One of my players asked if the next campaign could be extra with the realism and grit. I got possibly the worst thing to torture my players: weapons and armor durability! It'll be like playing LoZBotW and LozTotK with the big possibility of weapons and armor breaking. Welp, here's the homebrew rule:

This optional rule keeps players on their toes on keeping care of their equipment and getting replacements of their weapons and armor time through time. This rule is used in gritty and heavily realistic campaigns, though it can be daunting and sometimes annoying to players.

Weapons

All melee weapons have a max durability points equal to their weight times 50. All ranged weapons have max durability points equal to their weight times 200. If a weapon is made of metal, add the multiplication factor by 10. If the weapon is not made of metal, such as wood, subtract the multiplication factor by 10. For example, a longsword’s weight is normally 3Ib., and since it’s made of metal, it’s max durability points is 180 (3*(50+10)=180). A club’s normal weight is 2Ib., but since a club is usually made of wood, it’s max durability points is 80 (2*(50-10)=80). 

A weapon loses durability points for each damage of its type it deals. For example, a longsword with 180 durability points deals 5 slashing damage to a goblin. The sword then loses 5 durability points. Ranged weapons only lose 2 durability points despite the amount of damage it’s ammunition dealt, but they do lose durability points equal to bludgeoning damage when used as an improvised weapon. If a weapon’s durability points reaches 0, the weapon permanently breaks. Wooden weapons have vulnerability to fire damage.

A metal weapon that deals slashing damage can regain 5 durability points when a character uses a short rest to use a wheatstone or similar tool to sharpen the blade; likewise, any wooden weapon can recover 5 durability points when someone uses a short rest to carve the weapon back to shape. If a character has proficiency with carpenter tools and does this, the weapon regains 10 durability points.

Magic weapons have a max durability points increase to normal weapons. The increase varies by the weapon’s rarity, as shown in the Magic Weapon Durability Increase chart below. Artifacts don’t have durability points.

Magic Weapon Durability Increase

For example, an uncommon longsword would have max durability points of 360 (3*[(50+10)+60]=360). Magic weapons don’t lose durability points unless they do damage to a creature or object wearing magic armor that has equal or superior rarity to the weapon. If that armor has any damage effect when hit, the magic weapon loses durability points equal to the damage. For example, a +2 javelin (a rare weapon) dealt 5 piercing damage, but the +1 plate armor (a rare armor) that got hit were to spew fire that dealt 5 fire damage to the attacker, the javelin would be part of the blast losing 10 durability points (since the javelin is vulnerable to fire)–thus it would lose a total of 15 durability points.

Armor

All armor has durability points equal to their AC boost (excluding any dexterity addition to the boost) times 200. If the armor is made of metal, add the multiplication factor by 50. If the armor isn’t made of metal, such as leather, subtract the multiplication factor by 50. For example, leather armor has a AC bonus of 1, so it has max durability points equal to 150 (1*(200-50)=150).

Armor loses durability points for each damage conflicted towards it. For example, a character wearing leather armor with 150 durability points was to lose 5 hit points, the armor loses 5 durability points.

Magic Armor has a max durability points increase to normal armor. The increase varies by the armor’s rarity, as shown in the Magic Armor Durability Increase chart below. Artifacts don’t have durability points.

Magic Armor Durability Increase

For example, a +1 leather armor (an uncommon armor) would have max durability points of 520 (2*[(200-50)+110]=520). Magic armor doesn't lose durability points unless a weapon that hits the armor has a rarity equal or superior to the armor. For example, a +2 javelin (a rare weapon) can deal damage to an enemy, but the enemy’s +2 leather armor (a very rare armor) doesn’t lose durability points.

If you got any questions, I might answer them. I'll be glad to see any suggestions to make this better in any way.

Given that this will reduce the utility of weapons and armor, I would recommend a low-magic campaign as this would further widen the gap between martial characters and spellcasters, so the party mage would always outshine the other players. The other issue with implementing durability rules tends to be the amount of bookkeeping it requires, especially if the amount of durability it loses is based on the damage dealt. I think you could make this easier by creating magic items and using the number of charges as the durability points, so players can easily decrease the charges during combat and you can check the durability at any given time. One thing you might want to consider is how to deal with weapons that break during combat - unless the players always have an excess of weapons, you could end up with a slog of weak unarmed strikes and/or improvised weapons. You could say that a weapon enters a fragile state which makes it deal less damage, but doesn't break until combat is over. 

Some edge cases with your durability rules:

• Is it based on the amount of damage the weapon deals or the that the creature receives from the weapon. For example, if the creature is resistant, does the weapon lose the durability for the full strike before the resistance (and same question for armor, but reversed). Similarly, if a creature has 1 hit point and you get a critical hit that would deal 40 damage, does it lose 1 durability point or 40?
• How do bonuses to the weapon affect durability? For example, do sneak attack and great weapon master further count as the damage dealt? What about spells like Hunter's Mark, Hex, or Conjure Minor Elementals?
• You mentioned wooden weapons having vulnerability to fire, but I'm confused as to how that applies - can enemies directly target the weapons and, if so, are you using the Object Armor Class rules to determine the AC? If a player gets hit with a Fireball, does the weapon lose durability? Is this true if it's not a wooden weapon?
• I think having the magic weapons and armor only lose durability when confronted with an equal or higher rarity would be a mistake because monsters rarely have those items specified, so you'd need to figure out what the equivalent of the armor and weapon attacks are for each enemy. You could create some CR-based rulings, but I think it's just easier to give your magic items higher durability. Similarly, you'd need to decide how spells affect the armor.

One of the things that I've found to make a campaign grittier is to have things with lasting consequences. The Fomorian and the Shadow are good monsters for this. Curses/Cursed Weapons can also be something that has this impact and can be useful narratively while the party rushes around to try to find someone who can remove them. These could be easy things to include with your durability rules as they won't require any bookkeeping. 

Honestly - I am not sure how many combats per session you run. If its a RP campaign with one combat and you have a bunch of folks that like math sure. 

For an average session with at least 2 combats the amount of math to keep track of all this would be staggering in an average party.  Lets not even get started on mini boss fights - boss fights - dragon fights - I mean the permutations of spending 10 minutes deciding how much a weapons durability went down by are endless. 

You would end up with a bunch of Caster / Monk / Barbs to just avoid the hassle imo. 

Given that most DM's look for ways to streamline combat and move it along - IDK perhaps consider another way to add gritty realism. 

Just my $.02