Trying to Understand Sturdiness

[The Minted Lady]

I am building a modest, 3x2 home out of grass. First level is solid grass walls with a door. Second level is mostly windows with three solid walls. Then I tried to cap it with a clover roof, but several of the roof parts are saying there's insufficient sturdiness. I'm not quite sure what's causing it. Most of the roof is finished except for these two parts that say that.

Do I need to upgrade the walls on the bottom level to weed stem material? If so, is there a way to do that without tearing the walls down?

[Zap]

Hi, I haven't found a way to replace pieces of building without recycling one piece and then rebuilding.
If you look at a piece of building material it will tell you how many other pieces it's supporting. The numbers go into the red when they're supporting close to too many.
recycling a supporting piece will cause everything it's supporting to go *clunk* and the recycle.
hope this helps.