Durability is a property of armor, tools and weapons. It decreases when the item is used. If the durability reaches zero the item has to be repaired before it can be used again.
There's a durability property in saddles but except for a few exceptions (Roll Rat Saddle) it has no meaning there because it never decreases. Due to this, crafting higher quality saddles with only durability increased is a waste of materials.
Consumption[]
Ranged weapons(including Chainsaw and such stuff, which is logical) lose durability throughout firing, melee weapons and tools only lose durability when it hits a valid target.
The rate of consumption of durability varies for different weapons, like Metal Hatchet loses less than 1 durability per hit, Crossbow and Fabricated Sniper Rifle lose exactly 1 durability per shot, and Rocket Launcher loses more an 1 durability per shot.
All equipped armor pieces on a survivor will lose 1 durability altogether per about 16 damage absorbed. If a survivor is wearing armor without armor value(for example, naked with a Gas Mask only), damage won't be absorbed and there will be no durability consumption when the survivor is hit.
Some creatures have attacks that can break armor quicker than ordinary attacks. A good example is Arthropluera, which causes 32x durability consumption for its acid spit, and melee weapons also suffer tremendous durability impact when hitting them while alive.
Repairing[]
The repair cost is calculated by taking the percentage of lost durability and multiplying it with the half of the initial building costs (Scales with increased costs caused by Item Quality). The repair costs are always rounded up.
Special Gears[]
Some gears' durability decreases over time when worn, and some of them don't need all the materials which are used to craft them (Ex. Night Vision Goggles)
On Aberration, the Hazard Suit Armor loses durability gradually over time when worn in areas with radiation.
Inappropriate Usage[]
Several items will break much faster due to inappropriate usage.
Whip - Using whip on structures and animals decreases its durability much faster than gathering resources- Climbing Pick Using the climbing pick while falling.
- Roll Rat Saddle Colliding into Obstacles.
- Mining Drill Having it overheat.
Durability can also be increased by Item Quality.