Documentation describes room event data returned by room.getEventLog() for event type EVENT_BUILD as follows

https://docs.screeps.com/api/#Room.getEventLog

EVENT_BUILD	A creep performed build in the room.
targetId - the target object ID
amount - the amount of build progress gained
energySpent - the energy amount spent on the operation

the actual event data in game contains the following:

targetId,
amount,
structureType
x,
y,
incomplete: boolean // gets set to true, if this action completed the site build