How to Install Twin Wall Flue Through a Ceiling or Wall

A twin wall flue system is incredibly versatile - you can run it through walls and ceilings, both externally and internally, to suit your needs!

However, as with any chimney installation, safety always comes first. When a professional installs your twin wall flue system, there are a number of regulations and recommendations they need to keep in mind.

There are also plenty of specialist parts you need to find, too - all of which you can find online at Trade Price Flues!

Want to get a better idea of how to pass a twin wall flue through a ceiling or wall? Keep reading...

Use twin wall pipe only to pass through a ceiling or wall

You can ONLY use twin wall pipes to pass through a ceiling or wall. This type of pipe is highly insulated, so is designed for this purpose. A single-wall, non-insulated stove pipe will reach an extremely high temperature, so is not safe for passing through a wall or ceiling.

You must also be sure you convert from single wall stove pipe to twin wall flue pipe at least 150mm before passing through a wall or ceiling.

Check the required distance to combustibles

When installing a twin wall flue through a ceiling or wall, it’s essential you know the required distance to combustibles. This is vital for preventing hot surfaces from escalating into house fires.

As twin wall flue is so well insulated, it will have a smaller requirement than a single wall pipe. This will be specified by the manufacturer who produces your flue.

Installing twin wall flue through a ceiling into another room...

When you are taking your twin wall flue pipe through a ceiling into another room above, you need to use ventilated firestop plates.

This is made up of three pieces, as shown below. The top plate sits on the opposite side of the ceiling - the floor of the room above. It should rest on the floorboard joists and be screwed into place.

The bottom plate is then screwed onto the ceiling below - a bit like a sandwich!

The twin wall pipe goes through the hole in the middle, and is held in place with the clamp, which sits on top of the top plate.

Pictured: Ventilated Firestop Support Set - Stainless Steel

Avoid joining two pieces of twin wall pipe together in the void between the ceiling and floor below it. This is covered by building regulations, as they must be visible for inspection.

Installing twin wall flue through a loft space...

When fitting your twin wall flue through a ceiling into your loft space, your firestop plates don’t need to be ventilated - assuming the beams are exposed and there is no floor surface! If you do have a flooring installed in your loft space, then you will still need ventilated plates.

It is required that your twin wall flue be ‘boxed in’ when passing through a loft space. This doesn’t have to be anything extensive - often a simple frame with mesh covering is used. Just make sure it outlines the required distance to combustibles.

Installing twin wall flue through the roof...

Now you are passing your twin wall flue through the roof, you will need to use a roof support. This gives some support to the system via the rafters.

Pictured: Roof Support - Stainless Steel

Once you have placed the twin wall through the roof with the right support in place, you will need to seal it with the appropriate chimney flashing. This prevents rain, snow, hailstones - or any other any type of moisture! - from getting into the gap left between the chimney and roof.

At Trade Price Flues, we have a range of different types of chimney flashing to suit all roof styles, whether you have roof tiles, slate or a corrugated surface. You can choose from flexible roof flashing, lead flashing or even our unique Dura Flash.

Pictured: Seldek Aluminium Black EPDM flashing

We even have a blog post with everything you need to know about chimney flashing which has more information!

Installing twin wall flue through a wall…

A common way of installing a twin wall flue system is to place it through the wall of the room the stove is installed in. It is then run straight up the side of the house, meaning there are often fewer obstacles to deal with along the way.

When passing a twin wall flue through a wall, you can only use an angle of 45 degrees or less. You can use a Tee piece of twin wall flue to turn the flue back to vertical easily. Building Regulations also ask that you use a metal sleeve around your flue when passing through the wall. This is to stop the pipe becoming damaged through potential heat expansion.

Remember, you are only allowed a total of four bends in your twin wall flue system - this will use up two of them!

Pictured: Stainless Steel Twin Wall Long Branch Tee 45 Degrees

It’s also recommended that you use a base support at the bottom of your flue once you have passed through the exterior wall. These provide good structural security for the rest of your flue system.

For an external twin wall flue system, you should then use an adjustable wall bracket at least every 2 metres.

Pictured: Base Support Set - Stainless Steel

While not a regulation, it is best practice to allow the first section of pipe that exits the stove to run completely vertical for at least 600mm. This creates the best draw for the chimney. It’s also advised that you use no more than 2m of single wall stove pipe. This is because the stove pipe isn’t insulated, so your flue will be losing more heat than necessary which can lead to poor draft.

Don’t forget, as we said earlier, you need to convert your single wall pipe to twin wall pipe at least 150mm before passing through a wall, too. To do this, you will need the right adapter to join them together.