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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My ceiling is sagging. Because at some point a previous owner/ bodger took up all the upstairs floorboards and filled the joists with concrete. Which has sagged, pushing my downstairs ceiling.

What!
concrete !

Blimey I thought the last owners of ours were bad

Yes, I'll try to attach pics.

Lime plaster is horrible stuff-it will burn your skin badly, let alone your eyes. I wouldnt want ti be working with it overhead!

House built in 1901. DD’s ceiling was sagging. “Not a problem” said the builder who was working on the house.” I’ll put it on the list.”

2am it came down! Missed DD by a whisker. She was woken by what she thought was the sound of rain ( it was plaster dust) then half the ceiling hit the floor! Luckily not the half over her bed. DH and I thought the house had exploded! Poor lamb was bolt upright in bed covered in dust!
Next morning I rang the builder….” You know that ceiling…?”
We weighed some of the pieces 10kg!

Yes the plaster was often mixed with horse hair.
The plaster is extremely heavy and over time gravity does all the work to bring down the ceiling. You don’t need water damage or another external cause. It’s quite scary to think how much weight is above your head in these old houses!

Mouldings and roses can all be replaced. Either pre made off the shelf or you can still get people in to do it in situ.

If it’s listed and you are required to replace like for like I’d be surprised if they would allow you use steel mesh embedded in your ceiling.

Loved all that @PigletJohn!

Also, I had a ceiling replaced in my bedroom. As he was pulling the old bits of ceiling down, the poor plasterer was nearly brained by two heavy boxes of tiles which the previous owner, at least a decade ago, had nestled between the joists in the loft, and covered well in loft insulation. That exact spot had been above my head with a crack in it for a few years 😳

We've just moved into a 30s house. How can I tell if I have this sort of ceiling? There's no sagging anywhere.

@Nitgel you can only really tell for certain if the ceiling falls down or if you take up a floorboard upstairs and look a the other side if the ceiling.

No they’ll allow the mesh to repair
because it’s only small areas

Of course you can pull it all down in some properties but this thread is for people who are interested in repairing the lathe and plaster

as I think others have said too.

is it slightly bumpy.

if you tap it will have a hollow sound

you can lift a board from above to see from there.

tools and materials are a little puzzle for you. They are "mentioned" but not listed.

I have not worked with lime myself. A lime plaster ceiling is very thick and heavy, like concrete (which is why it is a far better sound barrier than plasterboard or modern lightweight plaster). If you have floors up, you can pack the gap with sound-muffling mineral wool, or with Cavity Batts which are not so expensive. Loft insulation is much lighter and less effective but helps muffle airborne noise.

Thanks 👍

(I thought I'd posted this but I can't see it now....)

old laths are liable to be rotted, worm-eaten and cracked, and their fixing nails to be rusted through. I have seen some that fell down, and a child could snap them with his fingers. You might be lucky, but you cannot rely on them.

that crack looks to me like L&P. Plasterboard cracks are usually in straight lines, and not wide.

Thanks piglet

good point

we know ours are fine luckily as I’ve seen them from above
we have problems only because of workman dropping stuff etc when working in the roof space

I’ll post a few pictures when we start jacking up the floors to do the work maybe after Easter
( assuming the conservation officer gives us the go ahead quickly )

looking again at your pics, I see an electrical socket. So I deduce we are looking at a screeded floor, not a plastered ceiling.

There's a characteristic appearance on the other side of lath and plaster - as the plaster is forced up onto the laths it should squeeze through and make what are called 'nibs' or 'snots' on the other side. These help to keep the plaster attached. I've attached a picture which may take a while to appear.

Someone mentioned horsehair/animal hair and this is important too. It gives the plaster flexibility so in an old house it can move as the house settles, and some cracking is absolutely fine. Lath and plaster is also breathable and helps with damp as it doesn’t trap the moisture in like gypsum plaster.

It's very sad that these ancient techniques used for centuries have been lost - although it’s much easier now to get tradesmen to work in lime, or in modern lime-adjacent materials. It’s being recognised that it was used for a reason.

I know all this because we bought a 400 year old house (though weirdly, not listed) with lath and plaster ceilings and replaced them! We also went on a course to learn how to do our own small repairs.

ETA: @PigletJohn mentioned the 'nibs' in his OP. The photo shows what they look like (when it appears)
Further ETA : that's not our house but we do have some old ceilings like that. Also some which are L & P but probably around WW2-era. It was used routinely for many centuries.

That is very clean looking compared with all the ceilings I have seen!

Thank you for this PigletJohn.
Due to now remedied water ingress the corner of a bedroom ceiling is cracked and hanging down.
About 2 inches towards the corner where the water came through a valley above.
Daylight can be seen through in the loft.
Is it feasible to cut out that corner of the ceiling and replace with board screwed to the joists?
Someone has suggested this.
It can then be roller painted?
I don't care what it looks like.
Just to make safe. Thanks.

Is the ceiling plasterboard, or L&P?

You could bodge it, though cutting laths may make more fall down, or you could repair it as described, which for a small section is not a huge job.