Why wouldn't this script actually ignore the collision?

I’m trying to create an effect much like any old 2D sidescroller where a player can jump through the bottom of a platform, but collide with the topside of it. I’ll be adding some more restrictions to it as this doesn’t account for a number of other factors, but I want this to get working as the foundation first.

Right now, there are no errors or warnings when I run Unity. This script is located on the player which also has other movement related code on it. The player is located on layer 8, (the same as this code snippet) and the platform with which he is colliding is located on layer 13.

    float playerVerticalSpeed = Input.GetAxis ("Vertical");

    if (playerVerticalSpeed > 0) {
        Physics2D.IgnoreLayerCollision (8, 13, true);
    }

Right now, the player will constantly collide with the platform whether or not he actually has a positive vertical speed.

Where is my thought process failing me, and how do I fix this?

What comes to my mind first is you might have set your layer index false. Note that index starts from 0.
Also you can try this function.
Physics.IgnoreCollision

And this for debugging:
Physics.GetIgnoreLayerCollision