Fibers, such as those cut from short lengths of twine can suggest a palm-frond roof. For a plank roof, you can make the roof of balsa sheet instead of posterboard, painted and scribed as for a door. You can add exposed roof beams by poking holes in the front and rear walls, just below the roofline. You can add a balsa ladder up to the roof, or build balsa stairs along one side of the building. For the ladder, be sure to glue a posterboard ground-extension at the base of the wall to support and protect the ladder.
Many tropical buildings have sunshades or awnings.
Poke two holes into the walls and insert balsa strips for braces. Let the glue dry. Then dampen the tissue with water and flow on grey paint. The paint will stiffen the tissue and make it droop like cloth. When the awning is dry, drybrush with lighter warm grey.