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Flue outlet / termination heights

doc j

Flue outlet heights - in Document J, pages 30-32

Stove building regs pages:
Stove building regulations intro
Notice plates
CO Alarm
Installation and Building Control
Stove hearth sizes
Flue outlet heights
Flue sizes
Ventilation for stoves
Flue pipe - distance to combustibles and heat shielding
Flue pipe - recommended length
Twin wall flue - boxing in
Changing from single skin flue to twin wall at a ceiling
Lining existing chimneys
New chimneys

Flue outlet / termination heights in general

The flue gases from the stove must be able to discharge freely and not present a fire hazard, whatever the wind conditions. In order to discharge the flue gases freely you need a good draw.
To get a good draw you need to raise the flue outlet away from the roof - a surface (like a roof) creates wind eddies and slows the wind down thus reducing draw, so by raising the flue outlet you bring the top of the flue up into unimpeded air flow.
To make sure that there is no hazard the flue outlet needs to be sufficiently far away from the roof, adjacent buildings, and roof windows.

Where to measure the flue termination / flue outlet height from

where to measure the flue termination height from

The flue termination is measured from the very end of the chimney / flue pipe where the flue gasses discharge. Rain caps, birdguards, etc, are not included within this height.

Flue outlet heights for standard roofing materials

flue outlet height diagram
flue outlet height table

These are the recommended heights that you can use as a way of meeting the building regulation requirements.

Very simplified these recommendations can be summed up in the following way:

The horizontal distance from the top of the flue pipe to the roof should be at least 2300mm or above the ridge
The flue termination is within 600mm horizontally of the ridge in which case the flue should be at least 600mm above the ridge.

Flue outlet height unsafe areas diagram

flue outlet height box

The grey area in the picture shows where the flue must not terminate on a roof with standard (non flammable) roof material.

The information in the diagram above was supplied and verified by Barie Meridew (flue and chimney consultant).

Typical flue outlet heights

Where the flue is not close to the ridge this rule gives the following flue outlet heights:

Roof pitchFlue outlet height above roof penentration
25 degrees1070mm
30 degrees1330mm
35 degrees1610mm
40 degrees1930mm
45 degrees2300mm
50 degrees2740mm

In some cases the requirements may still be met by using flue outlet height lower than the recommended height, whilst in other cases a higher flue outlet height will be needed to give a good draw.

Verified for Stovesonline by Mike Loud (independant flue consultant):
"Flue termination heights above the roof are a classic example of where comformance with the recommendations does not guarantee that the requirements will be met and where installations falling short of the recommended heights function perfect satisfactorily."

Flue outlet heights for combustible roofing materials

When your roof is made of combustible materials like thatch or shingles the recommended distances should be increased to those shown below:

flue outlet heights combustible roof

This means that below 38 degree roof pitches that the height of the flue outlet should be at least 1800mm above the roof when not close to the ridge, and above 38 degree roof pitches the flue outlet height should be 2300mm horizontally back to the roof (flue height figures are given in the table above) where the flue is not close to the ridge.

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