Deterioration

Deterioration is the gradual loss of the Hit Points of certain items (anything that can be hauled by a colonist) outdoors, unroofed, marsh, sea, shallow water, swamp, water. When an item's hit points reach 0, it is destroyed.

Deterioration is akin to an item taking damage from projectiles, explosions, or fire since they all affect the same hit points stat. Affected items include clothing, armor, weapons, and most resources. Deterioration is applied to an item that is not placed in a safe place.

A shelf protects any item.

Mountains have an indestructible roof making them well-suited to protect perishable items (but can be prone to roof collapses, destroying everything underneath).

Items that do not deteriorate

Special notes

Other than market value (see next), an item's loss of HP does not affect any other stats, including damage output, accuracy, armor value, heat or cold insulation, nutrition, flammability, beauty, comfort, medical potency or quality, etc.

As an item loses hit points, it may also lose its market value. This only applies to items which have not been configured to ignore health when determining price. The XML tag for this is healthAffectsPrice, which defaults to true if not explicitly set in the def or a parent of the def. This is set, for most items, in the base definition from which the item inherits its properties.

In the base game, items that do not deteriorate include^{[Is this an exhaustive list?]}:

raw resources (stone blocks, steel, silver, etc.)
textiles (leathers, wools)
plant matter (hops, psychoid and smokeleaf leaves; and also including wort)
raw animal products* (e.g. milk, insect jelly)
(including kibble)*
drugs
advanced components
chemfuel

(* food and meals (but not kibble or insect jelly) are still subject to spoilage)

If an item loses value with hit points, it loses that value very quickly. An item retains 100% of its value down to 90% HP, then loses 1.67% of its maximum value per 1% HP lost between 90% and 60% HP, then loses 4% of its maximum value per 1% HP lost between 60% and 50% HP, then loses 0.2% of its maximum value per 1% HP lost between 50% and 0% HP. The item will have 50% of its maximum value at 60% HP and only 10% of its maximum value at 50% HP.

Raw food (i.e. non-meals/pemmican/kibble) is subject to spoilage as well as any deterioration (see previous).

Deterioration is also applied to apparel (both clothing and armor) when worn.

A colonist does not receive negative thoughts for using deteriorated weapons.

Currently equipped weapons do not deteriorate.

Deteriorated ingredients do not influence a finished product made from those ingredients in any way.

Artillery shells that deteriorate to 0 hit points will explode.

Deterioration Rate

An item's deterioration rate specifies how many hit points are lost per day. Depending on the type of item, it ranges from 0.25/d to 10.0/d. This stat is listed in an item's information tab. If this stat is not listed or listed as '0.00/d', then the item will not deteriorate. Rain greatly affects the deterioration rate. For items with the quality stat, a high-quality item will have a lower deterioration rate than a poor quality item.

Immunity

Metal and stone are immune to deterioration. However, items crafted from these materials do not carry over that immunity.
Other immune items:

Impact on human's mood

Main article: Mood

A human wearing any apparel with hit points 50% or below will get the -3 thought Ratty Apparel.

If any worn item is below 25%, the thought will worsen to a -5 Tattered Apparel.

Compare them with -6 Naked thought.

Tips

Trees and roofs are more valuable after some update

Version history

In the past, deterioration lowered the effectiveness of weapons and armors, but that is no longer the case.