Is there such as a too much armor?

[Abomination713]

So I'm level 46 right now and I'm a Marauder Juggernaut. My whole gimmick is armor stacking to be a tank. I use a shield and spec into it on all the nodes I can, I spec into armor/element defense, have 2/3 endurance nodes on the tree unlocked, build endurance nodes like no ones business, use Determination aura (raises the armor me and the people around me by 38% so far), and even use Fortify (20% damage reduction) on hit and have gotten the nodes on the tree to make it even stronger. I am at 70% (76% with Determination) physical damage reduction just by walking around, but can hit 90% with ease thanks to endurance nodes and Fortify. So I still have plenty of points to put into nodes on the skill tree and am wondering if I should bother to put any more into armor since I can already hit 90%. Is there a point to keep stacking armor? Or does it become harder to and harder to hit the 90% cap as I fight stronger enemies so I should keep stacking it?

[Drake]

It doesn't really work like that, not just about armor, but about any defencive stats. POE defence is kind of bound by 2 laws :

1. every defence stat has a fatal flaw (it's by design), armor only protects against physical damage (being attack or spell), that means that any elemental or chaso attack or spell deals 100% of damage if you don't have resistance to them.

2. there is some sort of cap to any defence stat, preventing you to be completely immune to something. In the case of armor, it's real simple, the bigger the hit, the less armor effectively works. They thought funny to add diminishing returns on armor stacking, so even with high armor values, a crit from a very high physical boss will still oneshot you.
Armor is good for countering repeated small physical damage from trash mobs.

Then in the case of armor specifically, the % value is an average based on the current monster level you're hitting, it's gonna drop when you get to endgame becasue the stacking isn't going to keep up with the monster damage. And don't mistake it with real physical damage reduction, real physical damage reduction doesn't get diminishing returns, so a basalt flask that has flat 10% physical reduction can be way more powerful late game than some armor bonus.

That said, there is a build that uses armor, it's the molten shell build. Because molten shell skill scales of your armor value, so there is that.

To keep it simple, armor alone can't protect you, you have to combine other types of defences, like block, dodge, resists (elemtnal resists need to be capped, they are the most important secondary defence type, you'll need resists more than you'll need armor or evasion) etc. And most importantly, life. Nothing in the game is as important as life, because that's the only stuff that works on potentially everything. Stacking it as much as you can is what will make you stay alive, because there will always be a mob that will bypass one type of defence, and if you can't deal with it, it's oneshot everytime.

[Ratio decidendi]

Pretty much as above. But I'll add a few more things related to physical damage reduction.

Armour only protects against hits-based physical damage, so it will not work against physical-based DoTs. The percentage indicated on the character sheet is inaccurate, because it does not factor the specific level of the creature attacking you and other relevant map modifiers.

To elaborate on the point made by Sajah on "bigger the hit, the less armour effectively works", this is the relation between armour damage reduction and incoming damage.

Damage reduction = Armour / (Incoming Damage * 10 + Armour)

It's easy to see that as the amount of incoming damage increases, the damage reduction from armour decreases.

Note that when computing the actual damage taken, the damage reduction from armour is added with flat reduction like Basalt flasks and Endurance charges. Flat physical damage reduction sources work with both hits-based damage and DoT.

[Ghin]

You might want to think of the armour stat as protection from white monsters. Anything worse is going to punch through and kill you. Shield block and evasion are both great at stopping physical hits. I usually get all 3 if I want a tank character. Spell block is amazing if you can find it.