The majority of wood species in Slovenia are prone to fungal or insect attack,therefore they need to be chemically protected in order to be used in construction applications, where we want to minimize damage caused by insects or fungi. In the wood industry there are many chemical preservatives that are efficient at protecting wood, although wood treated with these products may present a potential hazard, as some preservatives are hazardous to the enviorment. In this graduation thesis we focused on researching a material that may exhibit fungicidal properties and be completely harmless to the enviorment, clay. We studied the fungicidal properties of clay according to the standard EN 113:1996. Norway spruce samples were pressure treated with different concentrations of a clay suspension in water and exposed to two types of brown rot fungi, Gloeophyllum trabeum and Poria vaillantii. Based on sample weight loss we determined the effectivness of clay as a wood preservative. Our study shows that clay is a poor wood preservative as none of the samples showed any improvement with respect to the control sample. In the case of G. trabeum the weight loss was around 30 % with no significant deviation in any of the samples. P. vaillantii samples showed a lesser degree of degradation, only around 7 %, with only minimal deviation in any of the samples.