Here's a very effective technique for tying into the existing roof with a perpendicular porch roof. Note that the porch ridge intersects the main roof below the main ridge. Likewise, the eaves of the porch roof extend lower than the main eaves. Lower porch eaves limit headroom when entering the porch but work fine here because of the tall ceilings in the main house.
Intricate Lily Langtry Spandrels contrast elegantly with the simple architecture of this lovely old home.