How to repair a sagging old ceiling (Lath and Plaster)

[PigletJohn]

This method of plastering ceilings was mostly gone during the 1950's, but there are lots still remaining that have not fallen down yet.

The construction is thin wooden strip "laths" which on later houses might be sawn, and on older houses split away with a sort of knife. I seem to recall that they were often made of chestnut. Often about an inch wide and a bit over a quarter of an inch thick, and nailed to the joists of the floor above at intervals of around an inch (I have not got one handy to check)

The plasterer mixed up a large bucket of powdered lime with water, and spread it on the ceilining, pushinh hard so it bulged up between the laths and flopped over. These flopped bits (the nibs) between the laths, once set, supported it and prevented in falling down, as did the adhesion to the laths.

In the hundred years or more since they were built, two things happened.

The plain steel nails holding the laths rusted away, especially over steamy kitchen, bathrooms and washhouses

In the Unpleasantness around 1940-45, must British towns and cities experienced numerous large explosions, which shook the houses and created blast that rocked the ceilings up and down.

It is therefore usual to find the ceilings are cracked and damaged, and many of the nibs have broken off. Many of them stay up from force of habit, and if reminded of their precarious condition, will fall down at whim. As well as sackfulls of thick lime, which is rather like mortar, and heavy, there will be a hundred years of dust, moths, woodworm and grime that will fall down. It is spectacularly dirty.

If you are a DIY homeowner with sufficient energy and fortitude, you can repair it yourself

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Yes it's lath and plaster.
Can clearly see it from below through the crack.
1930s semi.
I wondered how it could be cut out without risking more of it coming down.
I don't think my handyman would be able to do your method and he has no one to help him. I cannot except in very minor ways.
I avoid going into the loft.
He has suggested patching it with putting some wood across the crack and screwing that to the joists ?
But I feel any pressure on that crack will cause it to open up further into the centre of the room.
And cause more mess.
The room is also full of things most of which cannot be moved..
You're not near middx are you???
Could he cut a section out from below without cutting the laths ?

I don't really know what I'm talking about.
As you can probably tell.
It just causes me such stress.
My priority is stopping it getting worse.
I cannot clear the room.
I don't care what it looks like as to patching etc.
If it can be done. Just to be safe. Or safer.
Your method sounds like heritage restoration work.
But there are no assistants here.
And part of my problem is that handyman who is really a carpenter is the only person I can bear to have in the house.

"He has suggested patching it with putting some wood across the crack and screwing that to the joists ?"

He would have to clean the upper surface and remove the broken nibs first

Screwing wood to the joists would lift the sagging section back into place. The entire loose section needs to be supported or it will crack again.

But leaving the wood there would look much more unsightly than fixing it from above.

I suppose somebody could have a go at it later if they wanted.

Show him my guide and see what he says.

OK. Thank you.
? What would the edge of the crack look like as it doesn't look like I imagined plaster would.?
The cross section which has descended looks heavy and is grey.
About an inch thick.
Could it be some sort of board someone has put up.
It looks like a solid piece.
I imagined plaster to be white or pink ish and more powdery at the edges.
Not sure if I am explaining this.
What does a cross section of a dropped L and P ceiling look like.?

Lime plaster I have seen has always been off-white. It might vary with mineral content in other parts of the country.

Gypsum plaster can be grey, pink, or (if damp) chocolate brown.

Plasterboard cracks are straight, and show the shape of the rectangular boards they are made of, except where somrone has stepped on a ceiling from above. Regular lines of nail heads may show. It is typically about 10mm thick and has a coarse paper coating on each side.

Post some pics. Include something to show scale.

Yes it's a picture of the upstairs floor, which "erupted" for want of a better word, and has caused a sag in the ceiling underneath it.
My house was built in 1660 with an extension on the back dated about 1720.

Cement screeds over wooden floors are not rare, though IMO they are very undesirable. I think you need to pick it off from above.

It might have cracked due to movement of the timber, or there could be something causing expansion. Sulphur content can do that, as when fly ash from furnaces is used, or mixed rubble that includes gypsum plaster. Are you in Scotland, or in a former steelmaking or coalmining area?

Interesting you say that @PigletJohn , about 3 miles as the crow flies from Scunthorpe steel works.

Sometimes furnaces slag was mixed into concrete, and subsequently caused cracks and heave. We don't have it near me. An experienced local builder should be able to say.