Roof rails, roof racks, kayaks increase frontal area. No way to do that without a hit in efficiency. If you are only driving at speeds of 50 mph or below, a fair characterization could be "tiny" hit. Above that, tiny just doesn't apply.
For example, the FE (no roof rails) and LX highway EPA ratings are 49 and 46 mpg respectively. That is 3 mpg just on the small amount of area posed by roof rails.
I can promise you the hit is much more at 70 mph. Try 70 mph with your kayak on and off the car. At a guess, you will probably take a hit of 15/20%. Kind of subjective if you find that a big hit or not, but I think that 6% or 3mpg hit represented by the roof rails is big. A kayak/bicycle is huge!
2018 Kia LX HEV Metal Stream with Advanced Tech