Constructive Knowledge is where a Person is Legally Presumed to Know Something, Because he Should have Known it
Sometimes a court will find that a person has constructive knowledge or awareness of a condition, even though he lacked actual knowledge.
A person with actual knowledge is someone who, in reality, knows something or is aware of something. For example, if a person hosts a party at his house and knows that the roof of the shed in his backyard is rotting, he should not allow his guests to enter the shed. If the roof collapses and the host did not warn his guests about the dangerous roof, he might be liable for their injuries. The host would be liable because he knew the roof was rotting – – he had actual knowledge – – but failed to protect his guests.
Most people would agree it is fair and prudent to impose liability on a host who knows of a dangerous condition on his property and does not protect his guests.
Constructive knowledge is different because sometimes a person might lack actual knowledge of a condition, however, it makes sense to treat the person as if he did know about the condition.
Constructive Knowledge: Example
Stores are responsible for maintaining a reasonably safe environment. The store should protect customers from known, dangerous conditions at the store.
Let’s say a supermarket’s employees are unaware of a spill in one of the aisles of the store. However, as shown in the clip to the right, the spill has been on the floor for a long period of time, was plainly visible, and someone got hurt.
Under these circumstances, where the danger was apparent and existed for a significant amount of time, a jury may determine that the supermarket had constructive knowledge of the spill and hold the supermarket responsible. Although no one who worked at the store actually knew about the spill before the accident, it was there for a long enough time and was sufficiently obvious that the store should be deemed to have constructive knowledge of the dangerous condition.

Courts may apply constructive knowledge in other circumstances, too.
For example, if a person engages in a transaction, but knows enough facts about the transaction that he should be concerned that there is a fraudulent purpose behind the transaction, a court might determine that the person had constructive knowledge of the fraud. If the person deliberately ignored warnings that the transaction was part of a fraud, he may be deemed to have constructive knowledge of the fraud, even though he never acquired actual knowledge.