Thread: Plaster Ceiling Systems - Hidden Danger

I am a plaster restoration contractor in Greensboro, N.C. After more than 25 years of plaster restoration, I have concluded that there is a common defect that can indicate a real safety issue involving certain plaster ceiling systems.

Under-engineered Attachment System as a Singular Cause of Failure and Collapse

The fact that plaster ceiling systems fail* and collapse* is not a new issue. However, my theory that under-engineering (which is a widespread condition) is a major factor in attachment failure and ceiling collapse…often the only factor…is new.

Though plaster may collapse and break off of from ‘traditional’ wood lath and cause damage, my concern is with plaster-over-gypsum lath ceilings, especially ones that used 4 x 8’ gypsum lath. (I refer to these ceiling systems as ‘transitional’ ceilings because this combination of materials and techniques was employed after the time of ‘traditional’ plaster over wood lath, and before modern drywall.)

Why is this a Safety Issue?

These ceilings concern the home inspector because when they collapse, it tends to be complete, sudden and monolithic. Therein lies the safety issue. Such a collapse involves hundreds of pounds of material, in addition to insulation which may also contain asbestos. (The failure and collapse occur at the point of attachment to the joists.)

My first experience with a ‘failing’ plaster-over-gypsum rock ceiling system was here in NC. A client called me to come and look at the ceiling in his grandmother's room. One-half of the ceiling was hanging at a 5 degree angle, leaving an 8" gap the length of the room. The ceiling attachment system (between the lath and joists), was in an advanced stage of ‘failure’.

We removed the ceiling completely and dry-walled it. ** Fortunately, the grandmother had been moved out of the room and the ceiling had not collapsed on her.

The construction was three-coat plaster-over-rock/gypsum lath. It was attached by means of a ‘clip’ system to the joists, but had weakened and relaxed over time with the weight of hundreds of pounds of plaster.

Over the years, I have seen and remediated numerous such ceilings in the ‘process of failing,’ though they had not collapsed. I have also seen various attachment systems for that 'transitional' time period; the three most common being smooth nails, angled metal clips and wires. I have observed all three methods of attachment showing signs of failure.

What Should a Home Inspector Look For?

The age of the home should be your first clue. Transitional ceilings span roughly the period from the 1920’s to the 1960’s.

One way attachment failure can be observed is to look at a ceiling from the attic side (especially towards the middle of the ceiling), moving back the insulation. There should be no space between the joists and the back of the gypsum rock. Any space there indicates the extent to which the attachment system has been compromised.

A more obvious indication of attachment failure is the presence of straight cracks, either running the length of a ceiling (the most serious safety problem) or following a zigzag pattern. Because the transitional period for plaster-over-gypsum rock spans almost 40 years and different sizes of gypsum lath were used, one sees different patterns of cracking in these ceilings, following the joints in the gypsum rock.

Rarely, a ceiling may show circular or curving cracks. This usually will be a ceiling in the later part of the transitional period and also indicates a serious condition.


Another indication of a failing attachment system is ceiling repairs that do not last. This is because the plaster or drywall contractor did not understand the underlying cause of the crack and did not properly anchor the ceiling back to the joists before taping and mudding.

Signs of failure can be detected by home inspectors with a trained eye in most cases. As with any potential safety issue, recommendation should be made for further evaluation/remediation by a knowledgeable plaster repair person as these ceilings have been known to collapse.

Can Collapse Be Predicted?

Though these cracks are indications of attachment failure, one cannot predict when or if these ceilings will 'collapse'. However, the state of failure, if not remediated, is a progressive one. Given enough time, collapse will eventually occur.

An early client, M. Rothrock of Greensboro, N.C., shared a story with me as I was remediating the living room ceiling in her father's home (this one was attached by wires and was about 3" away from the joists as seen from the attic side)

The neighbor's house, built at the same time in the 40's by the same contractor, experienced a total collapse of all the ceilings in the house shortly after construction. Besides a number of similar stories from clients, I have found a number of instances online of property being damaged and in one case, a small child being killed by what appears to be this type of sudden monolithic ceiling collapse.

Visit my website at Transitional Ceilings For pictures and links to stories of pertinent ceiling collapse cases. In some, engineers or architects describe the cause of particular collapses and corroborate my conclusions here.




* For this discussion, a ceiling attachment system that has failed, or is in a state of failure, means that it is in a compromised state where it is observed to be in the process of failing at what it was intended to do; that is, hold the ceiling securely to the joists. When I use the word collapse, I am referring to a ceiling or part of a ceiling that has completely fallen to the ground.

** Estate Plaster Inc. has developed remediation techniques that do not require the removal of the ceilings in most cases. If a ceiling can be re-secured, it saves time, trouble, as well as money for a client. Proper remediation may also decrease failure and collapse due to contributing causes, such as water damage, storms or factors related to heating/cooling cycles and the affects of vibration or impact.


Philip LaMachio
Home

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I've found a picture of some of what I believe you are calling transitional lath. This first pic is from a 1956 wall cavity, looks like 16" gypsum strips. No serious cracking was seen in this house that I recall.

The other 2 pics are from a 1960 house. I pushed up on both sides of these cracks and saw no movement. Certainly, I am looking more closely now. Thanks for the heads-up.

John,

You're right. Clearly gyp lath.

The first clue is the date of construction (1920-1960), then look for type of plaster system (some houses did not have plaster over the gyp lath, perhaps to save $$$. )

Then look for the obvious signs of failure, ie: cracks on ceiling (walls are not really an issue).

Follow up with gentle pressure around crack to determine how secure the system is, or measure from floor to ceiling at crack and at edge of room to determine extent of drop.

Visual inspection from ceiling side may indicate extent of sagging, failure or even type of attachment.

Thanks for your interest,
Philip LaMachio
Home

First was plaster on stone/rubble.

Followed by plaster on brick/masonry.

Followed by plaster on wood lath over framing. Wood lath was originally wood boards with splits started in from the ends and the boards were pulled apart, expanding the wood, creating open slits between the splits for the plaster to mechanically key into. Later small wood slats were individually cut and nailed up as wood lath.

Followed by plaster on metal lath over framing (which is still in use today).

Followed by plaster on rock lath (not much used today other than restoration).

Followed by veneer plaster on plaster board (also known as "blue board" as the blue paper is designed to allow veneer plaster to bond to the paper. Veneer plaster is also known as "one coat" plaster as traditional plaster on wood/metal lath over framing consists of a scratch coat where the plaster is actually mechanically scratched into the metal lath/between the wood lath, followed by a brown coat which builds depth of the plaster, followed by the finish coat. One coat plaster is basically equivalent to the finish coat of three coat plaster.

The above being laid out very basically, and it will be easy to pick apart with details should anyone want to, and they can fill in precise time-lines when doing so.

Before he became a successful building contractor my late father-in-law was lather, then lathing contractor. When I was a young carpenter and had never seen wood lath installed he gave me a demonstration that blew my mind.

You have never seen anybody install anything faster that an experienced lather. They carried the nails in their mouth and spit then out like a machine gun and hammered home the wood lather with little blue lathing nails faster than many use a nail gun today. Also, as an aside, upper end homes had all their walls and ceilings canvassed after being plastered so you hardly ever saw any cracking unless there was some serious framing movement/failure.

Since we are delving into plaster history, here's a story.

A fellow plasterer and friend of mine who was apprenticed in Gr. Britain has worked on historical castles and Ann Hathaway's cottage (wife of William Shakespeare) located at Stratford-on-Avon.

That cottage, as did many structures at the time, used inter-twined twigs upon which the plaster was laid. The 'system' was called 'wattle and daub'.

I have plaster history books which have a lot of interesting info, but they are packed away at the moment.

I think though, I will have a plaster history page on my website. It is a very long and interesting history.

I also have a job coming up where the transitional ceiling is sagging at least 3 inches and will get photos (hopefully from attic side as well) of before and after.

As a P.E. in the Boston area, I've had the opportunity to investigate many of these collapses over the years, usually for the insurance company.

As dumb luck would have it, on every occasion the building happened to be unoccupied and there were no injuries. In my research, I ran across an interesting book (just in case anyone is interested): Metal lath hand-book... - Google Books

I'm always interested in these old construction textbooks (I know, I need to get a life)

By the way, if you haven't discovered it yet, Google Books is a great place for free research

Nice link, Steve but I don't get the free book.

Here's what a prolonged roof leak did to a wood lath ceiling.

Yes, John. Water and plaster don't mix, except when you are in the application process.

I bet that plaster outlasts drywall in the same situation though.

Originally Posted by Philip Lamachio

Yes, John. Water and plaster don't mix, except when you are in the application process.

I bet that plaster outlasts drywall in the same situation though.

We don't encounter mold growing on wet plaster, that's one good reason for keeping it up.
I found another 50's house with extensive ceiling cracks. This house has major foundation settling, sloping floors. Gypsum lath appears to be well anchored, so far at least.

We don't encounter mold growing on wet plaster, that's one good reason for keeping it up

Right. The lime inhibits mold growth. However, water will deteriorate the plaster over time.

found another 50's house with extensive ceiling cracks. This house has major foundation settling, sloping floors. Gypsum lath appears to be well anchored, so far at least.

Did you try pressing on the ceiling on either side of the cracks? It is possible the attachment is fine in this case, but seasonal expansion and contraction of the joists and rafter system exerts pressure on the ridged ceiling matl. and causes these cracks to form along the lath lines.