In Valheim, building height and stability are determined by structural support. When holding your building hammer, hover over any structure to see its color: Blue is grounded, Green is strong, Yellow is medium, Orange is weak, and Red is at its limit.
Using Core Wood Pillars
To build higher, use core wood pillars harvested from the Black Forest. These offer significantly higher vertical support limits than standard wood beams. Always ensure your roof tiles connect directly to a core wood frame to prevent collapse.