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Hairyloon

Repairing a lath & plaster ceiling.

A friend has had a velux window fitted in the bathroom and the job did not include making good on the plasterwork which is an old lath & plaster construction (well, not old on the scale of these things.
I have read that it is best to repair with a lime mortar similar to what was originally used, but I have never worked with that so I know nothing much about it.
Any top tips? Or is it just easiest to use modern plasters?

Either way, what needs to be done in respect damp proofing, with regard to it being in a bathroom?
dpack

modern plaster is easier and more damp resistant ,avoid bonding coat/browning as the stuff is a sponge.

non hydrolic lime plaster ready mixed is around 10 for 25 kg and would do the job but

this stuff is super sticky and sets very hard which is ideal for patching around a velux at around 25 a bag

other patching plasters are available but if you go for one of them ask the builders merchants if they are suitable for A around a window frame and B in a bathroom and C will it stick to lath .

re sticking , wire brush the lath then prime with waterproof pva diluted 3 to 1 allowed to almost dry will help.
Hairyloon

modern plaster is easier and more damp resistant ,avoid bonding coat/browning as the stuff is a sponge...

So use an undercoat plaster that isn't bonding or browning?

Quote:
non hydrolic lime plaster ready mixed is around 10 for 25 kg and would do the job but

Sorry, but you're being ambiguous: lime plaster is the traditional stuff, so is that your recommendation if I want to take the traditional route?
dpack

traditional = non hydraulic lime , 2 coats let the deep one set slowly keeping it damp then do a thin smooth top coat to level out any shrinkage and cracks, let it set slowly keeping it slightly damp .

non traditional but lime based = use the super plaster stuff one coat troweled level, let it set slightly damp.

another option is a bathroom rated one coat patching plaster, fill and trowel, let it set

browning and bonding are cheap (ie good for big deep areas) and usually end up nasty , the only thing they are good for are internal upstairs walls and even then i avoid them.

EDIT i would go for onecoat patching plaster unless a lime plaster was specified for a listed building which seems unlikely as they have a new velux Laughing
Ty Gwyn

I presume with the opening cut for the velux there is a minimum of making good to be done on the lath work?
Hairyloon

traditional = non hydraulic lime , 2 coats let the deep one set slowly keeping it damp then do a thin smooth top coat to level out any shrinkage and cracks, let it set slowly keeping it slightly damp .

non traditional but lime based = use the super plaster stuff one coat troweled level, let it set slightly damp.

another option is a bathroom rated one coat patching plaster, fill and trowel, let it set

Not at all bothered about being traditional, but it has to go on pretty thick, so I'm not planning to do it in one coat.

I presume with the opening cut for the velux there is a minimum of making good to be done on the lath work?
Pretty minimal, yes.
Ty Gwyn

Thistle mixed with sand for the first coat,well pressed into the lath`s,then finish with a thistle coat,just as good if your not fussed on a traditional finish. Hairyloon

Thistle mixed with sand for the first coat,well pressed into the lath`s,then finish with a thistle coat,just as good if your not fussed on a traditional finish.
Damn. I've strimmed all the thistles. I'll have to wait for them to grow back...
Confused

On a more serious note, how much sand?
Thanks both for the tips.
dpack

Laughing Ty Gwyn

Thistle mixed with sand for the first coat,well pressed into the lath`s,then finish with a thistle coat,just as good if your not fussed on a traditional finish.
Damn. I've strimmed all the thistles. I'll have to wait for them to grow back...
Confused

On a more serious note, how much sand?
Thanks both for the tips.

3 to 1
Hairyloon

Thistle mixed with sand for the first coat...

On a more serious note, how much sand?

3 to 1

Is that 3 sand to 1 thistle or the other way about?
dpack

3 thistle to one sand might be a bit less crumbly Hairyloon

And after all that, it is looking like a change of plan. The homeowner is thinking to simply screw plasterboard over the top...
Aside from the difficulty of getting the screws through and into something solid enough, I can't see any real problem with that...

The "aside" looks easier said than done mind you...
Mistress Rose

We took down the lath and plaster ceilings at our first house before replacing them with plasterboard. Built a little bank in the garden with the plaster and used the laths on the fire. Think it would have been a bit tricky getting the plaster board on top. Still, might be worth a go, although if it doesn't work, could end up with the whole lot down and start again. Slim

I ended up covering up some plaster with drywall a couple years ago.... Make sure you've got a decent thickness, and lots of good attachment. dpack

drywall or even sheets of waterproof mdf are very useful in the right places

flexiply can do some very unusual shapes

can you show us a photo of the hole?

from the description dawb on a hawk and a trowel still seems the best option
Hairyloon

can you show us a photo of the hole?

Alas no: I've trowelled it full of daub now...
dpack

if it is daubed out a skim of multi finish should complete the job quickly ,easily and properly for minimal cost Hairyloon

if it is daubed out a skim of multi finish should complete the job quickly ,easily and properly for minimal cost

That's what I thought, but I forgot to mention that the homeowner is a crazy person...

Also taking off the wallcovering to allow for the daubing exposed a damn great big crack. If it were up to me I'd investigate with a view to filling, but...
dpack

Laughing Slim

Sometimes I feel that the best advice I was ever given was "quit early, and quit often"

(with the caveat that the advice was not being offered as an excuse to give up)
Hairyloon

Sometimes I feel that the best advice I was ever given was "quit early, and quit often"

(with the caveat that the advice was not being offered as an excuse to give up)

I think perhaps that "quit" is too strong a word: it doesn't suggest any intent to go back to it... Or do I misread it?
Slim

No, that's how I intended it.

But from the perspective of you trying to help someone that is "crazy" and may not want to do things as you suggest would be proper. Can't push a chain.
Hairyloon

Can't push a chain.

Challenge accepted... Wink

Changing the subject a bit (but still in the bathroom), the question currently under consideration is the value of putting insulation into the end wall: ie a sheet of celotex type board behind the plasterboard.

On the one hand, some insulation is always better than none, but on the other, is there any point doing that little bit and none of the rest of the house?
Slim

Will the other walls ever be opened up in the future? Piecemeal can be okay for refurbishing....

What is celotex made out of? Will it be okay if moisture makes it in? I would think modern rockwool would be appropriate....

The bathroom exterior walls are where I would expect you'd most likely find humidity condensing within the wall. A bit of insulation may help to reduce that, but I know that these things quickly get tricky balancing a desire for breathability to dry things out and moisture barriers to keep humidity on one side or another.....
Hairyloon

Will the other walls ever be opened up in the future?
Probably not.
Quote:
What is celotex made out of? Will it be okay if moisture makes it in? I would think modern rockwool would be appropriate....

Polyisocyanurate. As I understand, this is a standard way of doing this kind of thing...
Quote:
I know that these things quickly get tricky balancing a desire for breathability to dry things out and moisture barriers to keep humidity on one side or another.....

I couldn't begin to guess which side would be which. I decided to not worry about it. Confused
dpack

as above re damp issues etc

as shown recently in london the stuff burns and if it does a breifcase sized lump produces enough one lungful toxic gas ( mostly hcn but a few other choice compounds ) to need a noddy suit in a large house

on the outside is potentially dodgy, inside why not use rockwool slab Wink
Hairyloon

Still leaves the question of whether to use anything... dpack

improving ventilation is my best advice for condensation damp

re insulating a cold wall where water vapour condenses a simple skin of suitable boards over rockwool slabs or mineral panels with suitably sized battens will make the inner surface a lot warmer and less prone to "dew "

it does make the room a bit smaller which can be an issue especially if you have to move the bathroom suite in a bit
Hairyloon

improving ventilation is my best advice for condensation damp
That wasn't actually the problem that we seek to address.

Quote:
it does make the room a bit smaller which can be an issue especially if you have to move the bathroom suite in a bit

The suite is coming out anyway and the room is big enough that it can spare a few inches off the end.
dpack

if chilly is the issue extra insulation ain't as good as a holistic approach when retrofitting houses but good heating, good ventilation and good insulation can make a home cosy

"it always feels quite chilly" in a room often has a damp component Wink
Hairyloon

if chilly is the issue extra insulation ain't as good as a holistic approach...

I don't recall any complaints about chilliness. The plasterboard was deemed the easiest route to something to put tiles on.
The insulation is very much just a case of "why not?"
Slim

Whoa, tiles going on gypsum board and not on cement board?

That's the way it used to be done here, but not for a long time.

If it's worth tiling, it's worth backing with cement board. (I believe that's the present thinking)
dpack

yep, waterproof board is sensible,

cement based, waterproof mdf or marine ply are suitable behind tiles
Hairyloon

yep, waterproof board is sensible,

cement based, waterproof mdf or marine ply are suitable behind tiles
You don't rate the moisture resistant plasterboards?
Slim

I've heard that those can be okay for areas that you don't expect to get wet, but that they're still not the best choice dpack

moisture resistant is ok for kitchen/bathroom walls/ceilings etc but if it is a "wet" area ie it needs tiles it is best to use a properly waterproof substrate board.

for tiles it needs to be ridged as well so good battening (pre treated with preservative ) and a thick enough board combo is best.

the trickiest bit is getting the front face of the battens shimmed out to a flat surface to take the back of the boards. a long bubble/long strait edge and chalksnap plumbline are handy to set the edge battens and then to work in from the edges making sure everything fits tight to the plane of the boards using the strait edge.

any mistakes at that stage will cause issues later
Hairyloon

moisture resistant is ok for kitchen/bathroom walls/ceilings etc but if it is a "wet" area ie it needs tiles it is best to use a properly waterproof substrate board.

I'd instinctively agree, but I've just part-dismantled the old shower cabinet and it seems to have been made of ordinary gyproc plasterboard, tiled over: doesn't seem any the worse for wear from being the back of a shower for many years. Confused
dpack

sometimes it is ok, if all the plumbing,tiling,grout and sealing is perfect to start and repaired as required and the house does not move causing even a small hole somewhere important it can be ok.

i have put a lot of horrible black messes in skips though and always try to use waterproof substrates if any tiling will ever get wetter than a damp cloth
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