Building a Stove Recess & Chimney Stack using Isokern Liner System

Building a Stove Recess & Chimney Stack using Isokern Liner System

Building a chimney stack should be a simple affair, it is covered well in building regulations document J and many drawing examples are available at the click of a button, You construct a masonry (brick or block) chimney breast to the dimensions to suit your design, room size and type of appliance and size the flue accordingly. However we have found over the years between the builder and the architects very rarely is it done correctly for the customers needs. Here we will run through a few key points of vital information we are frequently asked when discussing flues with customers, also hopefully cover a few myths as well. There is a useful video also. 

Lets start from the ground up.


All Masonry Chimney breasts to be built must have a constructional hearth (or sub hearth). In simple terms this is a concrete slab that is 125mm thick, project 500mm in front of the chimney breast face and be 150mm wide than the builders opening / stove recess. This is an old regulation requirement really, aimed at open fires as they put significant amounts of heat downwards and could easily set fire to any timber beneath. However modern stoves will generally not get the hearth beneath over 100°c so are mostly now approved to only stand on a superimposed hearth of at least 12mm thick, which does not require a constructional hearth when in a free standing situation..

Constructional Hearth Dimensions

A superimposed hearth (or decorative hearth) is the non combustible material on top of sub hearth, there are still requirements for this to be non combustible, and of suitable dimensions to suit the appliance manufacturer's requirements. Generally it is 300mm in front of the appliance and 150mm either side wider than the appliance, you can get this down to 225mm when installing a closed appliance and the stove manufacturer also states that 225mm is suitable. Hetas do recommend that the hearth should project more than the door opening size, but this is only guidance. When there is a discrepancy between regulations and manufacturers instructions You should always go with the most stringent requirement. If the stove does not get the hearth over 100° then normally in a Free Standing situation (no chimney breast) then a sub hearth is not required and often a 12mm thickness will be adequate.

Superimposed Hearth Dimesions

The fireplace recess (builders opening or chamber) should be constructed as per the measurements and guidance below. If the fireplace is on an internal wall then it must have a back of at least 200mm thick, if on a cavity wall or back to back with another chamber this can be 100mm.

The recess dimensions will be dictated by the appliance type to be installed. For example, a 5KW wood burning stove generally you will create a recess of approximately 900mm x 900mm x 350mm deep. Recesses that require a finished plaster look should be fully lined in A1 rated non combustible materials (not pink plasterboard!) such as Promat Promafour, normally the recess will not be skimmed/plastered as this will normally craze or fall off and look unsightly. You can finish in many materials including cast iron panels, Vermiculite panels or simply porcelain or natural stone tiles.

Flue diameters are a constant discussion with chimney construction, mainly it depends on what appliance and what fuel you will be installing whether it is a closed appliance such as a stove or and Open fire, such as a dog basket. Open fires should have a flue with a cross section of at least 15% the total opening size (unless full flue performance calculations are undertaken). A fireplace opening of 550mm x 550mm should have a flue diameter of no less than 200mm in diameter for solid fuel. We are often asked to supply flues for open fires with opening of approximately 1000mm x 1000mm, this often results in flues of around 450mm in diameter to comply with building regulations, which can be quite costly, very large and are made to order. To reduce this the only realistic way is to reduce opening size or install a large firebox or cassette stove to bring the flues down to between 150mm - 300mm in diameter (appliance dependant) . Stoves and Closed appliances, Generally you will maintain the flue collar or outlet diameter on wood burning stoves that are Defra Exempt, so most commonly now 5" or 6" in diameter, if your appliance is multifuel stove with a 5" outlet you will increase to 6" where it converts from the stove outlet pipe to chimney system. 6" diameter flues and above you will normally maintain the outlet size.

When constructing a masonry chimney you need to make the decision of what type of appliance are you going to be installing. Will it be an open fire? like a dog basket, or a closed appliance? such as a free standing wood burning stove or an inbuilt stove. With an open fire you will need to incorporate into the construction a gather block, normally concrete or pumice, the gather block is designed to funnel the products of combustion smoothly into the flue system, this increases flue performance and minimises the chances of spillage from the fire. The gather block will normally support the weight of the flue liner also.

Concrete Gather Construction Example 

Flue liners is a term that causes confusion within the construction industry, as it can carry dual meaning, so its critical to know which one you are talking about, as it is possible to line and lined chimney!. For all new build houses, chimneys have to be constructed with a rigid liner system and cannot be a flexible liner. So it is the most cost effective way to build your chimney stack with the most appropriate liner from the start, as this can minimise liner and construction cost and also reduce stove installation costs too. Flexible liners can only be installed in a existing masonry chimney system, as they are classed as a temporary repair. It is not permitted to use a flexible liner as part of a new construction or outside of a masonry construct of 100mm in thickness surrounding the liner.

A mid feather is a masonry dividing wall between two flues, within a chimney construction. When constructing a chimney with more than one flue contained within it, a midfeather must be present throughout the length of the construct. Mid feathers are present to separate flues from one another, this creates a firebreak for additional safety under chimney fire situation, gives constructional rigidity around the liner and laterally for the chimney stack, and acts as another barrier should any products of combustion be released under liner failure for any reason. Lastly a mid feather also gives your chimney port something to sit of properly at the top. You must include mid feathers in all chimney constructs with more than one flue.

Wall Thickness including mid feather