When most people offer guests a tour of their home, they give you a spin through the kitchen, living room and maybe their yard. At Mark and Lucia Freeman’s home, an elegant mid-century modern residence in D.C., you might also get an invitation to climb a ladder and check out the roof. It has become the talk of the neighborhood — and occasionally of social media — since the couple installed an array of native plants up there last year and added a bracing system to keep it all from sliding off.