This is specialist installation scope. At every penetration, a firestop plate — a non-combustible collar sealed around the twin wall pipe — is required by Approved Document J. The minimum clearance from combustibles must be maintained at every level: typically 50–70mm from the outer surface of the twin wall to any combustible material, including timber joists, floorboards and roof insulation.
Where the run passes through living spaces, boxing in is standard. The boxing must not compromise the required clearance and must remain accessible at every firestop position.
External and internal installations follow distinct processes but share the same compliance and certification requirements.
All twin wall installation — external and internal — is self-certified against Approved Document J of the Building Regulations under the HETAS Competent Person Scheme. A HETAS compliance certificate is issued on every installation. The local authority is notified through HETAS. No separate building control application is required.
Internal twin wall carries specific Document J requirements that go beyond the general standard. The firestop plate requirement at every penetration, the combustible clearance rule, and the heat shielding specifications are all detailed in Approved Document J. These are not advisory — they are building regulation requirements. Our internal twin wall installations are confirmed to Document J specification before the compliance certificate is issued.
HETAS twin wall flue installation across Romford, Havering and surrounding areas.
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