During the excavations within the fortification near Borgsum on the North-Frisian Island of Föhr, a massive wooden substruction was unearthed. The early medieval structure is covering an area of about 8 x 4 metres, and is preserved due to high humidity in the ground. Large house timbers in secondary use lie side by side with thick branches and rough stems in one level and formed the basis for a floor of clay sods. These revealed a large amount of hoof imprints from cattle and horses, thereby indicating that the building once present was presumably a stable. By means of drone photography and structure-from-motion technology a three-dimensional documentation shall be carried out. Sampling for species-determination and dendro-dating complete the investigations, preserving the structure otherwise in situ.