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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First, gather the tools and materials mentioned later.

move furniture, curtains and anything you want to keep out of the room, and lay plastic sheet over the floor and anything you can't move, in case it falls down while you are working.

put large sheets of ply or chipboard under the ceiling, pushed up to meet it, but not enough to push it up, yet. Brace it up with "deadmen" which are wooden props standing on the floor and wedged up against the boards.

the boards will, for the moent, mean that if the ceiling starts to come loose, it might only fall an inch or so, and it might only sag.

go into tjhe upstairs room or loft sabove the ceiling, which you have previously cleared of carpets, furniture and boxes of Christmas decorations.

Start lifting floorboards above the sag. Try not to use a hammer at any point until after the repair is complete and hardened. Hammer blows will wake up the ceiling and remind it that it wants to fall down. instruments like jemmies and a broad iron bolster will get most boards up. If you are lucky some of them will have been screwed down by plumbers and electricians working under the floor, and you can unscrew them. At the verry least, you should be able to see which boards have been up before, and will be relatively easy to lift. Try not to smash or damage them, because you will want to put them back later, and they will not be the same thickness as boards you can buy off the shelf.

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you can now see the upper surface of the ceiling. Observe that it is thick with dust, dirt, fluff, dead mice, hidden pornographic magazine, Victorian sex toys, and pieces of plaster. Very rarely, sacks of sovereigns or diamond bracelets. Pick them all out and have your assistant carry them away in buckets or rubble bags (not plastic bags, which will be torn). The stairs and carpets need to be covered with dustsheets due to the filth.

Using your builder's canister vac (not a domestic vac) hoover the loose material off the top of the ceiling. The dust is very heavy, and your assistant will not be able to carry the hoover when it is full.

You will now observe that some of the dead mice and plaster nibs have crawled into the gap between the laths and the sagging plaster. In some cases, under the joists. You must remove them, otherwise, when you jack up the boards to lift the ceiling, they will obstruct it and cause it to break.

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You can now call on your gang of assistants to slowly raise all the boards so that they push the sagging ceiling up into place. It can then be wedged up against the joists, as hard as you like.

Make them tea and hand round cake.

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the ceiling should now be in its proper place.

All you have to do next is to secure it so it doesn't fall down again.

The laths, with luck, will not all be rotted and broken, and some of the nails might not have rusted away to nothing. But you can't depend on them. So you need a support that can be fixed to the joists, and to the plaster.

You can do this using Expanded Metal Lathing, which is a sort of lightweight galvanised sheet, resembling a very coarse mesh. It is fairly easy to cut with your snips, and to bend between two square wooden battens. You want to bend and cut it into trays, sized and shaped so the bottom of each tray fits betwen the joists and lies lightly on the plaster, and the sides rise about half way up the joists so you can screw them into position (no nails remember, as hammers are not allowed) .

When you have done that, you will probably feel like knocking off for the day

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You are now ready to fix the lathing to the plaster.

Using your large builders bucket, tip about one third full of water from your watering can, and shake in plaster from your sack. You want a thick, creamy pouring consistency. Mix it and pour it quickly or it will set. You can use fine finish plaster, or Joint Cement (which is used for plasterboard jointing, and repairs). You have to pour it to fully cover the plaster and the expanded metal, it must be thin enough to run through the holes. You could probably also use wire mesh.

as well as sticking the plaster into place, it will also run through and cracks and holes, and set against the boards. I should have mentioned that the boards should be covered with thin plastic sheet so the boards don't get stuck to the ceiling. You can peel off the plastic later.

give it a day or two to set, and then you can release the deadmen and cautiously lower the boards. With a bit of luck it won't all fall on your head.

When working on joists where there are no floorboards, make some stepping boards to walk and kneel on. you can screw battens to the bottom of these boards to fit over the edges of the joists so they don't slide out of place.

did I mention it was labour intensive and uneconomical to try to get a builder to do it?

Please write a book on home diy piglet John. I would buy it. We only found an installation manual for a very old telephone and some cigarette butts in our last sub floor adventure.

Thanks that's interesting

I second this indeed 🙂

Thanks @PigletJohn

When our lath and plaster ceilings sagged, we had them pulled down and replaced. I can’t see the point of keeping them.

Or just overboard it.
Or pull it down and plasterboard it.

Indeed. The sheer weight plus time means its
the best solution all round to replace them.

I have no objection to that.

Some ceilings are very fancy, with cornices, coving and roses.

Thanks. I don’t believe we need your permission.

nor, indeed, do I need yours.

Ignore the PP being argumentative! This is very helpful of you @PigletJohn

I have a sagging lathe and plaster ceiling so this was an interesting read!

Mine are! I hope they won’t fall 😱
We didn’t manage to save the (undecorated) ceiling in the attic when putting insulation in. The builder wanted to, but it all just crumbled; they were cracked even before. However, it was probably something else than lath and plaster, I think it was horsehair mixed with lime. Also nailed to the joists with a million of nails.

Well that’s peak unsolicited advice!

Having been down this road myself I agree with pp that the better option is to pull it down and plasterboard it. It’s messy but quick and easy for a diy-er to do the first part. Can even do the second part yourself if you are capable and have an assistant - did. Then get someone into redo your cornicing and ceiling roses in real plaster. Main thing is you have piece of mind your 2 ton hundred+ year old
ceiling isnt going to come crashing down on your head one day.

My house was built in the 1930s so I don't know if I have these ceilings .
I've got coving and original dado rail and picture rail ( ver' ver' untrendy I know but meh )
I do know however , if any work needs done we'd get someone in.

DH is repainting the bathroom at the moment , it is a saga .
He won't be doing ceilings ,

Thank You @PigletJohn , I'll read these posts and it will stiffen my resolve to not let DH loose with anything sharper than a brush

This was very much not unsolicited @Weekmindedfool
I started a thread yesterday asking for ceiling repair advice and piglet John has posted it here seperately to make it easier for people to find it should anyone else to know how to repair a ceiling. Although I can see how this post looks a bit weird if you haven't read my thread.

This is, as always, very helpful advice @PigletJohn- which I wish I’d read before our bathroom ceiling did in fact fall in when we had our bathroom redone. If nothing else. I’d have been known the signs to look out for.
it had actually been over boarded previously it seemed, and layers of ceiling came down, and in the end we replaced loads of joists too. We searched and searched for the source of a leak that could have caused what we assumed was water damage. Interesting that actually it may well have just been the age.
out of curiosity, when they took down a lathe and plaster wall (same bathroom, lots of problems) the plaster was full of fibers, they’d told us it was likely horsehair to bind it together - does that sound about right for a 1930s house?

Solidarity, my fellow 30’s homeowner. I absolutely adore our 1930s semi but I am constantly on edge about what new problem I might find (not helped by the years of previous DIY fixes that have taken place)

Thanks @PigletJohn

Didn't think of using mesh

presume I can use lime plaster to join the mesh to the old lathes

I won’t be allowed to use anything else
Do you foresee any issues ?

You mentioned the list of materials but didn’t post it

some of us asked pigletJohn about lathe and plaster ceilings on another thread we were on
About ceiling repair

Hence This thread
Not all properties can have the lathe and plaster ceilings removed or overboarded
eg Listed buildings and
some with mouldings and roses etc