Building is fiddly and takes time to learn. The game is more interested in what you are snapping to that the item itself. Building starts with the item being blue when you look at it in hammer mode. It goes then to a couple shades of green, yellow, orange, light red and dead red. Dead red always breaks. If you don't start your architecture with the item actually touching the ground and being blue or if one beam is green and rest blue, that carries up and that one will break first. If all of this is too confusing there are some great videos on understanding the system. Watch one.