I've just removed a gas fire from my living room and am going to block off the opening .I'm proposing to screw timber just inside the opening and then fit a piece of plasterboard and plaster over the board flush with the wall. I was going to use multifinish or finishing plaster .It's not a big opening ....about 12" by 18" . How far back should I sit the plasterboard ....10mm be all right ?
If you're just skimming with finish coat (which is all you need on plasterboared), it wants to be about 3-4mm if you are doing two coats. You can't put finish coat on any thicker than about 2mm per coat.
The flue must be vented top and bottom. If it's on an outside wall, vent to the outside by installing an airbrick through the back of the fireplace, and I would suggest putting thermal insulation behind the plasterboard. If it's not on an outside wall, you'll have to vent to the room with a grill through the plasterboard (and there's no point in any insulation in this case).
At the top, if it's uncapped, that's fine. If it's capped, a top vent must be fitted too. Otherwise the flue will slowly attract more and more water, eventually damaging decorations on the chimney breast and causing your timber support to get dryrot.
As an alternative to closing off the fireplace, you could just close off the flue with a piece of plasterboard (still needs to be vented above as described above), leaving the fireplace as a storage or decorative area. If it backs onto an outside wall with a single skin of brick (a half brick wall), line the back with 25mm or 50mm cellotex to prevent condensation, and plasterboard/plaster over this.
Another thing to bear in mind... There will be a slow but steady drop of dirt from the flue. Make sure any vent is raised well off the floor, so the eventual pile of dirt doesn't block it. Suggest you also nail an offcut 12" strip of plasterboard along the back of the timber frame at the bottom, to keep said pile of dirt from resting on the back of the plasterboard facing the room, as it may stain through or distort it. Having the flue swept before boarding up would help, but won't prevent dirt dropping down the flue.
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