Property Types & Audiences
Plenty of the houses we work in — post-war builds, extensions, converted outbuildings, garden rooms — have no chimney at all. That does not rule out a stove; it just means the flue becomes part of the project rather than something you inherit.
How a twin wall system works
A twin wall flue is a manufactured insulated chimney: an inner stainless liner, an insulation layer, and an outer casing. That construction keeps flue gases hot along the entire route, which is what gives the stove its draw without a masonry stack around it.
It is assembled in sections, with the height and terminal position set so the outlet clears the roof properly. Get that wrong and you build downdraught problems into a brand new installation.
Internal route
Running the flue up inside the building and out through the roof is thermally the better choice — the system stays warmer, so the draw is stronger and tar accumulation slower. It also looks intentional: a black flue rising through a room reads as a design feature rather than an add-on.
The trade-off is disruption. The system passes through ceilings and possibly a bedroom or loft, with fire stops and clearances to combustibles at every penetration, and it takes up floor area in the rooms above.
External route
Going out through the wall behind the stove and up the outside is less invasive internally and often quicker. The flue is supported on brackets against the wall and rises to the required height above the eaves or ridge.
The compromises are visual and thermal: the flue is visible from outside, and being exposed it runs cooler than an internal route, which slightly reduces draw. In conservation areas or on listed buildings, external flues may face restrictions, so that is checked before anything is quoted.
What to consider before committing
The practical questions are worth thinking about ahead of the survey, because they shape the design.
- Where in the room the stove will sit, given clearance to furniture and walls
- Whether the hearth needs constructing to the required size and thickness
- Room ventilation, particularly for appliances above 5kW
- What is directly above the proposed route — bedrooms, loft, roof structure
- Whether the property is listed or in a conservation area
- How the flue will look from the street or garden, if external
Garden rooms, outbuildings and conversions
Twin wall systems suit outbuildings and garden rooms well, because there is no existing structure to work around and the route can be planned cleanly. The same clearance, fire stop and terminal height rules apply — a summerhouse is not a lesser installation, and the timber construction typical of these buildings makes clearance detailing more important, not less.
Frequently asked questions
Is a twin wall installation more expensive than lining a chimney?
Generally yes, because you are supplying the entire chimney rather than a liner inside an existing one. Cost depends mainly on total height, the number of components and whether the route is internal or external.
Do I need planning permission?
Many external flues fall under permitted development, but conservation areas, listed buildings and prominent street-facing elevations can change that. We flag it at survey so you can check with the local authority before work is scheduled.
Will it look out of place?
Modern black powder-coated systems are reasonably discreet, and internal routes often become a feature. It is worth deciding at survey stage rather than after installation, since it is not a cheap thing to reroute.
Can a twin wall flue go in a flat or upper-floor property?
It depends entirely on the building, ownership of the external walls and roof, and any factoring or title conditions. It needs looking at case by case rather than answering in general.
No chimney? Book a free survey
We will assess the route options, explain the trade-offs and quote in writing.