Fitting a Twin-Wall Chimney System in Dorset

When installing a twin wall system you will nearly always want to start from the bottom and from here work your way up, as the straightest way is the most efficient way for a chimney system.

It is important to always calculate the chimney system before you start, this can be done your self or we can do this for you using our twin wall calculation software. Often the most efficient route if the stove is against an external wall is to go out through the wall and straight up and around the soffits.

Stainless steel twin wall system we fitted a couple of years ago near Poole in Dorset

Fitting a Twin Wall Rigid Chimney system going out through the wall

As stated already when planning and installing a twin wall system we start from the bottom and go up.

So the first step is to Lay the hearth, as discussed this can be a 12mm hearth as long as you are using a Stove that is 12mm hearth compatible.

A double-sided stove can make a great centrepiece.

Next, we position the stove so as to ensure it is the correct distances away from combustibles and that it has a minimum 225mm in front of the stove for spillage we recommend 300mm where possible.

From here we can work out exactly where we want the stove to go through the wall by building up the first few components such as the stove to twin wall starter piece, the first length and then the elbow. From here you can see the line that the pipe will need to take to exit the wall at a 45-degree angle.

Checking that the hole is in the correct potion before and after the hole making process.

Remove the starter components and position the wall sleeve over your marks, draw around the wall sleeve , cover up the stove or remove fully and drill holes around the wall sleeve at a 45-degree angle , this will give you the perfect hole if you’re not sure on your angles then you can always ask someone to hold the wall sleeve next to where you are drilling to help you. You will need a power drill with a long drill bit to keep the angle going through the two skins of brickwork and the cavity,

I find that it is worth going out in one or two places fully such as the top and the bottom and then remarking from the outside and doing the hole from there, this way you won’t chip off too much brickwork, we use a décor ring on both sides to hide the imperfections but they only give you a few inches for mistakes.

Once the hole is made, check that the wall sleeve will fit and then put the stove back and build the twin wall system up and out through the hole,

Log Burner Fitting Poole

Building a fake breast around a twin wall install for a more traditional look.

be sure to check that everything is level before you start adding brackets to the outside wall, the bottom bracket taking most of the weight needs to be a table bracket.

From here you can build up your twin wall system quite easily by putting brackets up as you go, you might need 2 more 45-degree bends and different length brackets if you are going around the soffits.

Be careful when attaching to the facias as they can we weak if there is nothing behind them, you might find that you need to use a bracing bracket from the roof if the height is over 2m unsupported depending on your system specifications.


Twin wall Chimney in Poole Dorset

External twin wall chimney


Once the twin wall chimney is up and well suported you can fill in the hole using bits of brick and waterproof mortar to whether it in, it is also worth using a decor plate to cover up the hole and this also helps with water penetration if sealed up well with clear heatproof adhesive.

Fitting a Twin wall Rigid Chimney system going straight up through the house

Going straight up through the house if it’s on the bottom floor can be quite a bit more work. You might have two floors to go through and also a roof if it’s a 2 story house but then again it might just be a single roof if it’s a kitchen extension or a conservatory.

Going through a ceiling you will need to be aware of where the joists are as you will want to avoid these if possible. It might be necessary to cut and support a joist but it’s not something you should attempt unless you know what you are doing.

When going through a roof or ceiling you will need a vent fire support as well as ceiling and rafter supports.

Coming through the roof again is a very precise activity,  you most certainly don’t want a leak or worse a fire in the loft.

It’s worth bearing in mind that when you pass through a level you will often need to re-route your pipe to go through the next set of rafters. To do this two 45degree bends will come in handy.

twin wall 45 bend

Twin wall often needs to go around bends fo this you will need to 2 45-degree bends

You will need rafter support to secure the pipe while going through the roof. Where possible its best to find the area that you need to go through from the inside and then with the help of a screwdriver pushed through the felt go around onto the roof and remove the tiles.

Once you have the area cleared you can add the flashing over the pipe If you use a flexible flashing and storm collar then you will find that the roof angle doesn’t need to be an exact match whereas a lead flashing has a lot less room for movement. A flexible flashing also will give a tighter seal.

Seal with heatproof silicone and cut and replace the tiles.


Lead flashing for log burner

The lead flashing needed adjusting as it wasn’t the exact angle


You might need to get a roofer in to do the flashing if you’re not confident.

When doing a flat roof you will need to seal it with a torch on felt, for this you will need to hire a gas torch.

Flat roof flashing for twin wall in Poole

Making water tight a twin-wall system using aluminium flashing

If you go through a room you will most likely want to box in the pipe as it’s not that attractive, alternatively, you can fence it off with wire but this looks worse unless its the industrial look that you are going for. You will need to do one or the other so as to stop people leaning combustible materials against the pipe.

Cage for twin wall chimney

Caging in for a chimney system instead of boxing in.

Ensure you have vented fire stops and supports in place and that the pipe is at least 60mm away from any combustible material. You will need to add inspection hatches every 1m of any boxing in.

Going through a loft it’s important that you add a wire cage to stop people leaning anything against the pipe, as this is a potential fire hazard.

The termination point

You need to ensure that you finish your twin wall system terminates 2.3m away from the roof or any windows when measuring horizontally.  Or 60cm above the ridgeline to ensure that the turbulence doesn’t create any downdraft. Be also aware of surrounding buildings and trees.

The termination point height should ideally be 4.5m min from the top of the stove for an adequate draw.

Twin-wall chimney fitted in Lilliput Poole

Using bracing brackets as the roof was really steeply pitched.

Twin wall bends allowance

While you want to go as straight up as possible you will have to go around things from time to time but make sure you never use more than four 45-degree bends, and of those 45-degree bends, you need to ensure that no more than twenty per cent of the total length is at a 45-degree pitch.

If you are planning on taking on a twin wall system remember to get your plans checked by the building control officer before you start.

For calculations and further information about installing your Twin-wall system contact us on 01202 748 445

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