Archaeological studies in the forest-tundra zone of Western Siberia are highly relevant to studying the material culture, social structure, and ethnic history. The presence of permafrost ensures the unique preservation of organics in cultural layers, including timber, which makes it possible to conduct dendrochronological studies (calendar dating of samples, determination of species composition, typological analysis, and the source of the timber origin). In 2011–2012, during the excavations at Fort Nadym, 347 samples of wood were selected for the assessment of the age of wooden structures. The results showed that most samples belonged to three species of trees: Siberian larch (Larix sibirica Ledeb.), Siberian spruce (Picea obovata Ledeb.), and Siberian pine (Pinus sibirica Du Tour). The typological analysis revealed that walls were mostly built from spruce, pine logs, and half-logs, whereas the floors were made from larch and pine. To assess the origin of wood, a new methodological approach was proposed. As a result, it was demonstrated that the main building material was driftwood. This has allowed us to make more accurate interpretations and to specify the years of construction. The analysis indicates three periods of construction / reconstruction: the 1450s–1460s, 1470s–1480s, and 1520s–1570s. The new approach can be applied to other wooden monuments located on the banks of major water arteries of the Siberian forest-tundra zone.