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I've had a gas fire removed and the pipe sealed off by a plumber, I want to cover the hole over now (about 50cm wide by 70cm tall). I've bought a piece of plasterboard and some plasterboard adhesive already but I've never used them before. I've got loads of offcuts of timber I could use to make battens if I needed to.

The main thing I don't understand how to do is to make sure the new piece of board is flush with the existing board on all three sides (left, right and top) because the board is the same thickness (12.5mm) but the existing board is on with dot and dab so is a few cm away from the blockwork that forms the chimney /gas fire hole. 

The only idea I have at the moment is to cut away enough of the existing plaster to show a good few inches of block wall around the fire place, then perhaps screw lengths of wood to the back of the plasterboard to make it be the same depth as the existing wall - but just not sure how I'll ever get it bang on (or near enough). Once it's in I'll use filler around the crack.

Any ideas!!? Thanks

Edit: the other idea I had, and this might be over-complicating it, was to temporarily screw a timber all the way across the front of the new board and the old board, screw it into the existing wall and the new piece whilst the plasterboard adhesive sets, in the hope this would hold it in place (flush), then unscrew it once it's set and fill the holes in...

It doesn't have to be totally flush as you will be adding scrim tape across the joins and skimming with 2/3 mm of plaster.

I would just run glue/screw a couple of battons to the back side of the existing plasterboard, all the way across the hole, and use that as a mounting frame for the new piece. That'll  be totally flush if all the board is the same thickness as you say.

Could you not just cut away enough of the existing plaster ( as you suggest) and then just dot and dab around the outside to attach the new board. You should be able to get it flush.

Screw wood battens to the wall, use plastic shims behind the batten to set the depth. You can go in and out with a screw in a normal plastic wallplug quite a few times and they still hold fine.

Then screw plasterboard to the battens.

If your chimney is still open at the top, install an air vent in the board. 

scrim tape and air vent added to the shopping list...! thanks 

I never thought to add battens to the back of the existing plaster board but I could do this - but I'd need to fairly carefully knock out / chip away some sections of the dot and dab stuff that's holding the current plasterboard on. It's all the way around the fireplace hole at the moment, there are no spaces really. I could do that though. I think I'll try this 👍

Normal stud spacing is 600 mm so if you're at only 500 mm hole between the dot/dab fixing you should be fine strengthwise.

Bolster chisel will remove the dot/dab easy enough.

I've done exactly that when repairing some holes in plasterboard.

I had to do similar once, but on a ceiling with lathe and plaster.

You could cut some small bits of plasterboard to use as alignment spaces for getting battens in. That's probably what I'd do, but I'm not that familiar with dot&dab. So, screw your little section of plasterboard onto the batten, put batten in place with board edges aligning, fix batten (potentially with some kind of grab adhesive, as getting masonry screws mm accurate is always a challenge) and then repeat process. Finally install new section of board.

Another option so save having a vent in your wall is to make a recess where you can put something decorative in (candle, lamp, etc.) with the vent in the new "ceiling" of the box?

Works well when the plasterboard is fixed to battens, but would there be enough space with dot & dab? 

Fit a frame and screwed plaster board 20/25 mm back from room plaster face.

Cut plasterboard to shape . Dot and dab this price to back piece.

Use a straight timber to push exactly flush or 2mm back for skim/ top coat of finishing plaster.

Without plastering it will be very visible I'm afraid.

Another thing to think about is how well insulated are the rest of your walls? If the back of the new piece of plasterboard is cold an exposed to un-insulated external walls, you may be creating a cold spot that could be a future damp and mould issue. If none of the house walls are insulated then it won't make any difference.

Forgot to add. Make sure you have some ventilation to the  old fireplace / flue.

Are you going to skim it or tape and fill the seam? It sounds like the latter.

That's basically what I do (by various rough and ready means), hold it in place with temporary screws, batons, wedges and props, whatever it takes while the glue sets. Foam glue shrinks a couple of mm as it sets if you let it so plan for that if you're using it. If you're skimming the whole wall or taping and filling, fit the new board flush. If you're just skimming the board and blending it into the old plaster then chamfer the edges of the old plaster so they can take tape, PVA them so it actually sticks and fit the new board 2-3mm low.

I mostly use temporary drywall screws for spacers as they give infinite adjustment to help me pull a patch in unevenly to fit non-flat walls, I put some screws just through the board being careful not to strip the tenuous thread they cut in the board, the tips of those press on the bricks and space the board out. I drive some more right through into the mortar pulling the board down. By fiddling with them carefully you can get a nice flush patch even on a wonky wall. I use foam adhesive which has quirks, cement type dot and dab adhesive is firmer and doesn't shrink, you can probably just press a small board in slowly to the right depth so long as the dots are sensible sizes (use batons to spread the load and act as depth stops).

Taping, filling and sanding the seams/screws will probably give you the best DiY finish unless you have experience plastering, it's tricky blending new into old. It's not impossible to get a good finish like this but it won't cover up any error in setting the board depth.

Add a vent so the chimney can still breathe.

jk

You'd need thin enough batons to match the d&d thickness, but, as OP is covering over a void, you could use whatever you've got and just thin out the ends as necessary.

Not sure without having seen the hole in question whether you need battons or dot/daub some plaster to stick it on, or use some board adhesive. But the general approach is

• Make the hole straight edges to make it easier to get the replacement plasterboard just right
• Cut the replacement board exactly the right size and shape for the hole
• Fix the replacement plaster somehow (this is what I can't advise on without seeing it), could be dot and daub by the sound of it?

BUT...  plaster is a pain in the rear. When you have a joint between 2 different bits, like sheets of plasterboard in a wall or ceiling, the junction/corner of a wall and a ceiling, a repaired section like this. It has a terrible tendency to look absolutely lovely... for a while and the slightest difference in expansion/contraction or any movement creates hairline cracks (harmless but look awful) at the junction, so maybe in mid winter you will notice you can see a hairline crack showing the outline of your repair. 

"...Once it's in I'll use filler around the crack...."

It is *essential* that you correctly tape the joint with a non stretch (fibreglass) tape, or at a later date, that hairline crack will appear. I think correctly taping it is the thing you need to get right, more so than how to fix the repair board. Even professional builders can get this wrong, I'm looking at a long hairline crack right now and I know who didn't tape it properly (no, it's not me!)