Metal bar screens as part of the water intake structures has been normally designed by means of a mathematical procedure and then a wide security factor is applied. By doing this, any kind of screens with bar thickness and separation according to the national normativity fits the hydraulic design for the required intake discharge. Trying to rationalize the design, an experimental study is conducted over a bottom intake whose screen is usually located below the low-flow spillway. The experimental work considered round bar screens of commercial diameters to evaluate the behaviour of the flow through the screen. Tests were made over a natural scale model (prototype), and some of the hydraulic variables that have more influence on the flow behaviour through this type of screen were manipulated to propose a more rational hydraulic design procedure.