Valheim structural stability can be simply described as "distance from the ground". You can see the structural strength of various items by stacking them head to toe. Each new piece gains more distance from solid ground(measured through the construction pieces), until it breaks. Other factors like weight, thick/thin beams, number of pieces, etc. have no effect on construction.

Most building materials have similar structure strength, and horizontal distance comes with a penalty that hits stone blocks very hard. The strongest building materials are wood-iron beams, and iron-gate doors. Hide them as skeletons inside larger builds.

Trees (normal and swamp) are very tall and count as "solid ground" to build with. Terrain can also be raised/lowered to create anchor points to build from.