Short answer. All of the owners do, in proportion to the value of the storey each of them owns — but only where the titles and any agreement between them are silent. The Civil Code supplies default rules for shared parts of such a building, and they apply just in the absence of agreed terms.
What the law says
The main and party walls, the roof and the other things used in common, shall be preserved at the expense of all the owners in proportion to the value of the story belonging to each
Civil Code, Article 490 — Storeys Owned Separately. Read the full provision →
The rules apply only by default
Before anything else, read the opening condition. The rules operate if the titles of ownership do not specify the terms under which they should contribute to the necessary expenses and there exists no agreement on the subject. So the first place to look is not the Code but the documents: the deeds by which each storey was acquired, and whatever the owners have agreed among themselves. Where those speak, they govern. The article exists for the very common situation in which a building was divided among relatives or buyers and nobody ever wrote down who maintains what.
Shared parts, shared by value
For the parts nobody owns alone — the main and party walls, the roof and the other things used in common — preservation is at the expense of all the owners in proportion to the value of the story belonging to each. Note the measure. It is not equal shares and not floor area, but the value of each storey, so a ground floor used commercially may carry more of a roof repair than a small unit above it. Owners of upper floors often assume the roof is theirs alone to fund, or theirs alone to control. Under this rule it is neither.
What each owner carries alone
Against that, each owner shall bear the cost of maintaining the floor of his story. The private part is his own charge. The article then returns several shared elements to the common account: the floor of the entrance, front door, common yard and sanitary works common to all, shall be maintained at the expense of all the owners pro rata. The pattern is consistent throughout — what serves everybody is funded by everybody, and what serves one storey is funded by its owner, with the boundary drawn by use rather than by physical location.
Stairs, and why they are apportioned oddly
The staircase rule is the neatest illustration of the principle. Stairs from the entrance to the first storey are maintained by all with the exception of the owner of the ground floor; those from the first to the second by all except the ground and first floor owners; and so on successively. Each flight is paid for by the owners who must climb it, and an owner stops contributing once the stairs rise past his door. If a dispute is brewing, the documents to gather are the deeds, any valuation of the storeys, and the record of who has actually been paying for what.