Wooden mantels are usually one finished item ( legs, frieze and shelf are all attached).
Stand the fireplace insert and mantel together. You need to check that they fit together with some overlap between the face of the cast insert and the internal opening of the wooden mantel.
You should now be able to measure the size of the opening that you need to make in the wall in order to accomodate the fire insert. If the mantel has a large rebate (the difference between the inside and outside returns - the gap where the fireplace insert will fit) you will need to remove less material to accomodate the insert.
If the fireplace is to be fitted on top of the hearth, the thickness of the hearth must be allowed for in assessing how big to make the hole to accomodate the fireback as it will usually raise the fireplace by 2-3 inches.
Draw roughly on the wall the size of the hole that you will require and then make the hole using a lump hammer and bolster chisel.
If you stand a mantel flat against a wall the gap on the inside where the insert fits between the mantel and the wall is a measure of the rebate usually about 1-3 inch for cast inserts.
For many original cast inserts it will be necessary to fit a lintel to support the central section of chimney breast.although in many Victorian houses in England the brick arch that was usually fitted around the 38 inch high level is still intact and will often do.
It can be noted that many Victorian and Edwardian houses in the UK have a standard builders opening built into the chimney breast during the original construction that is usually around 38 inch square by 14 inch deep (or the depth of the chimney breast)
If there is a piece of skirting board fitted try to remove it intact or cut it around the centre of where the fire opening will be and remove it in two pieces.
Once you have made the space in the chimney breast (or on a flat wall if the chimney is on the outside wall of the house) it is a good idea to put down the hearth into its central position and do a dry run, assembling the mantel and insert in position on the hearth. You should check that there is a clear path for the fumes and smoke from the firebox to escape up the main flue.
Cast Iron inserts should be fitted to the brickwork so it is best to remove the plaster to at least the extremities of it (usually about 38 inch square)
If everything now looks correct remove the fireplace and fix the hearth in place by bedding it on a thin layer of sloppy mortar about 1/4 inch- 1/2 inch thick should be adequate. The gaps around the edge of the hearth will need to also be pointed using the same mortar.
The space immediately behind the hearth can be now filled up to the same level as the hearth with mortar or broken brick etc with a mortar layer over it.
If the cast iron insert is the type that have tile sets incorporated in them, the tiles must be fitted before the fireplace is put in and cannot be easily changed afterwards. Both original and repro cast iron inserts will have some form of tile carrier or a bar behind which the tiles fit.
The easiest technique for fitting the tile sets (usually five 6 inch tiles on each side) is to start at the bottom and wedge each tile in turn using cardboard or wooden wedges. Bonding plaster should then be plastered all over the back of the tiles and the tile carriers or bar. After about half an hour this should be hard enough for the insert to be fitted
The next step is to put the insert with its cast back attached into position, central on the chimney breast normally, and hold it in place with the mantel, making sure that the mantel is flat against the wall and that the insert is pulled forwards to meet the inside returns of the mantel.
If the fixings for the mantel are mirror plates (which are usually made of brass and stick out like ears from the sides of the mantel ) you should mark the position of the central holes in them on the plaster at this stage.
Most cast iron fireplaces have an integral cast iron fireback. However some do not. If the cast iron insert has a separate fireback (a clay fireback or ceramic fibre-back ) this should be put into the opening before the insert is put in place and pulled forward to meet it after the mantel has been fitted and before the job is back filled .
Gently remove the wooden mantel leaving the fireplace insert undisturbed. Drill and rawl-plug the marked holes for fixing the wood mantel to the wall.
Wooden mantels should be fitted on to dry plaster. If the plaster has been removed or has fallen off the wall at the points where the outside of the mantel touches it, this should be replaced and the mantel not left fixed to it until it has dried considerably.
If the gap between the outer edges of the insert and the brickwork is around 1- 1/2 inch or less the insert can be sealed in this position by the use of bonding plaster. It is important to dampen the brickwork with water or a water PVA mixture first as very dry brickwork will absorb the moisture too quickly from the interface between the plaster and brickwork and will not adhere.
It the gap is larger than this it must be bridged using non combustible materials. A quick way is to cut bricks and build up to a few inches below the top of the insert at the sides. Then a lintel must again be used across the top. The essential point is that a gas tight seal must be achieved all around the two sides and top of the insert, right down to ground level
The wooden mantel can now be fitted by screwing it to the previously sited rawl plugs.
Any space behind the fireback must now be filled with non combustible material such as broken brick. The space should be filled level with the top of the fireback at the front edge and should slope up at the back and sides. Any soot which falls down the flue should roll into the fire opening rather than collecting on a shelf where it could ignite causing a chimney fire.
The loose rubble should be capped off using weak mortar ( 7 parts sand to 1 part cement ) and if a rear flap or damper is to be fitted this should be installed before the cement has hardened and checked that it can move freely.
With cast iron fireplaces there is usually no need to worry about the interface between the fireback and the insert fascia. as when heated up the cast fascia will expand together with the back, be it fired clay or cast iron. Fire cement can be used to fill any gaps between back and fascia. The skirting boards can be refitted
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