Partially Collapsed / Sagging Plaster Ceiling

I am trying to fix a plaster / lath ceiling. Near one corner, a section approximately 2' square has fallen out, and the rest of that corner is sagging visibly. It looks like the plaster has become detached from the lath there, but the lath and joists seem solid. I have a few questions about effecting a repair.
* Is it practical to only repair the damaged area? One option I was considering was removing the sagging plaster and screwing plasterboard into the joists below the sagging and the hole. However, it seems a reasonable assumption that if one part of the ceiling is damaged, the rest will be soon.
* Is it possible to screw entirely new plasterboard over the sagging plaster, and the hole, and the existing ceiling? This seems perfectly possible from reading http://www.diyfaq.org.uk/miscellaneous.html#repairing and similar, but the problem is that the existing ceiling has stippled plaster.
* Regardless of which option above is best, is it cost-effective to do the plasterboarding myself and then pay a plasterer to do the skimming, or should I just get the plasterer to do the whole job if I'm not planning on skimming it myself?
Thanks for any advice,
Dave
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My plasterer would only do it if I plasterboarded it myself first. He didn't fancy a bad back.
Christian.
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Dave Page wrote:

Correct.
It doesn't matter if it's stippled or not, plasterboard screws come in a few sizes and anything less than 3 inches is a POP*

Better to ask individual plasterers, some want the boarding some don't, if you lived near me and you boarded it and supplied the scrim and plaster I'd do it for 100. Expect to pay a plasterer 200..ish for supply and fit.
* Piece Of P1$$
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Phil L wrote:

OK, so would I want to screw the new plasterboard directly onto the stippled ceiling, or use spacers to leave a gap, or try to file off the stippling somehow before screwing new board down?
Dave
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On Thu, 09 Mar 2006 18:19:47 +0000, Dave Page

Use a marker pen to measure and mark out the positions of the joists (they will be spaced evenly so you just need to find out how wide they are, and what the spacing is). You screw through the old ceiling into the joists. Assuming by "stippling" you just mean a stippled paint effect, then don't worry about it. The plasterboard is 12.7mm thick, just screw it up nice and tight (but not too tight as the screws will go through the surface of the p/board), and make sure you're screwing into the joists and not the failing ceiling!
It's a two-man job though - the plasterboard is heavy and will need to be propped up firmly while you screw into it. I used a home made wooden prop - a 2.4m length of 2x2 with a large X, also made out of 2x2 screwed to the end.
--
Chris Cowley

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Dave Page wrote:

Unless it's scalloped (really deep furrows of plaster like olde worlde) then you don't need to do anything to the stipple. Find the joists and mark on the wall with a pencil the centre of each joist (sometimes they are not all evenly spaced, most of them are set at 16 inch, but if the joiner that day had 1 joist too many or not enough, they could be spaced differently. Once you've got a pencil mark on each wall, get someone to hold a piece of timber up and strike a line along, either with a pencil or use a chalk line, so that the ceiling has a clear line where each joist is.
Don't use spacers nor anything else, 3 inch plasterboard screws (watch out, they are *surgically* sharp) and a decent cordless driver are all you need.
If you are boarding it for someone else, don't leave any gaps between boards, also they must be 'staggered' like this :
___________________________________________ I I I HALF I IFULL BOARD I _____________________I______________________ I I FULL BOARD I I I I __________________________________________ I and so on..
and *not* done like a chess board, like this:
___________________________________________ I I FULL BOARD I I I I __________________________________________ I ___________________________________________ I I FULL BOARD I I I I __________________________________________ I
The dotted lines in the above samples are the short ends of 6X3 plasterboards, and you should have the short edges meeting on a joist, don't have the long edges meeting at the joists, otherwise you will end up trimming the length rather than the width when you get a tight fit and this weakens the board (the edge with paper all round it) causing it to crumble and you don't get a good fixing.
HTH
(If the above diagrams don't come out too clear in this post, I'll do a jpeg and put it up somewhere)
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And it didn't display properly, here's what it should have looked like:
http://i2.tinypic.com/r10oht.jpg
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Phil L wrote:

Agree with that; last time I had one done the plasterer wasn't at all happy with me proposing to board it as he said he knows how much gap he likes between adjacent sheets etc etc (ie. basically a crappy boarding job can make it much harder to skim plaster afterwards). In the end I didn't argue as the price for the all in job wasn't that much higher than for skim-only. Incredible how fast the guy was able to chuck the boards up, with his screw gun etc.
David
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On Thu, 09 Mar 2006 16:25:16 +0000, Dave Page

I plasterboarded over an existing cracked/sagging lath and plaster ceiling about 18 months ago. I didn't skim it at all - just used plasterboard jointing compound on all the joins, sanded to a smooth finish, PVA'd it and painted it (using some Polyfiller-branded "no cracks" ceiling paint). So far, so good; not a single crack or blemish.
If you don't mind a fair bit of mess, pulling down the old ceiling first would be preferable. It is a blimmin' messy job though.
--
Chris Cowley

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On Thu, 09 Mar 2006 16:25:16 +0000, Dave Page

Do what I had done in my kitchen about 3 years ago and today the ceiling is fine .It was L+P and to take it down would be both time consuming and extremely messy ....Fix battens across the joists through the existing ceiling at appropriate centres depending on what size PB you are using then screw fix the PB to the strapping .Tape and joing it ...It's relatively easy ...then seal and paint it ..Alternatively do the boarding and get it skimmed ..
Stuart
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