Building a fireplace and chimney from scratch

Many people decide that they would like an open fire or solid fuel appliance in their home only to find themselves handicapped by the fact that they have no suitable chimney or fireplace. Either existing fireplaces are iri the wrong place or—in the case of modern homes—they’ are not part of the original house construction.

The best solution is to construct, a new fireplace recess and chimney from brick. This is suitable for all types of inset open fire and. can be used also with most modern free-standing and the closed fires. As an alternative you may with to fit a prefabricated flue and chimney. This is fairly Complicated and advice should be sought before commencing work.

Building regulations

Whatever type of solid fuel appliances you install—wood or coal burning, open or closed—the fireplace recess, and chimney must-be soundly constructed so that there is no fire risk inside the house. It must also be carefully sited against a suitable outside or inside wall that is strong enough to take the weight, of the flue and the chimney stack.

If the fireplace is to be built on the ground floor, you should consider carefully how the chimney breast is to be fitted through the first floor, the upstairs ceiling and the roof.- This will involve cutting through roof and ceiling coverings as well as some of the joists.

Building regulations governing fireplace and chimney construction vary from country to country and even from one region to another so it, is essential to discover those which apply to you; your local building inspector or a competent architect can help if you are in difficulty. You should then draw up detailed plans of all the work and submit them to your local authority for approval before proceeding.

Fireplace construction

All brick fireplaces and chimneys are constructed in the same way, despite the fact that dimensions and materials vary according to the building regulations. The sizes and materials quoted below give rough estimates only and you should check carefully that these are acceptable before starting work: Fireplace recess: The box which forms the fireplace must have sides and a back of solid non-combustible material—usually brick—at least 200 mm thick. If you build the fireplace against an external or party wall, check first that this is thick enough to form the back. Either a solid wall 200mm thick or a cavity wall where each leaf is 100mm thick will satisfy UK building regulations.

Walls less than 200mm thick should have an extra leaf added directly against the back wall to make up the required thickness. Erect this before the sides and insert metal ties across both walls to hold and strengthen them. Hearth: Once the fire is in operation the floorboards, joists and floor coverings need to be protected by a hearth against accidental damage. This consists of two solid slabs, one beneath floor level—called the constructional hearth—and a second, which sits on top of it—called the superimposed hearth.

The constructional hearth must be at least 125mm deep and should project 500mm in front and 150mm to each side of the opening. Most constructional hearths are completely solid but if you are building a fire with underfloor air ducts you must allow for these on your building plan.

The superimposed hearth consists of brick, concrete or some other non-combustible material at least 48mm thick. It should project at least 300mm in front of the opening and 150mm on each side of the fireplace.

Unless you are installing a fire with underfloor ventilation, a solid concrete back hearth must be fitted inside the recess and finish level with the top of the superimposed hearth in front of the opening. Chimneys: Great care must be taken when building the brick chimney and forming a flue inside it. The brickwork around the flue should be at least 100mm thick—except where the chimney is built against a party wall when the brickwork between the flue and next door must not be less than 200mm thick.

You may need to build the chimney with a bend or crank in it—to avoid internal projections or built-in furniture, or to make sure that the chimney pot ends up on the correct part of the roof. Keep bends angled at less than 60° to the horizontal and remember that an angled chimney is much more difficult to build than the straightforward upright variety. Chimney stacks: A hole may need to be cut in the roof to accommodate the new chimney assembly. The chimney stack can then be built above this and a pot cemented on top. The chimney pot must be at least 200mm in diameter internally and should be sited at least 1m above any nearby ridge of the roof to avoid down draughts. Fireplace opening: The size of opening you build will vary according to the type of appliance you are installing and the area you particularly want to heat up. You may, however, wish to construct a larger opening so that almost any type of appliance can be installed in the future without the need for complicated structural alterations to the chimney breast.

The brickwork above the opening must be supported by a reinforced throat forming lintel made of concrete. This is triangular in shape and is fixed so that the chamfered side faces inwards towards the back of the opening—an arrangement which allows the fire to draw air but avoids unnecessary draughts.

The size of lintel you use will vary according to both the design of the fire and the size of the opening, but it should be at least 300mm longer than the opening so that it can bear on the adjoining brickwork by 150mm.

To allow the fire to draw correctly, the lintel must be wide enough to create a gap of around 100mm between it and the fkeback. Fit a throat restrictor—this will reduce the throat size when necessary for better performance, and even shut it off altogether to reduce draughts when the fire is not in use.

The back and sides of the opening need protection from the heat of the fire. This is usually provided in the UK by a fireback ; in Canada the opening is lined with firebricks laid in high temperature cement. Clearances: Even with the thickness of masonry required by the regulations, combustible material must be kept clear of the outside of the construction. Clearances vary, but are often around 50mm.

Preparatory work

You are bound to create a lot of disruption and mess during building, so ensure that you clear rooms as much as possible and cover furniture you cannot move.

You could start by cutting holes in the floors and roof to accommodate the chimney; do not forget to allow the necessary clearances. However, if you cannot install a temporary cover over the roof opening, build up to near roof level before cutting it. This will avoid the risk of rain or roofing materials falling on to the working area.

Before you cut away any of the floor or roofing timbers, carry out careful checks by removing floorboards to see which way the joists are running and how they are supported on the adjoining walls. You’may have to erect temporary props to take the weight from below while you cut away the timbers and secure them with trimmers.

To cut a hole through to the outside of the roof you need to get into the loft and cut away any roofing felt and battens before removing slates and tiles. If any rafters are in the way of the chimney, you must cut them away and add a false rafter to take the necessary weight.

Constructional hearth

Although it is possible that a solid ground floor will meet the requirements for a constructional hearth, it is more likely not to—and you will certainly have to construct one if you want to build a fireplace in a room with a suspended wooden floor.

Start by carefully marking out the area to be covered by the hearth. Remove the floorboards inside and around the edge of this area, continuing until the two joists on each side of the proposed hearth are clearly exposed.

The intervening joists must be cut away and replaced by a single wooden trimmer along the front edge of the hearth. Temporarily support them by building up with bricks from the ground to the underside of each joist.

Once you have sawn away the joists and worked them loose from their fixings on the wall, cut a trimmer from wood of the same dimensions long enough to span the gap across the front of the hearth. Skew nail it to the ends of the joist you have sawn through and to the full joists on both sides of the hearth.

To build the hearth, first erect timber formers around the outside of the hole. Fill the base of the box with hardcore and tamp it well down. Make up the rest of the hearth with a concrete mix of one part cement to six of 10mm all-in ballast and level this off with the existing floor. Leave the concrete to set for at least three days before building on top.

Building the recess

If the wall behind the recess is less than 200mm, your first step is to line it with an extra leaf of bricks to make up the required thickness. This should be built hard against the back wall and secured with metal wall ties mortared into the existing joints.

Fix the metal ties in place on the back wall before you start building.

Space them at regular intervals between every third or fourth row of bricks and about 300mm apart. Use a plugging chisel and club hammer to cut slots in the bedding mortar large enough to accommodate the ties. Then mix up fresh mortar and fix each of the ties in position, leaving one end protruding from the wall. Allow the mortar to set for two to three hours.

Build the extra leaf on the back wall to the exact dimensions required by the building plan. As you build upwards, keep each course of bricks on the new leaf in line with those on the back wall so that the ties can be incorporated in the joints. Leave half brick gaps at both ends on every second or third course, so that you can bond in the sides of the recess.

Once the back is complete, build up the sides and then the front according to the dimensions you have decided upon. Bond the sides into the back using the gaps you have left, and tie them into the existing brickwork at the front corners by chopping half brick gaps with a hammer and bolster.

Recess to chimney

The most difficult part of the recess construction is to provide a smooth transition from the wide opening where the fire burns to the narrow flue which carries away smoke and combustible gases. If this part of the chimney—known as the throat—is not built correctly, the fire will not burn efficiently and is likely to smoke.

Although the special lintel will form the front throat angle, this must be matched on the back wall of the recess by building up the rendering to form a mortar flaunching.

Once you have built the sides and front of the recess to the required height, lift the throat forming lintel into position and check the distance between it and the fireback. Allowing for the protruding mortar flaunching, work out exactly where the lintel must be placed to produce the required throat of 100mm. Make a note of the relevant dimensions then lay the lintel to one side until it is needed.

Continue downwards and fit the fireback in position. Most manufacturers provide simple instructions specifying how a fireback should be fixed and you should follow these carefully.

Before inserting the fireback, try it for size to get some indication of the amount of mortar render and packing material you will need to place behind it. Then fill in around the back and sides, starting with bricks and mortar if the gap is large.

Just as you approach the correct depth, wrap two thicknesses of corrugated cardboard around the back of both sections of the fireback. This will provide enough room for the fireback to expand without cracking once the fire has been lit. The face of the backing material can be given a thin coating of fire-proof render— consisting of one part cement to four parts vermiculite—to protect it from excessive heat.

Fit the bottom section of the fireback into place first, adjusting the backing material as necessary. Join this to the top section with fireclay cement and try to adjust the position of the top section so that it joins neatly with the bottom edge of the fiaunching.

Then form the concrete flaunching at the back of the throat. Start about 300mm above the throat and gradually thicken the render until it is deep enough to form an angle matching that of the lintel.

Once the recess is complete, mortar the lintel in place. Make sure that there is the correct gap of 100mm between it and the fiaunching. Leave the mortar to set for two to three hours before continuing work.

Chimney to roof level

The chimney construction starts above throat level, and consists of a flue liner surrounded with brickwork. Each section of flue should be fitted as you progress, and the brickwork built up around it.

The minimum internal flue size in the UK is 185mm for square linings, and 200mm for round linings. This is suitable for stoves and roomheaters, and for open fires with openings up to 500mm square; larger open fires should have larger flues. In Canada, minimum flue sizes for roomheaters are 150mm for round flues and 200mm for square ones; but open fires must have flues at least 225mm diameter if round, or 200mm x 300mm otherwise.

Start at the top of the throat and fit the first flue section in position. Then construct a brick box, of the required thickness, around this lining. Tie the box into the wall as you progress, either with metal ties or by chopping out bricks from an existing brick wall on every second or third course. To join sections of clay liner together, use fireclay cement or a 1:3 cement/sand mix.

The liner need not touch the chimney brickwork but the space could be filled with a 1:4 cement/ vermiculite mix to provide additional thermal insulation.

Continue upwards in this way until you reach roof level. Once the chimney is complete as far as the roof, make good the ceilings and floors. Note that floorboards, architraves and so on usually do not count as ‘combustible material’ for the purposes of the regulations about clearance distances.

Building on the roof

You should not start work on the roof without making sure that proper safety precautions have been observed. A securely tied ladder or scaffold tower as well as a crawler board attached to the ridge are essential pieces of equipment.

Continue the chimney above roof level using the same techniques as you used below. Where the chimney passes through the roof, the slates or tiles need to be carefully replaced and the flashing firmly fixed all around the brickwork.

Finish the brickwork so that the chimney pot can be positioned at the regulation height above roof level. The top two courses should project— or oversail—the others by 30mm so that rainwater is thrown clear of the brickwork beneath. Alternatively, you could use a prefabricated concrete capping of the type which incorporates a drip groove as well as an oversail.

Mortar the chimney pot into place next. Cut off the metal flue or top flue block as it appears through the top of the brickwork and fit a pot of the same diameter on top. Surround the base of the pot with a DPC of bitumenous felt and on top of this lay a thick, sloping concrete flaunching.

Finishing off

The work is completed by building a superimposed hearth over the top of the constructional hearth. This can either be built in brick to match the chimney or formed in concrete and covered in a mixture of decorative quarry tiles.

Erect formwork around the outside of the hearth and fill this with a weak mortar mix. Before the mortar sets hard, push a length of dust-suppressed asbestos rope between the hearth and the base of the fire recess to allow both to expand without cracking. When the hearth has set point over the rope with fire cement.

Once the grate or stove has been installed, the fire is ready for use. You can leave the chimney breast as it is, or—if you want it to match the recess—build a false breast in brick.

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