Introduction
Every house constructed will most likely require at least one fireplace and chimney. Fireplaces can be constructed for either open fires or closed appliances, i.e. stoves. To avoid future costly remedial measures, it is important that the basic good building practices on the following pages are followed.
Fireplace Recesses
At Party Wall
Where fireplaces are constructed at party walls, the thickness of the wall between fireplaces should be at least 200 mm.
Please Note: In houses where 2 fireplaces occur within the same house, the thickness of wall between the fireplaces and flues should be at least 100 mm throughout the entire height.
Care should be taken to ensure that all opes in places where pipes pass through blockwork are fully sealed. Opes not being correctly sealed allow smoke to enter the pipe duct, resulting in smoke ingression to the first floor rooms.
Diagram C103 - Typical chimney arrangement
Fireplaces On External Walls
It is important that either the thickness of the back of the recess in a fireplace is at least 200 mm or that whatever material within the cavity is non-combustible. The extent of the non-combustible area is illustrated above.
Diagram C104 -Typical back to back fireplace recess either side of a party wall
Diagram C105 - Typical fireplace recess on the external wall - plan view
Diagram C106 - Typical fireplace recess on the external wall
“Toothing in” is not permitted; chimney blockwork is to be built at the same rate as other blockwork.
Fireplace Openings
If the throating on a fireplace is badly formed, draughts will occur. Ideally throating should be at 45 degrees. Also, if the opening at the top of a fireplace is too high, the chimney will not draw properly.
Flue Liners
Good practice for flu liner installation:
Use of precast flue gatherers.
Use of spigot and socket or rebated flue liners with the socket facing up, fire resistant mortar should be used to joint the liners.
Flue liners should be packed with a mix of sand, cement and lime so as no part of the flue liner touches blockwork. The sand cement mix should be as follows - 1 part cement, 1 part lime and 12 parts sand mix.
Dry mix should never be used, ensure mix is wetted with water.
Only use flue liner with a diameter more than 194mm or a square flue of equivalent dimensions.
Diagram C107 -Typical installation of precast flue gathering lintels
Diagram C108 - Typical chimney breast detail at first floor level
I.S. EN 1457-1: 2012 Chimneys – Clay/ceramic flue liners – Part 1: Flue liners operating under dry conditions – Requirements and test methods, and I.S. EN 1457-2: 2012 Chimney – Clay/ceramic flue liners – Part 2: Flue liners operating under wet conditions – Requirements and test methods should be used to select the appropriate class of flue liner. Class A1 N1 should be the correct class in normal conditions.
I.S. EN 1457: 1999 states that all flue liners should be marked with: “EN 1457”, the manufacturers id and date it was created as well as the class number or numbers.
Fireplace Construction
Use of a preformed flue gathering lintel is strongly recommended as it is vital that the throat is correctly formed. In order to ensure smoking does not occur, fireplace dimensions should be checked and controlled such that the height from the top of the grate to the underside of the flue gathering lintel is less than 550 mm.
Diagram C109 - Typical section through a fireplace and suspended timber floor
Cookers - Installation
There are 2 general types of cookers, namely, oil-fired and solid fuel. In both cases, cookers should be installed based on the recommendations and requirements set out by the manufacturer. This is important to ensure that there is sufficient draw and also that the risk of condensation in the flue is reduced.
Oil-Fired Cookers
Attention should be paid to manufacturer’s instructions with regard to oil-fired cookers as it may be necessary to line the standard 200 mm flue with a 150 mm stainless steel flue to limit condensation.
Solid Fuel Cookers
There are 2 things that should be taken into account of regarding solid fuel cookers: the location and construction of the soot box and the type of fuel to be burned. Some materials have a larger moisture content than others. If condensation forms in the flue, it could result in a tarry substance running down the chimney. Turf or wood with high moisture contents increase the risk of condensation greatly.
Running the cooker at a low heat, especially for materials such as wood or peat, can result in the gases running up the flue cooling and condensing into liquid. This could result in a tarry substance flowing down the chimney and damaging the cooker. Whenever possible, burning at high temperatures is preferable to burning at low temperatures.
Gas Appliances
Only qualified persons should install gas appliances. Gas appliances should be installed in accordance with the manufacturer’s instructions.
Ventilation Note
Technical Guidance Document F of the Building Regulations outlines the requirements for room ventilation that must be adhered to.
Trimming At First-Floor Level
Combustible materials around the brick or blockwork chimney should be at least 200 mm from a flue, and 40 mm from the outer surface of a brickwork or blockwork chimney or fireplace recess as long as the width of the brickwork or blockwork is at least 100 mm.
All metal fixings used to fix the combustible material should be at least 50 mm from the flue. All floor joists that run into the chimney stack should be trimmed.
As illustrated, if using two joists to form trimming and trimmer joists, joists should be nailed at 450 mm centres 20 mm from the top and bottom of the joist. Joists may also be bolted at 1 m centres. One problem with using this method is that it can be difficult to obtain appropriate joist hangers.
Diagram C110 - Minimum clearances required for back to back chimneys
Diagram C111 - Typical joist detail perpendicular to the chimney stack
Diagram C112 - Typical joist detail parallel to the chimney stack
Please Note
1) Always support joists on single steel hangers; never build joists into party walls.
2) Maintain the dimensions for the chimney breast outlined opposite up to roof covering. Beyond the roof covering, the distance between flues can be reduced from 200 mm to 100 mm, as long as doing so does not affect the structural stability.
3) The dimension marked as * above is illustrated as being 200 mm to suit block courses. However, the dimension may be 100 mm if necessary/preferred.
The batten used to support the trimmer joist along with the nailing should be specified by an engineer. The engineer appointed must be qualified by examination, in private practice, and possess professional indemnity insurance.
Diagram C113 - Typical joist detail perpendicular to the chimney stack - Detail A & B
Diagram C114 - Typical joist detail perpendicular to the chimney stack - Alternative Detail B
Chimney Heights
The minimum heights for chimneys are as follows:
If the chimney stack is within 600 mm of the ridge, chimney must be more than 600 mm higher than the ridge, provided the pitch of the roof is greater than 10 degrees.
In other cases, the flue outlet should be at least 1 m above the highest point of contact the chimney has with the roof slope.
For flat roofs, the chimney pipe outlet should be 1 m above roof level.
Important note – A chimney flue located lower than the ridge level may not draw properly.
Diagram C115 - Technical Guidance Document J requirements for chimney heights
Flat Roof Extensions
The subsequent diagrams illustrate the guidance set out by Technical Guidance Document J of the Building Regulations.
As stated previously, in order for a chimney to draw properly, the stack should be at or above ridge level. The height and the width of the chimney should be sufficient to ensure stability.
Diagram C116a - Typical minimum height requirements for chimneys on flat roof extensions
Diagram C116b - Typical minimum height requirements for chimneys on flat roof extensions
Diagram C116c - Typical minimum height requirements for chimneys on flat roof extensions
Chimney Height & Width
Chimneys are generally not designed to accommodate satellite dishes or aerials.
Excessively slender chimneys can be liable to overturning in strong winds. In some circumstances, where information regarding location and wind speed may indicate the need to change the proportions of a chimney, the proportions can be changed and proven by calculations to be stable.
Diagram C117 - Height of chimney
EXAMPLE
The height (H) if a standard stack of width (W) 440 mm located on a site where the wind speed is less than 26 m/s should be less than 1760 mm (440x4.0). If the site was greater than 26 m/s, the height should be less than 1540 mm (440x3.5).
Bungalows
Bungalow chimneys should be a minimum of 4.5m above the top of the fireplace, this will ensure that the chimney draws properly.
Diagram C118 - Typical bungalow chimneys
Aerials and satellites should never be fixed to chimneys unless the chimney in question has been designed to accommodate the load. Generally, chimneys are not designed to accommodate such imposed loads.
The outlet of a chimney must be more than 1 m higher and 2.3 m away from any opening skylight as detailed below.
Diagram C119 - Minimum height of chimney/flue opening with respect to openings
Capping Of Chimneys
DPC should be installed below capping in a chimney to stop rain from penetrating into the top of the stack. The use of a precast capping is recommended above pouring of and in-situ capping. The below diagram shows a typical in-situ concrete chimney capping.
Diagram C120 - Typical chimney capping - in-situ concrete
A rendered stack with a precast concrete chimney cap and flue properly sealed on top is the most effective method available for ensuring rain penetration does not occur.
When placing the capping, ensure a gap is left between the underside of the capping and the flue liner socket below to allow for elongation of the flue due to the head.
Metal Tray DPCs
A metal tray DPC is a requirement in solid masonry chimneys. The purpose of the tray is to prevent moisture that is absorbed by the masonry in the stack from penetrating downwards.
The metal tray should be made from non-ferrous metal, lead (code 5), or stainless steel. It should never be made of galvanised steel or plastic. In order to prevent corrosion and avoid staining of surrounding materials, lead trays should be coated with bituminous paint in places it is in contact with mortar.
Appropriate stepped lead flashings should be installed along with the metal tray DPC. The metal DPC should be installed a minimum of 150 mm above the lowest point of intersection the stack has with the roof.
Diagram C121 - Typical dpc tray - metal
Diagram C122 - Typical location of metal dpc tray in a chimney stack
Drainage of water collected in a tray occurs via a weephole. Weepholes are can be created in two ways. One method is to leave open perpend joints in brickwork or, alternatively, proprietary plastic weephole vents can be used. The plastic vents are installed in the perpend joints during construction of the brickwork courses.
When weepholes and drainage channels are free from mortar, water drains from the tray over the lead flashing and on to the roof. However, if the weepholes and drainage channels become blocked, water will build up in the tray and result in dampness, as illustrated opposite.
For dormer roofs where the back of the tray is in a habitable area, an additional upstand should be included to prevent water ingression into the room. An alternative to this is to install 2 DPC trays.
Preventing Smoking & Defects
Highlighted in the table below are the most common faults associated with chimney and fireplace construction. Listed are the defects, the causes, and the methods of prevention methods.
Table C12 - Common fireplace and chimney defects and preventative measures
Incorporating a chimney cowl into chimney design can aid in overcoming downdraught as mentioned above. Illustrated are some typical purpose made cowls that are available. However, if the chimney is constructed properly, there should be no requirement for such measures.
