Short answer. A deposit is necessary when it was not freely chosen. The Civil Code names two cases: when the safekeeping is made in compliance with a legal obligation, and when it takes place on the occasion of a calamity such as fire, storm, flood, pillage or shipwreck.
What the law says
A deposit is necessary: (1) When it is made in compliance with a legal obligation; (2) When it takes place on the occasion of any calamity, such as fire, storm, flood, pillage, shipwreck, or other similar events.
Civil Code, Article 1996 — Necessary Deposit. Read the full provision →
Two cases, one idea
Article 1996 provides that A deposit is necessary: (1) When it is made in compliance with a legal obligation; (2) When it takes place on the occasion of any calamity, such as fire, storm, flood, pillage, shipwreck, or other similar events. An ordinary safekeeping is voluntary — you pick someone you trust and hand the thing over on terms you have agreed. A necessary deposit is the opposite. Either the law requires it, or events leave you no real choice about it. The Code thinks that difference important enough to give the category its own name and its own treatment.
Safekeeping required by law
The first case covers a thing placed in safekeeping because a legal obligation requires it, not because the two parties wanted the arrangement between them. The terms here are not really negotiated; the duty comes from outside the relationship. Two practical consequences follow. The person holding the thing cannot treat it as a favour he may drop whenever he likes. And the person who handed it over cannot always simply demand it back at will, as though the arrangement had been settled privately between them. What governs is the obligation that produced the deposit in the first place.
Calamity, and why it is treated differently
The second case is the vivid one: fire, storm, flood, pillage, shipwreck, or other similar events. Someone thrusts a box of documents at a neighbour as the water rises, or hands over a bag while fleeing a fire. There was no time to pick a trustworthy keeper, no time to write anything down, and no bargaining over terms. The list is not closed — other similar events takes in comparable emergencies. Because the owner was in no position to protect himself, the law declines to treat his silence about terms as though it were a considered agreement freely made.
What the article does not do
It defines the category; it does not by itself spell out every consequence. Other provisions deal with what such a keeper owes and how far he answers for loss, and the Code has separate rules again for things travellers bring into hotels and inns, which people often assume are covered here. Nor does it excuse a keeper from returning the thing once the emergency has passed. If you handed property over during a calamity, write down as soon as you can what you gave, to whom and when, and find a witness — proof, not law, is usually the hard part. Claims carry time limits.