Here are some PnP rules to work with:
Shield Bash Attacks
You can bash an opponent with a light shield or heavy shield, using it as an off-hand weapon. See Table: Weapons for the damage dealt by a shield bash. Used this way, a shield is a martial bludgeoning weapon. For the purpose of penalties on attack rolls, treat a heavy shield as a one-handed weapon and a light shield as a light weapon. If you use your shield as a weapon, you lose its AC bonus until your next action (usually until the next round). An enhancement bonus on a shield does not improve the effectiveness of a shield bash made with it, but the shield can be made into a magic weapon in its own right.
Shield Spikes
When added to your shield, these spikes turn it into a martial piercing weapon that increases the damage dealt by a shield bash as if the shield were designed for a creature one size category larger than you. You can’t put spikes on a buckler or a tower shield. Otherwise, attacking with a spiked shield is like making a shield bash attack.
An enhancement bonus on a spiked shield does not improve the effectiveness of a shield bash made with it, but a spiked shield can be made into a magic weapon in its own right.
Light Shield does 1d3 Bludgeoning. Light Spiked Shield does 1d4 Piercing.
Heavy Shield does 1d4 Bludgeoning. Heavy Spiked Shield does 1d6 Piercing.