Hi and welcome
Over here the often used practice is to leave the old ceiling in place and fix plasterboard under the old, screwed through the old ceiling into the joists with (lots of) long plasterboard screws.

Removing a lath and plaster ceiling is a lot of work and mess. Once the ceiling has been pulled down all the remaining nails have to be either removed or banged in unless battens are fixed to the joists to carry the new ceiling.

Battening over the existing ceiling is also an option as this can give better support to the old ceiling and the battens can be spaced to match the PB sheets to reduce waste and cutting. (I used 50mm x 50mm battens last time I did this but I have also seen 30mm x 50mm used (AKA roofing tile battens))

A consideration in what ever you decide can be driven by any decorative architrave you may want to keep.

We don't have lath and plaster ceilings here but have reed matting and plaster - same idea though. When I have repaired such I have used normal plaster successfully (a layered build up may be needed). Your picture shows that you might be missing some of the battens, in which case these should be replaced to give a better key for the plaster. Don't be tempted to nail replacement laths, use screws - much gentler on the remaining ceiling than the banging.

Depending upon the amount of ceiling that is sagging it is possible to screw back the laths with large headed screws to refix the laths but you have to ask if the whole ceiling might be going the same way so if more than a couple of places are involved a better overall fix should be undertaken.

Some time back when faced with a ceiling to do I bought a plasterboard hoist. Magic - it makes plasterboarding a ceiling a one man job and allows easy accurate positioning of the boards. Buy new or secondhand and sell on when done.

Plasterboard hoist (they fold up for storage)
https://alinda.hu/gipszkarton-es-osb-emelo-allvany?gclid=EAIaIQobChMIuLiu66XO_QIV-YxoCR2uyQcUEAQYASABEgKH1fD_BwE

+1 what Peter said about how the joist spacing will not match metric plasterboard edges, much faffing required...! Old joists are often more slender and flexible than the modern ones, so hairline cracks may appear at the joins between plasterboards.

Depending how big each blown section is, you can cut it back square as far as a joist, and insert a patch of plasterboard with packers to match the level, then tape and skim over.

If you have woodchip wallpaper or swirly artex it is easier to replace the whole lot with plasterboard than to remove the wallpaper!

Thank you for all the responses!

The message I am getting is that replacing it with plasterboard is viable (please correct me if I am wrong), but the mess and work for removing it completely might be the problem.

The site is messy already as I've been using an angle grinder with a stripping disk to remove old paint from the walls which creates tons of dust, so I also bought myself a JSP mask with filters. In this case, I might as well just take it all down so I can install the plasterboard straight onto the joists?

Posted By: philedgeI do regret the bits I left in place. Having said that, it is a *really messy job*.
Also, keep in mind that insulating on the 'dirty' side of the L&P using rigid boards will always leave air gaps in between.

Are those something that can be cleaned with time and patience? I am not in a hurry to get the job completed, so I wouldn't mind going around with a scrapper and pliers to remove nails and scoop out bits of plaster from the joists.

Also, is there a right plasterboard material to use? should ceilings (in between floors and loft) be breathable?

Posted By: bgasparotto
Are those something that can be cleaned with time and patience? I am not in a hurry to get the job completed, so I wouldn't mind going around with a scrapper and pliers to remove nails and scoop out bits of plaster from the joists

I was referring to the unfinished side of the L&P where the plaster is protruding from between the lathe strips. This is a feature. When leaving the L&P in place, getting rigid boards to fit to the unfinished side is impossible without leaving air gaps. This was my main reason for removing L&P on my (sloping) ceilings, for example.

Posted By: djhBetween floors it doesn't matter about breathability but breathable will be normal. What does matter between floors is noise insulation and if you're replacing the ceiling you should be able to improve that. We used acoustic rockwool for noise insulation. We also put resilient bars beneath our joists and haven't seen any sign of cracks. It might also help overcome your old-fashioned joist spacing.

Did you use rockwool as thick as the joist height? Mine are pretty tall (150mm) so I am wondering about the weigh it would be adding to the structure. Also, could I also use soft wood-fibre instead since I will be also using it in other places? I read it also has good sound insulation properties given its mass.

Great idea about the resilient bars! I was wondering how I would hold the insulation in place as I will install it from under the ceiling (I don't want to remove the floorboards above as they are the one thing that look nice here), so it seens I can just sit the insulation on top of the resilient bars?

Posted By: djhWe also put resilient bars beneath our joists and haven't seen any sign of cracks.

I second that. We put Res bars on all our ceilings and internal studwork as part of sound proofing between rooms. The secondary benefit was no visible cracks in the PB joints.

Posted By: bhommelsMake sure you use the right length plasterboard screws so you don't accidentally screw the resi bar to the joists. Seems obvious, but....

The idea is that you do not transmit sound to the joist via the plasterboard. I found it useful to predrill a 2/2.5mm pilot hole through the plasterboard and res bar as if the screw does not take up on the bar straight away it can push the bar away from the plasterboard and when eventually it takes up the screw is angled and ends up with the head countersinking deeper than desired. The pilot hole in the bar locates the screw. Use the screws designed for the bars. If fitting downlighters you will need top hats to lift the rockwool off the fitting. Ones made by Hellerman Tyton will fit through the light fitting hole and the legs spread out. I am sure other makes exist.

Posted By: bgasparottoDid you use rockwool as thick as the joist height? Mine are pretty tall (150mm) so I am wondering about the weigh it would be adding to the structure. Also, could I also use soft wood-fibre instead since I will be also using it in other places? I read it also has good sound insulation properties given its mass

No we just used the 100 mm required by regs. Our joists are 200 mm+ metal web joists :) Dunno about the woodfibre. It sounds like it ought to work but I'd check the weight is OK and that it's flexible enough to insert easily (the rockwool is bad enough - make sure your clothing is itch-proof).

Great idea about the resilient bars! I was wondering how I would hold the insulation in place as I will install it from under the ceiling (I don't want to remove the floorboards above as they are the one thing that look nice here), so it seens I can just sit the insulation on top of the resilient bars?

We installed from underneath. The insulation just sits on the resilient bars unless its held up by wedging between joists or over pipes, wires and whatnot. We didn't have any downlights so didn't need to get into top hats and suchlike.

I have never used flexi WF for any acoustic purpose, so I cannot comment on tat specifically, but in terms of ease of use it is great. Still loads of dust and fluff from cutting, so wear a mask, but really so nice to use and easy to fit. I've used it in retrofit under-floor insulation and, if it was not snowing, would be using it between timber posts in my extension. (160 rigid WF as EWI and 80 flex inside that. 280mm flexi in vaulted ceilings). Just cut slightly oversize, squeeze like an accordian, and pop it in. Gloriously self-supporting. Great stufff, and as expensive as all other WF products, *but it doesn't itch* (worth a lot to me!!).

bgasparotto
If you are going to replace all the ceilings with plasterboard then seriously consider a plasterboard lifter to take all the stress and strain out of the job
https://www.ebay.co.uk/b/Plasterboard-Lifter/184199/bn_7023424666?norover=1&mkevt=1&mkrid=710-153696-560033-8&mkcid=2&mkscid=102&keyword=&crlp=355246904622_&MT_ID=&geo_id=&rlsatarget=dsa-764521878696&adpos=&device=c&mktype=&loc=9106041&poi=&abcId=&cmpgn=2033188068&sitelnk=&adgroupid=71572596173&network=g&matchtype=&gclid=EAIaIQobChMIq-T_p6bR_QIV7JBoCR0SMAgzEAAYASAAEgLE_vD_BwE

Posted By: Peter_in_Hungary
If you are going to replace all the ceilings with plasterboard then seriously consider a plasterboard lifter to take all the stress and strain out of the job

Thanks for the suggestion Peter! I am definitely considering it after your first comment (sorry I didn't respond your first comment but I noted it :))
In fact it should be a good idea to get one of these as we will be doing this in the whole property, but I wonder whether it would work in narrow spaces such as hallways? Otherwise it will have to be me and the wife lifting it up :D

Posted By: Nick Parsons

Flexi WF: 50-55kg/m3. Not itchy at all, to me anyway.

Acoustic Rockwool 60kg/m3. If you're getting the SE to check re weight of WF then you need to do it more so for acoustic RW.

Thanks Nick! I was under the false assumption that some "wool" would be inherently lighter than anything else! I will double-check this with the engineer anyway since we will also be adding weight to the bottom floor with the WF + UFH system.

I leave them up and tack over them and skim if they're in poor condition.

Posted By: bgasparottoI read a fair bit about WF not itching as much as rockwool, and we will be using it for our underfloor insulation as well

Is the stuff you're using underfloor flexible as well? I'd have thought you wanted something that was load-bearing to some extent.

1200x450mm plasterboards are easier (IME) for ceilings when working alone or in tight spaces or if they need to be carried up stairs. Need to arrange battens/noggins spacing to suit.

Like Peter said, make full use of the tools that are available, sell on afterwards if you want. If you are doing a whole house then look at buying a collated-screw driver, you can put screws in one handed while holding the pb with the other. Get a couple of step-up platforms the right height to reach your ceiling from comfortably.

Check the size of your boards before you fit. I had 2 pallets of boards and they were spot on size. When I ordered some more my usual builders merchant sent me some of different make and the boards were 5 mm shorter on the width. The spec is +/- 5 mm so were in spec but on a big ceiling when you come to the end of the run you can be way out. 2 Boards and one is 10 mm out.

Posted By: revorIt will be worthwhile if a bit time consuming if you survey the "flatness" (will call it that rather than level) of your ceiling joist using very tight string lines or laser level. Find the most protruding point and make that your reference point (Zero) From there work out using lines to find the mm out other fixing points are and fasten a plastic spacer of appropriate thickness at the fixing point of the RB. The RB are usually very straight as long as not damaged so you may able to use the lengths as straight edges to work out the amount of packing thickness instead of using lines. If you do not pack out gaps, when you screw the RB in place, you will distort it. Where the plasterboard end meet you need a support cross member you have to be accurate with the position as there is not much bearing surface once you allow for the 12mm from edge screw fixing. Hopefully you get the drift.

I think got the idea, thanks! Interesting how the prep work seems to be as important as the finishing itself for us to achieve the end result we are expecting :)

Posted By: djhIs the stuff you're using underfloor flexible as well? I'd have thought you wanted something that was load-bearing to some extent.

It is soft, yea. I am planning to install a breathable membrane around the joists so the insulation can sit on and where the top of the WF is flush with the top of the joists. For structure, we are planning to get UFH boards with channels, cover the pipes (perhaps MDF? still unsure) then install our own flooring on top (mix of tiles and wooden floor reclaimed from the underfloor we are replacing for UFH).