Refined House Rule Armor Class in Old School Essentials

Facial hair is impressive, 
but does not
 contribute to defenses

I have thrown a bunch of ad hoc rules at D&D over the years. And my favorite and most workable is Armor Reducing Damage. In Unearthed Arcana, they had a suit of field plate armor that acted like a limited pool of hit points. I don't necessarily like giving the characters a way to purchase hit points. 

What I do is slightly different. I offer damage reduction based on how low the AC is, to a practical limit of AC 2 for non-magical armor. 

The AC scale is 9 to 2 for damage reduction.  

AC 9 - No armor, no damage reduction. 
AC 8 - Only a shield, no damage reduction. 
AC 7 - Leather armor, -1 to damage. 
AC 6 - Leather armor  + shield, -1 to damage.
AC 5 - Chainmail, -2 to damage
AC 4 - Chainmail + shield, -2 to damage
AC 3 - Plate armor, -3 to damage
AC 2 - Plate armor + shield, -3 to damage
AC 1 or lower, no further damage reduction except for magical armors which can reduce damage to -4. 

The damage reduction is a property and advantage of armor over speed, toughness and/or magic properties. A wolf or dragon does not receive a damage reduction because they probably don't have armor. An orc or horse in armor or barding does receive damage reduction. 

Wearing all of the armor
helps a lot. 

In exchange for this reduction of armor, the character must be fully dressed, meaning they have all ancillary parts of their armor for it to reduce: ie helmets, boots, greaves, bracers, gloves or gauntlets, etc. Having all parts covered simply removes the possibility of a light hit (a dagger or a punch) from doing harm. 

There are two weapons that are unaffected by this reduction - Long Bows and Crossbows. These arrows and bolts have so much mechanical advantage they simply won't bounce. They do skip off angled bits which is represented by a poor damage roll, not the quality of armor.  

There are two corollary rules to this. 

Each type of armor is made up of the lesser armor types. What this means is, chainmail is made up of a layer of leather armor plus the mail. Plate armor is composed of chainmail and leather. The end result is, your character's investment in an expensive suit of armor means you also have a functional lesser suit of armor in addition to the full set. Plate armor can be worn as plate, chain, or just leather. Also, you can save time by only suiting up to your comfort level. This can also come into play for retainers and followers, giving a soldier an ability to suit up in layers quickly. 

There are many stories where the hero only suits up to the first layer and fights to defend his page or squire as they suit up. This injects a bit of drama and heroism. 

History is full of examples where soldiers wore what they thought made sense at the time, say the undergarment but not the protective metal cover. Of course, what makes these commentaries notable is the soldier won or lost a battle seeming based on what they had on. 

Harald Hardrada's troops got caught wandering without their mail shirts but were also completely surprised by a massive army bearing down on them with no warning. 

Several times gladiators were pressed into service as soldiers in the Empire's legions. It could go either way. In the Year of the Four Emperors, the gladiators had the advantage of the heaviest armor but made a poor showing when thrust into traditional set-piece battles. However, in urban settings, they were dangerous in combat. Later, Marcus Aurelius pressed gladiators into the role of soldiers. The Empire was decimated by a plague so there was no lack of legionnaire standard armor for them. Or they served in a role where armor wasn't a factor. 

Numerous times, the legionaries got ambushed while wearing only their tunics but were holding heavy pickaxes and turf cutters. They destroyed heavily armored enemies. If there was one thing legionaries were more practiced in than sword fighting, it was using tools to make camp. 

Even power armor has limits, 
say if your feet leave the ground

This has an interesting social side effect on D&D which also has a good history at the table. Padded or studded leather, banded and ring mail are transitional types of armor that cannot be broken down like chainmail and plate types. They are all one piece with metal bits attached directly to the cloth or leather. They aren't layers and don't come apart. They have a place and are very descriptive of a specific type of character. A barbarian or cleric would be expected to have the heaviest but cheapest armor available, ring or banded types. A Thief or an Assassin looks like a ruffian, but never a guard. 

Back to the corollary rules for armor. I run with the idea that a person wearing armor is unencumbered in combat or movement until "one more thing" is added. Don't wear a backpack in armor. Don't walk in mud in armor. Don't let peasants jump on your back in armor. Don't get hit with a mancatcher or take a pilum to the shield. All of these will immediately encumber a character in armor in rather disastrous ways. 

When reading through those examples of historical battles involving mismatched armors, the side that moved smartly won. 

I have tried a couple of rounds of combat first level characters using the Old School Essential rules.. Damage reduction increases the loiter time of first level characters while not eliminating death. This is give combat an epic feel as one Fighter with 8 hp and plate armor can survive 1 good hit (more than 4 or 5 hp damage) or 2 average hits (3 or 4 hit points of damage) and a whole series of weak blows. 

When facing heavier damage, say 2d8 hp from a serious bite, the same 8 hp Fighter (or Dwarf) in plate is more likely to survive due to damage reduction but is by no means assured of it. An average roll would be 9 hp of damage reduced by 3 leaving just 2 hp left. On the other hand, reducing 15 points of damage does nothing. 

It is an interesting mechanic, if anything it can make your fighting types much tougher. You can try out Old School Essentials Characters, Magic, Monsters, and Treasures on DriveThruRPG.