I also, just today, put a porch floor down on a re do on my house. The floor consisted of 2" pine planks 12-16" wide, with a spline spanning 8' no joist. With out the spline the plank would clearly sag with my weight on it, deflect 1-1/2" in the middle, with the spline installed you would need to look carefully for the deflection, almost unnoticeable.