Because I had no intention of using the chimney, I chose to close the greater part of the opening using timber battens and plasterboard (drywall).
It was also convenient to fix a length of 100x100mm timber at the top of the opening to act as a lintel, thus allowing me to make use of those bricks I had earlier removed to close the opening above it. This arrangement would also provide some structural support for the brickwork above it.
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The plasterboard was screwed to the timber battens using rush resistant dry wall screws. I fixed a batten to the wall about 2 inches above the floor boards, this provided both a substrate and spacer which would allow easy installation of the skirting board. This just left the plastering; a layer of about 12mm was required to reach the level of that on the surrounding wall. For a thoroughly correct job one would have had to use lime plaster (calcium carbonate) to match the adjacent material. Unfortunately lime based building materials are difficult to obtain and can be expensive. Alas it is far more convenient to use a modern gypsum (calcium sulphate) plaster. The characteristics of this material will be different to the adjacent lime and cracking will inevitably occur at the interface between the two materials as time goes by. However I intended to put up wall paper so I was not concerned about this.
As an amateur I have discovered that the only way I can achieve a smooth finish is to build the plaster up in layers. I used two coats of blue hawk undercoat plaster, each about 5mm thick. The second coat I made sure was always about 2mm below my final level by using a long straight edge held against the surrounding wall. For the final thin coat (scratch coat) I used blue hawk finishing plaster. I have found the blue hawk brand of plaster more expensive but easier to work with than Thistle plaster (the other main brand available). The bulk of the plaster is always applied rapidly to cover all areas. The plaster is then smoothed over with a plasterer’s trowel to achieve a smooth surface. I use long strokes first covering the area in one direction, then again using passes at right angles to the first. I find it is also necessary to add small amounts to areas of the surface were there are holes or deep depressions. I continue to work the plaster like this as it sets and with perseverance a smooth surface is achieved. For a super smooth finish the plaster can be left to set for about 20 minutes and then worked over with the trowel again
The rectangular opening cut in the drywall and subsequently covered with a white plastic fly-screen, as seen in the final picture, is to allow for ventilation of the chimney. This is a requirement of UK building regulations and prevents the brickwork forming the chimney from becoming damp.
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