This essay concerns the interpretation of house plans in three-aisled longhouses and hall-buildings, in Scandinavia during Iron Age. Full scale house reconstructions need a better basis of interpretations to understand the patterns of roof supporting post holes in excavated house plans. This has led to the thesis, presented here, that the layout of house plans during Iron Age is based on geometrical proportions. In order to prove this, geometric house plans were applied and compared with excavated house plans in 11different case studies, including the fortification of Fyrkat Denmark. It was found that all house plans in the case studies indicate, that the geometrical proportions 1:2, 1:3 and 2:3 of a given circle must have been used for the basic layout of the roof supporting post holes, using a compass and a straightedge. Strings and stones were used in a field study.