How to install a wood stove chimney through a wall or roof (Non-chimney installs)

Quick Step Guide to Non-Chimney Installations

1) Start your system with a length of Single Wall, Vitreous Enamel Stove Pipe in the same diameter as the outlet on the stove.

2) Connect a Single to Twin Wall Adapter to convert from this Single Wall Pipe to insulated Twin Wall Pipe a minimum of 150mm before reaching any surface (ceiling or wall).

3) The remainder of the flue system must be in insulated Twin Wall Flue Pipes only from this point onwards.

4) For internal systems passing up through the property, you shall require Joist Supports and Ventilated Fire Stop Plates for every ceiling the system passes through.

5) For external systems rising up the outside of the property, this must pass through the wall at no greater than 45° with a Wall Sleeve lining the passage through the wall itself. Another 45° Elbow or Tee Piece can be connected once through the wall to turn the system vertically.

6) Joist Supports are sufficient for supporting an internal system with Adjustable Wall Brackets providing the support for an external system every 2m maximum.

7) For internal systems, a Roof Support, Silicone Flashing and Storm Collar are necessary for safely passing through any roof type.

8) Once through the roof Surface (internal) or past the roof/gutter line (external), Twin Wall systems must attain a minimum vertical height of 1m to create a minimum horizontal clearance of 2.3m.

9) Finish off any system with a choice of Rain Caps or Anti-Wind Cowls.

What Is a Non-Chimney Installation?

Planning a brand new flue system may seem daunting at first but with this guide we hope to remove the mystery of creating a twin wall installation to suit your home, avoid the common pitfalls and understand Approved Document J of the building regulations in language we can all understand (highlighted in bold).

If you do not have an existing chimney in your home to install a wood burning stove into, a new class 1 chimney system needs to be created using Twin Wall (or Double Skin) insulated flue pipes. This Twin Wall flue system will either run internally (up through the floors of the property and exiting through the roof surface) or externally (passes out through an external wall and rising up the outside of the building, clearing the gutter line), depending on your preference and the room in which you wish to locate your new stove.

Why Do I Need To Use Twin Wall Flue Pipes?

The three main benefits of starting a flue system with single wall pipes are they tie in with the traditional aesthetic of a log burner, can withstand the temperatures produced in the outlet collar due to the enamel and they transfer a lot of heat to the outside of the pipe, maximising the heat output produced by the stove. However, it is for this very reason that it is unsafe and even illegal to pass a single wall pipe through a wall, ceiling or roof as the outer surface will become far too hot and may transfer that heat into nearby combustible materials such as joists and furnishings.

This is where Twin Wall flue steps in. As the name suggests, these pipes feature separate inner and outer skins with a layer of insulation between to prevent heat transfer to the outer edge of the pipe, making them safe to run through any surface type and compliant with Approved Document J of the current Building Regulations. By keeping the gases within the flue warm, this encourages the draw to continue rising and prevents cooler air outside from negatively affecting this

How to start your insulated flue system

No matter the flue system you are going to create, they should all begin with a length of Vitreous Enamelled, Single-Wall Stove Pipe. This will measure the same internal diameter as that of the outlet collar of your stove (5” or 6” are standard but it may be 7” or 8”) and will slot inside the collar via the male spigot at the bottom end of the pipe. And the male, narrow spigot must only be at the bottom of the pipe and not the other way around, no matter if it seems to fit better upside down as you will not be able to connect your Single to Twin Wall Adapter to the wrong end. The diameter of the outlet collar is the minimum diameter for the entire flue system and you can never reduce this. Simply seal this connection with at least 1200°C heat resistant cement or silicone sealant. We supply industry leading brands such as Vitcas, Kos and Envirograf for this very job.

It is possible to skip this step and connect the Twin Wall flue directly to the outlet collar with an Adapter however, this may over time tarnish the finish of a stainless Twin Wall pipe or remove the coating entirely from a black Twin Wall pipe.

Also important to bear in mind; hot gases want to rise and therefore a straight vertical length at the start of the system encourages this. The sooner your gases hit a bend in the flue, the less able they will be to negotiate through it without depositing soot and possibly slumping back down the chimney so exiting the top outlet of the stove is ideal but you are allowed to exit the rear outlet via a Single Wall 90° Tee Piece if necessary.

Which Twin Wall Adapter Do You Require?

As much Single Wall pipe can be used from the stove as required (but should be kept to a minimum) but you must convert from this to Twin Wall insulated flue a minimum of 6” (150mm) prior to reaching the underside of your ceiling / roof or inside of your wall.

The crucial building regulation to consider at this point regards the internal diameter of your flue system. The minimum internal diameter for a flue system when you reach the Twin Wall section is 6” (150mm) and this is easily attained by selecting the correct Single to Twin Wall Adapter. It is a common misconception that a system increases when it converts from Single Wall to Twin Wall flue simply due to the change in the overall size of the pipe but this is incorrect. We are only concerned with the internal diameter of the system and so to maintain the same diameter throughout (i.e. 5” to 5”) you require a Single to Twin Wall Adapter and to increase the diameter (i.e. 4” to 5”, 5” to 6” etc) only then do you require an Increasing Single to Twin Wall Adapter.

The only exception to this rule is for DEFRA approved stoves. Due to the cleaner burning nature of these models, they alone are permitted to remain at 5” diameter for the entire flue system and still comply with Approved Document J of the Building Regulations.

Now your system has begun correctly and you have successfully made the conversion from Single Wall pipe to Twin Wall insulated, you must plan the route your flue will take; internal, external or a mixture of both.

How To Install a Wood Burning Stove Chimney Through a Wall (External)

The first common error made when installing a wood burning stove chimney through a wall is that you can exit out from the rear of the stove and run horizontally, directly through the wall to the outside. This is incorrect as there must be no horizontal runs totalling greater than 150mm in length in a flue system or 90° (right angle) turns. Again, this is due to hot gases needing to rise and if forced to travel horizontally, they shall lose their heat, deposit soot in the pipe and hit the right angle turn like a barrier and slump back down the system.

To comply with the building regulations, a 90° Tee Piece (or elbow) can only be used directly in the rear outlet of the stove and absolutely nowhere else in the system. All bends can be no greater than 45° to allow the gases to continue rising freely and use these sparingly as you are limited to a maximum of 4 turns per system.

There are 4 Twin Wall items necessary to safely pass your system through an external wall; 45° Wall Sleeve, two Finish Plates and a Fixed or Adjustable length of Twin Wall pipe, long enough to pass clear through the wall whilst rising at that crucial 45° angle. The Sleeve protects both the pipe and the material of the wall from the thermal expansion of the pipe and the Plates tidy everything up inside and out. This is only applicable for passing through standard, non-combustible cavity walls. Should the wall consist of combustible materials (i.e. a wooden structure, you must allow for a safe clearance all the way around the flue of 60mm although this does vary depending on the brand of Twin Wall used).

Now your Twin Wall system has successfully exited the property, we need to turn the system vertically which is easily achieved with (ideally) a 135° Tee Piece & removeable Cap or a 45° Elbow. The benefit of the Tee Piece & Cap is it allows essential access to the external run of flue for cleaning and draining of condensates and rain water. The 45° Elbow does not offer this access.

For the optimum draw on your stove and compliance with the building regulations, the overall length of your flue system must attain a minimum length of 4.5m from stove outlet to Rain Cap. This does not pose an issue for chimneys on two storey homes but must be considered for shorter installs such as conservatories and summer houses. Depending on the length of flue selected for running through the wall dictates how far outside the system will sit from the outer surface. Quite often, this is only 50-80mm between outer wall surface and the pipe for aesthetic purposes and use with the shortest (and cheapest) Wall Brackets.

There are two important points to consider when calculating the lengths of Twin Wall flue required to successfully (and legally) run the system up the outside of your property wall; 1) the overall height of the finished flue 2) do you need to negotiate around any obstacles such as windows or overhanging soffits? Remember building regulations only allow for a maximum of 4 turns in the entire chimney and 2 have been occupied already (before and after passing through the wall), therefore you can only use 2 more elbows to create an offset around an obstacle, usually the gutters / soffits. Should you find you have other obstructions then the chimney will need to be re-sited unfortunately.

The only form of support typically required for an external Twin Wall system are Adjustable or Fixed Wall Brackets secured to the outside wall of the property, securing the flue system every 2m at very most. These brackets can be either fixed at 50mm or adjustable such as 50-80mm, 80-130mm, 130-210mm (that’s distance from wall to pipe) or longer still but for long external runs of 4m or more, it is highly recommended to secure an Adjustable Base Support at the bottom of the system as an extra anchor for the long run of Twin Wall above in addition to the Brackets. For supporting a flue past the roof line, there is a selection of Guy Wires, Stays and Structural Locking bands.

How High Must a Flue System Rise?

The final aspect of your flue system is ensuring you attain the necessary overall height and choice of suitable terminal…

For an external Twin Wall system rising up any wall other than a gable end, you must consider two building regulation requirements:

  • The flue must rise high enough to create a 2.3m horizontal clearance between the top of the last length of pipe and the surface of the roof

AND

  • It must rise at least 1000mm once passed the roof line / soffits / gutter (basically, the lowest section of the roof surface) OR as high as the ridge

If rising up a gable end, the above still applies but with one caveat:

  • If the system passes the edge of the roof within 600mm of the ridge itself, then the system must rise at least 600mm above that ridge

A couple of additional points that often occur concern adjacent buildings and dormer windows:

a) If the system looks due to terminate below a window in a pitched roof or within 2.3m horizontally of any openable window, sun light or dormer, then you must extend the system 1000mm above the window

b) Any adjacent buildings within 2.3m of your flue system must unfortunately also be cleared. The flue will have to rise at least 600mm above the highest point on the adjacent building

And finally, simply secure a Rain Cap, Anti-Wind Cowl or any choice of Twin Wall terminal available in your range of Twin Wall. These are fairly straight forward to understand but please bear in mind, no Rain Cap is water tight and Anti-Wind Cowls are only advised for longer systems that create a sufficient draw and are located in open or exposed areas such as hillsides, by open land or waterfronts.

How To Install a Wood Burning Stove Chimney Through a Roof (Internal)

A typical internal Twin Wall flue system starts identically to that of an external system from the stove collar to the Twin Wall Adapter, as outlined above. From this point however, we are not heading through a wall but rather travelling up through the floors of a building to eventually exit through the roof surface.

For each ceiling / joist you are to pass through, there are 3 essential components to purchase; 2 Ventilated Fire Stop Plates and a Joist Support. Sometimes sold as a ‘Fire Stop Spacer’, Ventilated Fire Stop Plates must be secured into the ceiling or the floor whenever the system exits and/or enters a habited space. Basically, this boils down to requiring these plates everywhere except for the floor of the attic but even there it is recommended. And why ventilated plates? This is to ensure cool air can pass between the floors keeping the outer edges of the flue as cool as possible but still delaying the potential spread of fire. Solid Fire Stop Plates are for gas systems only. And as the name suggests, the Joist Support screws to each joist and forms a bracket for the system roughly every 2.5m. Therefore, in a typical two storey home, you shall require 4 Ventilated Fire Stop Plates and 2 Joist Supports.

No matter which type of system is being installed, any joints between pipes must be accessible and not buried within walls or joists and the Twin Wall must be boxed or caged in on the upper floors.

The most effective form of flue system is a horizontal, internal chimney however, you do have up to 4 bends or turns allowed within the system should you need to negotiate around an obstacle along the way in the form of 45° Elbows (never 90°) and a Roof Support should be used on the rafters as a final form of support.

It is now time to pass the Twin Wall flue through the roof surface itself and there are three important stages to this:

Firstly, remember there is a safe clearance to combustible materials that must be maintained around all twin wall flue and so a 6” Twin Wall pipe will require a hole measuring approx. 320mm across (60mm clearance + 200mm outer diameter of pipe + 60mm clearance).

Secondly, selecting the correct Flashing for your roof is critical. The Flashing itself is a silicone or EPDM rubber cone that sits atop a base (traditionally lead but more likely aluminium these days) and can be cut back to tightly grip around the flue to provide a weather proof seal on the roof. However, Flashings do not need to be confusing as they basically fall into one of only two categories; either suitable for a tiled/slate roof or not. Flashings for tiled roofs have a large base which needs to be secured underneath the tiles whereas Flashings for every other roof type (flat, corrugated, wooden etc.) has a small base designed to be secured on top of the roof surface.

Thirdly, combine your Flashing with a Twin Wall Storm Collar to sit approx. 100mm above the Flashing as an extra layer of protection against the elements.

How High Must a Flue System Rise?

And the final aspect of your flue system is ensuring you attain the necessary overall height and choice of suitable terminal…

For an internal Twin Wall system rising up through the roof surface, you must consider all four of the following building regulation requirements:

  • The flue must rise high enough to create a 2.3m horizontal clearance between the top of the last length of pipe and the surface of the roof

AND…

  • It must rise at least 1000mm once through the surface of the roof

ALSO CONSIDER…

  • If the system passes through the roof surface within 600mm of the ridge itself, then the system must rise at least 600mm above that ridge
  • If the system looks due to terminate below a window in a pitched roof or within 2.3m horizontally of any openable window, sun light or dormer, then you must extend the system 1000mm above the window

And finally, simply secure a Rain Cap, Anti-Wind Cowl or any choice of Twin Wall terminal available in your range of Twin Wall. These are fairly straight forward to understand but please bear in mind, no Rain Cap is water tight and Anti-Wind Cowls are only advised for longer systems that create a sufficient draw and are located in open or exposed areas such as hillsides, by open land or water fronts.

19 thoughts on “How to install a wood stove chimney through a wall or roof (Non-chimney installs)”

    1. Thanks very much for your enquiry. I have passed this on to our Customer Services Team who will respond to you directly and provide you with the advice needed.

  1. Im costing to install a Hwam 2360 in my bungalow with a top mounted flue direct up (no bends). I cannot find any information telling me if I should purchase a 5″ or 6″ flue system. Which one should I be costing?

  2. My ‘Orangery’ type conservatory will have a 500mm (external) brick pillar in the corners, including where the log-burner will sit. This is obviously a twin-course plus insulation space pillar, rising up from the dwarf-wall that goes all around the base of the conservatory.
    The roof is complex (all glass, double-hipped i.e. pyramid design) so, I want to take the single-wall pipe vertically from the stove-top, then kink 135 degrees, go through the small area of wall that it the brick pillar and kink 135 outside, back to vertical and go 1m above the Consersatory roof (this will be happening 4m from the house).
    I know from your site that I need to use a converter to connect twin-wall to my vertical pipe 4″ before I enter the wall.
    So, my question is, will the pillar be thick (wide) enough ?
    I could I ‘move the window-frames along’ and have another brick-width on the side on the pillar that I will put the twin-walled pipe through and 1 brick-width less on the pillar by the house wall (which is already 1.5m wide).
    Any other thoughts ?
    Thanks.

  3. Hi
    I think that I understand the installation of the flue and wood burning stove ,
    what I am unsure about is the type of house that we have got was built in 1986 app
    it has a open fire ,I believe it is called a fire box as it has got a damper there is a flue that appears to sit on on top of it .this I am told is a Parker flue system ,it looks like they are in metal sections,I have been told these are filled up with clay balls it also sounds very heavy at the top there is what looks like a square chimney stack I have not got a clue about this .
    How do I remove it ?
    Thanks for any suggestions.

    J P Rose

    1. Our advice in these situations would always be to contact a local HETAS registered installer to visit your property and advise on how best to remove the existing system

    1. When it comes to the compatibility of Twin Wall, you must stay within the same brand I am afraid and so this Convesa H-Cowl will not connect to a Dinak flue system.

    1. We offer an extensive range of stoves perfectly sized for a summer house and can advise on a safe and effective twin wall flue system

  4. The outside of my wall where the pipe passes through is cemented round the flue i hear bangs when it is cooling down is this ok

    1. Banging is not typical and I would advise you seek the assistance of a HETAS registered engineer. It may be that the incorrect plaster has been used and you are not allowing for the thermal expansion and contraction of that pipe.

  5. After my flue passes thru the wal does the chimney have to go staright up? or can it angle off for a few feet? Instead of going thru my roof overhang can I angle out to the edge of the overhang then up the rest of the way?

    1. You are permitted a maximum of 4 turns in any one flue system and typically as far as an external Twin Wall flue system is concerned, these would be used 1 inside and 3 outside; 1 to turn the sytem vertically and 2 to negotiate around guttering or soffits etc.

  6. Do you recommend using/ installing the ‘optional’ external air intake at the back of the stove, or is this not really necessary?
    I’m struggling to find out much info for this.

    1. It really depends on which stove you are interested in and where you are installing. External or Direct Air allows for a constant supply of fresh air for your fuel but not utilising it wil mot impact on the performance of your stove necessarily.

  7. My twinwall flue is going through a wall. When using a 45 degree wall sleeve, should I pack it with Rockwool between the sleeve and the flue pipe, and leave a 60mm airgap around the outside of the sleeve. Or should I leave an airgap between the sleeve and flue pipe and pack Rockwool around the outside of the sleeve?

    1. You can fill the gap between the Twin Wall Pipe and the Wall Sleeve with rockwool but most keep this gap clear.

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