As of November 2018, distinct layers cannot be composited for analysis. When a sample consists of two or more distinct layers or materials, each layer must be treated separately and the results must be reported by individual layer.
Plaster and stucco: If plaster and stucco wall or ceiling systems are layered, and the layers can be distinguished, then the layers must be analyzed separately. Where a plaster or stucco wall system is constructed in layers, and the asbestos-containing layer becomes a distinguishable but "non-separable" component of the wall system, the results of the analysis of the individual layer(s) may include a small amount of the other layers when analyzed (for example: a skim or brown coat layer may contain a small amount of the base coat layer in the analysis of the skim coat layer).
Add-on materials (surfacing): All materials "added" to wallboard or other base materials (such as sprayed-on materials, paint, ceiling or wall texture) must be analyzed separately, if possible. The results of the analysis of those individual layers of "add-on" material cannot be averaged with the result of the analysis of wallboard for a composite result, but must be analyzed and reported separately. Where a thin coating of one material is applied over another material and the materials cannot be separated without compromising the layers, the analysis may include a small amount of the base layer. If for example, a paint layer containing asbestos is spread over a wallboard layer, and the paint layer cannot be separated from the wallboard, then a small amount of the wallboard layer may be included in the sample of the paint.
Joint compound and wallboard: When joint compound and/or tape is applied to wallboard it becomes an integral part of the wallboard and in effect becomes one material forming a wall or ceiling system. Therefore, a bulk sample of the system that contains the joint compound, tape, and wallboard may be analyzed to identify the asbestos percentage of the system. Surfacing materials or other layers that can be separated must not be included in the analysis.