This picture is a shot of ribbed lath from the 1980s and the development of plaster keys.
Keying plaster means applying the plaster with enough pressure to assure that keys are developed. Without plaster keys scratch and brown coat lamina will fall off the wall. I visited a central California elementary school years ago at the behest of the maintenance guy. He asked me to go up into the crawl space of the 10′ wide ceiling above the kids walkway on several of the buildings. He got a ladder and held it for me as I climbed up and stuck my head into the crawlspace.
I saw keyed plaster just like this. Thinking I would check how hard it was, I rubbed my thumb across the keys and they turned to dust. I rubbed my hand along an 8″ round area and it all turned to dust. Then the ceiling showing on the front side broke loose and fell to the walkway. The keys were perfect, but they had not cured at all.
Later – during the frantic covering your butt stage of the process of fixing what was a dangerous situation for any kid attending the school – I found out the sand to cement ratios were almost double what they should have been, and the ceilings and walls had never been moist cured. Finish had been applied but it was acrylic finish, with no moisture added to the substrate prior to application.
This picture shows plaster over foam that was not bedded. Slamming the door of the home produced the condition you see here.
Inspection of plaster, using the Moh’s Hardness Test by scratching the wall with a penny, is an easy simple way of saving yourself major grief. During seminars I give on inspecting plaster, Moh’s Test is the first item on the list of ways to inspect plaster.
If you have an interest in scheduling a seminar or you have questions about this condition or other situations, call or email:
Portland Cement Plaster 
Cement plaster falling from a ceiling at a school is more than just scary. Nice job to help protect the kids . Since I work terrazzo now, the Mohs scale is in my daily vocabulary. They test marble chips all the time.
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