Chimneys are sometimes taken down above the roof line and terminated in the loft. They are still connected to the lounge and still work – but stop in loft.
When this happens, it is still possible to use the existing chimney by relining it with a flexible flue liner. At the point where the chimney stops, the liner will also stop. The liner is then converted to a twin wall flue system.
The twin wall flue is also cylindrical but unlike the flexi’s the twin wall is rigid. The twin wall flue also has two layers of stainless steel but in between each layer, rockwool is placed.
The rockwool insulates the flue and allows it to pass within a short distance of timber. Battens, rafters, and purlins are common features in roof structure so being able to pass relatively close to these timbers is important. A flexible flue liner isn’t safe to do this, but the twin wall flue is.
ADMIN: PHOTO – Adapter
ADMIN: PHOTO – adapter with flex and twin attached
At the point where the two flues meet, an adapter is used. It is commonly referred to as an anchor plate. This is a solid flat square plate. At the lower end a tube acts to attach the flexible flue liner. Above the plate, a small section of twin wall is fixed in place. Another section of twin wall can attach to this and continue upwards through the roof.
If bends must be used to bypass the main structural timbers, they will be used. Sometimes we can trim the structural timbers.
Trimming is a method referred to cutting back timber and adding more in a different place to allow the offending timber to be removed. It is often done in the install construction phase of a building – for example to allow for windows to fitted in a roof. However, once a roof is on, this process becomes more involved and access may be an issue too. Therefore, trimming timbers is not our first choice as this can mean much more work and time being involved which will add to the cost. We may have to remove a large portion of the roof to trim the timbers and add the correct supports.
But as with all flues the straighter they are the better they perform – bends reduce how the flue works and can cause issues with draw. Consequences of this is smoke spilling out of your stove during use. This is known as smoke spillage.
The gamble to use bends and slightly decrease the draw often outweighs what is involved with the process of trimming rafters and such like.
In so much that a straight flue is good for draw, a tall flue is also good. Short flues that are under 4.5m also have more issues with draw.
Another issue that can cause issues with draw is the position where the flue stops. Keeping it too close to a roof can lead to something known as downdraught and leads to smoke spillage. Therefore, we’ll want to ensure the flue is terminated in the correct place. Typically, this is at least 60cm to 1m above a flat roof and on a pitched roof, it can be at least 2.3m away from the roof or nearest object such as a tree or another building.
Therefore, there are lots of considerations to take on board when planning for a twin wall flue that is coming out of a roof – and more so for one that is very limited in position because of the existing position of a chimney.
Access may also be an issue as the chimney is likely tucked away in far reaches of a loft! And the only access may be through the roof.
Assuming we’ve managed to get your flue past the structural timbers using bends or even trimming the timber, we’ll also want to fix it in place. The anchor plate adapter can be fixed in place using concrete screws/bolts and mortar. But another brace is preferred that is attached to the roof timbers below the roof tiles or covering.
Once the flue is through the roof, a flashing kit is used to act to waterproof the system and prevent water ingress. Once again, this involves removing the roof cover and making good. Often this involves tiling, so working on your roof means access from ladders is tricky. We have an access tower that we bring along to this type of job.
The chimney at the bottom in the room below will have a fireplace. This fireplace will need to accommodate your stove. So, fireplaces are often modified to ensure building regulations are followed for a stove installation. This can include hearth modifications or even opening the chimney to accommodate the new stove. We have a page dedicated to fireplace modifications.
Consequently, this type of installation – that involves converting a flexible flue liner to a twin wall flue – can be one of the most complicated types of installation that we perform. It involves a fireplace, and two flue systems – often with terrible or limited access.
That said, we’ve performed many of these installations and as always have developed a good system ensuring the success rate is achieved for a safe, efficient, and aesthetically pleasing installation (it looks and works great!). In fact, we’ve regularly been called upon to do this work when other installation companies have failed or bailed on the job.
We’ve managed on several occasions to perform a complete fireplace rebuild including enlarging the fireplace, install a new single piece hearth, rendering the recess, skimming the chimney breast, install an Eco-stove to a flexi-twin wall flue for a two-storey house for under £5.5k and managed to do it within 2-3 days. This is typically the top end of what we’d charge as it involves the most amount of work, so it is there to give you an idea. Upgrading the stove and adding wood effect beams and such like will obviously add to the cost.
If you want more information, please check out this page and get in touch.