A pergola without a roof still cuts direct sun by roughly 40 to 60 percent, depending on how closely the slats are spaced and how they are angled. The open top is the point: you get filtered shade and airflow instead of the trapped heat that builds under a solid cover, and rain still reaches plants below.
Pergolas also do jobs a roof can't. The posts and beams define an outdoor room on an open patio, give climbers like wisteria or grapevine a frame, and hold string lights, a fan, or shade cloth. Many owners add a louvered top or retractable canopy later for full coverage on the hottest days, so the open frame is a starting point, not a limitation.
If you're unsure how one would sit on your patio, preview a few styles on your own photo with AI pergola design.