A sill plate, sometimes called sole or bottom plate, is the horizontal framing member at the bottom of a wall that the wall studs are connected to. It is typically 2”x4” or 2”x6” in residential construction, and if installed over a concrete foundation, it is probably treated with a preservative to prevent absorbing moisture from the concrete.

Because the sill plate is at the bottom of the wall, it is typically affected by a water intrusion as water migrates through the structure. It is critical to obtain moisture content readings from affected sill plates because if the sill plate is not treated, it is a great organic food source for mold.

If you record a wall moisture reading, such as drywall, it is a good chance the sill plate got wet and you need to ask for a sill plate reading as well. Always ask for penetrating moisture meter readings of sill plates to minimize the risk for false positive readings.

If the sill plate is treated with a preservative, like the blue wood in the photo above, it may give false positive readings because some preservatives can give moisture readings even when not wet.

The U.S. Dept. of Agriculture wrote an article years ago stating that, “The usefulness of electric moisture meters, therefore, is severely limited by wood preservatives that change the electrical properties of the wood.”

You should still ask for moisture readings of treated sill plates, but be sure the technician is using a penetrating moisture meter with the 5/16” metal probes. It is not necessary to take readings deep into the core of treated sill plates, and those readings are typically not reliable.

The wood preservative is used to make the wood less susceptible to mold growth, thus less likely to be a problem and may give a higher-than-normal moisture reading even when it is not affected with water.

Remember, if wood materials are not treated with a wood preservative, destructive mold growth could begin to occur at 20% moisture content or greater, so it is critical that moisture content readings are provided to prove they were left in a state that would inhibit the growth of mold.

To take sill plate readings, the technician should use a hammer probe or paddle attachment if it can’t be reached with the normal 5/16” probes, angled below the drywall into the bottom of the sill plate.

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