When building a base or a house in Valheim, you'll need to pay attention to the building's stability, or structural integrity. With the Hammer equipped, place your cursor over a building part that you placed down. The colour of the part illustrates how strong or weak it is. Blue is strongest, followed by Green, then Yellow, then Orange, then Red.

If you see parts that are yellow, orange, or red, that means the part isn't properly supported by the ground or by other building pieces. If you build too far out in the air without supporting your build, then after a certain point, newly placed parts will just collapse.

Any wooden building part that isn't covered by a thatched roof tile will get damaged over time if it rains.

All buildings have a durability or health value, and if a wooden piece is rained on it will continue to decay down to a minimum of 50% health. You'll know if this happens because the wooden piece will turn a slightly different and less healthy colour.

The decay rate is fairly slow, and you can repair a building part using the Hammer tool. Another little tip is that it never rains in The Plains biome, so you can leave wooden tiles uncovered in that biome and it'll never decay.

All this info was practically ripped from Ollie Tom at RockPaperShotgun.com