Plaster Ceiling vs. Gravity
DEAR TIM: I live in a 70 year old home. The plaster ceiling in my living room fell to the floor without warning. It ruined my furniture and floor. What caused this to happen and how can I check to see if my other ceilings are in danger of falling? G. H.
DEAR G. H.: There can be many reasons as to why your ceiling fell to the floor. Water damage, vibration, house settlement, structural member failure, etc. are among several of the possibilities. However, I think the most common cause of this type of failure is seasonal expansion and contraction of the wood framing members of your ceiling and the wooden lath boards which are nailed to the ceiling joists.
The plaster is attached to the ceiling joists by means of thin pieces of wood called lath. These lath boards are nailed to the ceiling joists with approximately a 3/8" space between them. When the plaster is applied to this wood lath system it oozes thru the spaces and spreads out on the top side of the lath boards.
When the plaster dries, it is 'locked' into the wood lath system. This system works very well as long as nothing moves. However, the wood lath expands and contracts at a different rate than plaster. In the summer months, the wood has a tendency to expand because it picks up moisture from the humid air. The wood lath contracts in the winter months when the air is drier.
This movement between the two materials causes the plaster to crack where it oozes thru the wood lath strips. When it cracks in enough locations, the weight of the plaster causes it to completely detach itself from the wood lath and fall to the floor, often with catastrophic results.
You can check the other ceilings in your house for this problem. Examine the ceilings and look for cracks. Using your hand push lightly against the crack. If the plaster moves up and down, it has detached from the wood lath and may be in danger of falling. Be sure to wear safety glasses or goggles when doing this, because small particles of plaster can fall into your eyes.
