When Is It OK to Redecorate Wet Plaster?

Robert Grupe / April 2019

Q: I recently had a roof leak and as a result, have wet plaster walls. Can I use a moisture meter to determine the moisture content of the gypsum?

A: Gypsum can withstand cyclical wetting without degrading the mineral and physical properties of the finished product. For many years, gypsum was used as a poured-in-place gypsum roof-deck. To prove the prior statement, some manufacturers would repeatedly place a sample of the roof deck in a pail of water overnight. Once the sample dried, it retained all of its properties. The dried-out sample could be re-immersed into water with the same results. However, prolonged immersion will degrade the product. In gypsum panels, if the core gets too soft, the facers will tend to delaminate. In plaster, the material will get soft to the touch. The point is that wet plaster may not require complete removal. If allowed to dry out, the material will retain all its working properties.
A visual inspection of either gypsum plaster or gypsum panels is usually adequate to determine the serviceability of the product. If the material is visually solid, not soft or chalky, it will most likely be good for future use. There is no fear of failure tomorrow should the material get wet today. However, what can’t be determined is the status of what is behind the gypsum. Failure of any water-sensitive material such as wood framing may still be an issue.
Moisture meters are not recommended by the Gypsum Association to determine an accurate moisture content in gypsum. In their pamphlet, “Assessing Water Damage to Gypsum Boar,” they state, “Even moisture meters specifically intended for use on gypsum board do not generally provide accurate percent moisture readings.” A 20 percent reading for wood is acceptable and common; however, that same reading in gypsum is meaningless. Should the actual moisture content in gypsum be 20 percent, the material would be too soft to remain in place. At best, meters will give a relative reading and provide the user an indication of how far the moisture intrusion has migrated from the source.
There is an ASTM standard on moisture meters specific for gypsum products. The scope of this specification designated C1789, Standard Test Method for Calibration of Hand-Held Moisture Meters on Gypsum Panels, is for the calibration of “handheld moisture meters for gypsum board, glass faced gypsum panels and fiber-reinforced gypsum panels by means of electrical conductance and dielectric meters.” Experience has indicated that accuracy is not guaranteed even using this procedure. Thus, the readings taken from a meter cannot provide an accurate value and therefore determine a safe level of moisture within gypsum.
Assuming the plaster is still solid and just damp and there is no evidence of mold growth, when can the plaster be redecorated? This level of dryness is important if the desired redecorating is paint. If the redecorating is a wet product such as plaster or a Venetian finish, then it’s more important that the original plaster is solid. The first step in any moisture mitigation process is to find the source of the moisture. Any leak must be repaired or sealed so there will no longer be any water getting to the plaster. The next step is to investigate and remove any mold. A good source for information on this topic is from Foundation of the Wall and Ceiling Industry, “Preventing Losses from Moisture and Mold During Construction” (www.awci.org/foundation).
The next step is to make sure the plaster is dry enough for redecorating. A common mistake is to use temporary heaters to facilitate drying. This may introduce excessive moisture into the room and ultimately to the plaster. The combination of dry heat and ventilation will effectively remove any excess moisture in the plaster.
Plaster with a high degree of moisture content will feel cool to the touch when compared to plaster that is dry. This is the method recommended by at least one plaster manufacturer to determine if the plaster is ready for redecoration. Moisture migrates from areas of high moisture concentration to areas of low moisture concentration, which is similar to the phenomenon of heat transfer where hot air migrates to cold air. This is the principal employed with the next simple test, which is taping a piece of polyethylene over an area of the plaster in question. When left for 24 hours, the area under the plaster may appear darker than the adjacent areas, or moisture may appear on the polyethylene. This moisture indicates that more drying time is required.
Some plaster and paint manufacturers recommend that all new plaster, old repaired plaster and old existing plaster surfaces should be primed prior to painting. A quick review and comparison of published specifications of plaster and paint manufacturers will determine if this is a requirement.
Wet plaster does not require complete replacement. Hand-held moisture meters do not provide accurate information as to moisture content in gypsum board products. Following some simple procedures may bring the plaster back to the performance characteristics it had prior to becoming laden with moisture.

Robert Grupe is AWCI’s director of technical services. Send your questions to grupe@awci.org, or call him directly at (703) 538.1611.