Sagging Ceiling Plaster . . . What's Happening?

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The quick answer is ... gravity! Of course you knew that, right? So the real question is: why are MY ceilings sagging?

Problems with plaster ceilings

Four probable causes:

1. backing failure

2. water

3. plaster failure

4. weak framing

Backing failure. Wood lath can come loose. The nails over time may work their way out some as the wood framing dries out and shrinks. The lath pulled by the weight of the ceiling plaster follows gravity and you see a droop or bulge.

Water. A roof or plumbing leak can weaken old plaster and cause it to sag.

Plaster failure. This is usually the most common problem with old ceiling plaster. Over wood lath, plaster depends on the strength of its keys for security. Plaster keys form when the fresh plaster was first forced into the narrow spaces between the lath. The plaster curls over and locks into place behind the lath, providing the anchorage to hold it in place.

Over time, due to vibration and other factors, the keys can break. When this happens to enough of them in a localized area, a sag develops in your plaster ceiling.

Weak framing. I am often amazed how underframed some old ceilings can be. You will sometimes see a plaster ceiling up to 10 or 12 feet wide framed with two by four joists. Plaster lath with its plaster overcoat is HEAVY, way more than such light framing should have to support.

I have seen old ceilings like this sag in the middle by two inches or more. It is the framing that is sagging, not the plaster per se which may still be fairly secure.

In this case, someone who knows what he is doing, like a skilled carpenter, needs to go up into the attic and put in place a strongback member to attach everything to once the ceiling has been jacked up to a level position. Only then should any broken or cracked plaster be dealt with.

Another problem I often encounter is found in more modern plaster, in other words, ceiling plaster that is applied over rock lath.

Here the problem is that the rock lath itself is sagging. Rock lath became a popular substitute when wood lath fell out of favor. It is typically three-eighths of an inch thick, often too light for the weight of the three coat plaster usually applied over it.

If the plaster work was done at a time of high humidity and the three coats applied in rapid succession, without sufficient drying time between coats, the lath is soaked with moisture and weakens.

Everything sags a little between each set of joists. The sag is locked in place when the plaster hardens. The result: ceiling waves , most noticeable when the light comes in at an oblique angle (morning,evening).

Remedies

When plaster over wood lath fails and a bulge shows up, you have to remove the loose plaster and replace it. This can be done either with new plaster or with thin drywall glued and screwed into place to fill the depression over the lath. how to fill holes

It is a good idea first to reattach the old wood lath in place if needed, using small galvanized shingle nails. Larger nails will likely split the lath, which you certainly want to avoid.

Sometimes the best remedy for bad ceiling plaster is to remove the bulges, apply furring strips over where the joists become visible to bring them level with the surrounding plaster, and overlay the whole ceiling with three-eighths drywall. Screw the rock into place, don't nail it. Tape and finish the joints and texture to suit your taste.

If your sagging ceiling plaster is backed by rock lath, and you want to get rid of the ceiling waves, you will need to fill the low spots and bring them to level with the sagged areas. Fortunately, joint compound is relatively light material (when dry) and you will not be adding a lot of weight, even though you will be putting on mulitple coats to level things.

You may end up coating as much or most of the ceiling by the time you fill in all the low spots. So you might as well lightly skim the remainder and retexture the whole thing to suit yourself.

If the plaster is sagged in a localized area due to water damage, you can either replace the loosened veneer coat (last coat of a three coat system), or if the lath is sagged also, cut the whole bad area out and replace with drywall brought level with the surrounding plaster.

If the sags are extremely pronounced, the only remedy may be to remove the ceiling entirely and start over with new plasterboard and veneer plaster. Probably a job for the plastering professional.

This is the newer and superior form of modern ceiling plaster. Less expensive too than the old three coat system.

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