What is a builder's opening?
This is the rectangular opening set in a chimney breast to take a solid fuel open fire which would have incorporated a decorative fire surround at some point. In the UK and Ireland this opening is usually approximately 22" high by 16" (560mm x 406mm) wide, subject to the age of the house, and at the time of construction would have also had a shaped concrete fireback built into it. The builder's opening can often be easily re-opened by removing the fire surround and backplate to make a recess to take a wood burning stove.
Check that there's a suitable lintel in place by removing some of the plasterwork above the opening until the lintel is exposed and then ensure that brickwork above the lintel is sound before removing the fireback. In very old houses it is not unusual to find that mortar joints just above the lintel have perished because of continuous intense heat and the effect of acidic condensates building up from decades of inefficient coal burning. Any loose mortar will need to be removed and the brickwork rebuilt. This is one reason why estimates for 'knocking out' an old fireplace can be subject to change as an installer will not know the state of the chimney breast or lintel until they have started the job and removed some of the plasterwork. It's also one of the main reasons that most stove installations incorporate flexible flue lining which eliminates potential combustion gas (including carbon monoxide) leaking into living spaces through perished brickwork joints.
Expanding the builders opening
Whilst the standard builders opening is suitable for many inset stoves there is not usually enough space for a freestanding stove which will also have a significant heat output. This is because of the requirement, to not only accommodate the stove in the opening, but to also provide sufficient air space at the sides and top so that radiated heat can circulate. Failure to provide this air space could cause heat build-up inside the appliance which would eventually damage some of the stove components and possibly any brick and plasterwork in close proximity to the stove. In these situations the builders opening can usually be expanded but, in our opinion, this is a job for a professional as it will most likely involve the use of props to ensure the integrity of brickwork above the proposed larger opening before the new, wider lintel is positioned.
In The Stove Yard's experience it is usually possible to create a larger opening of approximately 900mm x 900mm (3'x3') without too much work in most chimney breasts. This will leave sufficient airspace at the sides and top for typical 5kW output stoves, except for some of the wider 5kW slimline stoves. It is important to leave at least a full brick length at each side of the opening as this is not only structurally more stable and is in line with building regulations but also looks much better. It goes without saying that the depth of your new opening will usually depend on the depth of the original builders opening which typically will extend to the back of the wall. Remember too that the width and depth of the replacement hearth must conform to the requirements of the width of the new opening and the final position of the stove.