Chimneys and Flues: A Guide to the Building Regulations Advice Sheet Chimneys and Flues: A Guide to the Building Regulations

Any existing chimney and flue must be cleaned and visually inspected to ensure it is
structurally sound, clear of any blockages and obstructions from fallen masonry or old birds’ nests, and that it is of the required size? for the new stove, fire or fireplace being fitted. It may also be necessary for the chimney to undergo a smoke test, using a smoke bomb, to check the integrity of the chimney lining and its gas tightness. If the lining is found to be porous and leaky you will need to either have the chimney re-lined, fit a new prefabricated liner or use a manufactured flexible metal liner. Do make sure the liner is matched to your new fire or stove and suitable for the fuel you will burn.

Chimney & Flues a guide to building regsYou will find the following additional Information useful and helpfull!

The Essentials to Understanding, Fireplace, Chimney and Flue Sizes

I Don’t know, I’m Fluless. Which Gas Fire, Which Flue (Class 1, Class 2 etc)

Chimney Height and Draft

Anatomy of a Chimney


Chimney or Flue Size (Cross Section)  

Make sure the diameter of the flue is matched to the appliance you are having fitted. You should always consult the manufacture’s handbook to find the recommended flue size. If you are fitting a stove, the general rule is the flue should never be smaller than the stove’s outlet collar. The building regulation guidance notes state the minimum flue size for stoves is:

Up to 20kW burning smokeless fuel – 125mm diameter 

Up to 30kW burning any fuel –minimum 150 mm

If the stove is registered as Defra exempt and has 125mm outlet collar then a 125mm flue may be used. In effect the regulations recommend a minimum flue size of 150mm for any stove including stoves in which wood can be burnt, unless the stove is Defra exempt link to Defra exempt stove suppliers. Many stoves are manufactured with a 125mm outlet collar which then has to be upsized to 150mm upon installation. In essence any stove that is going to burn any other fuel than smokeless fuel or is Defra exempt has to have a minimum cross sectional flue size of 125mm and many installers advise all stoves be fitted with a flue with a minimum cross sectional size of 150mm. Even if your stove manufacturer recommends a 125 mm flue and liner please check that you HETAS  registered installer is willing to fit a 125mm chimney liner before purchasing as many will not and opt for the 150mm liner. 

Where a 125mm stove outlet is upsized to fit a 150 mm flue liner it is possible for the flue to accumulate a lot of soot and deposits before the performance of the stove becomes impeded alerting the homeowner the chimney needs sweeping. So it is important if you do install a 150mm flue liner to a 125mm stove outlet you keep to a regular chimney sweeping schedule to prevent the danger of a chimney fire. ( see Essential  Must Do's of Solid Fuel Fires)   

For open fireplaces the flue or chimney size should be a minimum of 185mm square or 200mm in diameter.  If bituminous coal is to be burnt as the main fuel it is recommended the flue size should be no less then 225mm in diameter.   

It is important for the fireplace opening to match the chimney size. As a rule of thumb the chimney, fireplace opening, should

be not less than 1/7th to 1/8th of the area of the fireplace opening if the flue or chimney height is above 6 metres. A 225mm diameter flue of a similar height will support a fireplace with a dimension no greater than 550mm by 550 mm (22inches by 22 inches).  For lower chimney or flue heights such in the case of a bungalow the ratio should be reduced by a 1/6th. Factors other than the chimney height may also need to be taken into consideration such as, the construction of the smoke chamber, and the location of the chimney. Larger fire openings will require bigger flue sizes for example a flue for doubled sided fireplace should be calculated taking into account both sides of the fireplace.   


Chimney or Flue Size (Height)  

A height of 4.5 meters is recommended as a minimum height for a chimney stack or flue although other factors need to be taken into consideration such as the roof height, adjacent buildings, roof windows and the material the roof is constructed from.   The top of the chimney or flue should be above the height of the building to prevent any wind eddies from interfering with the draw of the chimney and the evacuation of the smoke (see diagram 1 and corresponding table 1).   Chimney height is measured to the termination of the chimney or flue and does not include any rain caps, bird guards or spark inhibitors etc that may be fitted.

Diagram 1: Flue and Chimney Outlet Heights for Standard Roofing Materials


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Table 1. Flue Outlet Heights for Standard Roofing Materials

Point where flue passes through weather surface1 & 2

Clearance to flue outlet


At or within 600mm of the ridge.

At least 600mm above the ridge.


Elsewhere on a roof (whether pitched or flat).

At least 2300mm horizontally from the nearest point on the weather surface and

a)       at least 1000mm above the highest point of intersection of chimney and the weather surface;


b)       At least as high as the ridge.


Below (on a pitched roof) or within 2300mm horizontally to an opening roof light, dormer window, or other opening.

At least 1000mm above the top of the opening.


Within 2300mm of another building whether or not beyond the boundary. NB 3

At least 1000mm above the adjacent building.

NB .

  1. The weather surface is the building external surface, such as its roof tiles, or external wall
  2. A flat roof has a pitch of less than 10 degrees
  3. The clearance given for A or B, as appropriate, will also apply

In summary in order to satisfy the building regulations if a flue or chimney exits the roof more than 600mm from the ridge of the roof then the horizontal distance from the top of the flue or chimney should be at least 2300mm above the roof or ridge.  If the chimney or flue exits the roof within 600mm horizontally of the ridge then the flue or chimney should be a minimum of 600mm above the ridge. Diagram 2 shows the area on a traditionally constructed non-flammable material roof where the flue must not terminate.  The flue or chimney outlet must be above and outside of the shaded area.   

Diagram 2 Flue or Chimney Outlet Highlighting Unsafe Shaded Area


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The height of the chimney or flue will be determined by the pitch of the roof. Table 2 offers an indication of the different flue and chimney stack heights necessary to give the mandatory required roof clearances when the flue or chimney exit point is beyond 600mm of the ridge.    

Table 2: Typical Flue and Chimney Heights by Roof Pitch

Pitch of Roof

Termination height above exit point from roof

25 degrees


30 degrees


35 degrees


40 degrees


45 degrees


50 degrees



For buildings with flammable roofs such as felted, thatched, or shingle roofs the flue or chimney the regulations are different. The flue or chimney must terminate at a minimum1800mm above the combustible surface as shown in diagram 3 below.  This means if the pitch of the roof is below 38 degrees


Diagram 3: Diagram 3: Flue and Chimney Outlet Heights for Flammable Roofing Materials


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degrees the height of flue or chimney stack will need to be at least 1800mm above the roof when not close to the ridge. If the pitch of the roof is 38 degrees or greater the termination point of the flue or chimney will need to be 2300mm horizontally from the roof line and roofing material.    Although the building criteria must be met to satisfy the local building regulations this does not guarantee the chimney or flue will perform satisfactorily. The chimney must be of an adequate height and free from turbulent air to draw properly for more information regarding these issues see the accompanying articles found on the Contents by Theme Index


By Phil Cleaver


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