Construction Consulting Questions #4 – Are cracks in plaster walls bad?
Many older homes throughout the East Bay and Bay Area in general have plaster walls and plaster ceilings instead of the newer gypsum sheet rock material.There are benefits to both covering systems as well as issues that detract.
First, let’s look at a few comparisons. Plaster take time to install since you have to preinstall the “lathe” backing board strips prior to putting on the two to three coats of plaster whereas sheet rock installs quickly and requires minimal work to finish the seams and apply a smooth or textured final coat. Plaster, along with the lathe backing, can dry out if it gets wet due to water intrusion whereas sheet rock with its paper backing can become a source of mold when moisture conditions occur on the backside of the material. Plaster is more rigid and crack from structural movement. Sheet rock is more resilient and can “flex or give” more readily.
Why do plaster walls and ceilings crack more easily than sheet rock? There are several reasons, but let’s start with the basics. Plaster is relatively brittle and sheet rock is not. Sheet rock is simply attached to the wall or ceiling framing with nails or screw fasteners. Plaster is applied over lathing strips (which look like yardsticks with a small space between them) and becomes rigid after it cures. Note: the reason that there is a space between the lathing is so that the plaster (after it is mixed) will “ooze” inbetween the lathing and “roll out” onto the backside of each adjacent lathe and form an elongated ridge which is wider than the gap, yet integral with the main or base coat of the plaster itself. In this manner, the initial plaster coating is gripping the back sides of the lathing along the edges at the gap. (Make sense?)
So, with this brief knowledge of how plaster is installed, we’ll quickly talk about cracks you might be experiencing in your walls and ceilings.Cracks in walls generally occur from the differential movement of your framing due to building distortion from settlement or framing deflection etc. (and seismic events of course).Ceiling cracks can have the same issues as walls in addition to the fact that it is on the ceiling (let me explain).With the plaster being applied to the underside (downside of the ceiling) along with the fact that the initial coating is being held in place from the “gripping effect” of the plaster on the back side of the lathing, the weight of the ceiling plaster can cause some slight bowing of the ceiling framing.
Over time, ceiling framing (the joists) can droop or bow due to “long term bending” such that the center of a room’s ceiling might be a little closer to the floor than the ceiling which is nearer the walls. (Get the picture?) Now with this in mind, you can begin to see that the length of a ceiling joist which is bowed is actually longer along the bottom side as compared to when it was originally installed. This lengthening does not fare well with the more rigid plaster and as result can cause the cracking (or stretch marks if you will). Simple cracking due to the gripping portion of the plaster is also a potential culprit due to inadequate plaster being installed to form the “grip” etc.
Are these ceiling cracks bad? Not necessarily so, but usually it is a good practice to physically see just how loose the plaster has become so that you don’t get sizeable areas falling on you. The fix for most homeowners is to either patch the cracks (best method for patching is to utilize wide permeous cloth and sheet rock mud), install sheet rock over the existing plaster and lathe, or better yet, remove all the plaster and lathe and replace with sheet rock. Cracking to the walls are usually not as prevalent as ceilings and patching is more often than not the remedy than complete replacement as it is for ceilings.In closing, “Are cracks in plaster bad?” The answer lies in the arena of “Why are the cracks forming in the first place?” Understand why they are occurring and you will get a better understanding of your choices on how to rectify them as well as have a better feeling as to whether there is something “bad” or not with respect to framing issues, settlement or perhaps seismic activity.
Anderberg Construction Consulting, Inc.
4100-10 Redwood Road #368
Oakland, CA 94619-2363
(510) 482-2779
tomanderberg@gmail.com
