Can you answer this riddle?

He is too short and can only reach the button to the 7th floor. On rainy days, he carries an umbrella that he can use to poke the button to the 14th floor. He doesn't carry an umbrella with him on sunny days. Hope that helped :P

The dwarf/umbrella thing is probably the expected answer, but Buzzard is correct that it is a fundamentally stupid explanation.

A better answer might be that he usually takes the stairs part way for exercise, but on rainy days his shoes are wet so he avoids the stairs in case he slips and falls.

Another possibility is that he is a vampire (or extremely photo sensitive) and it's a glass elevator at the side of the building which is exposed to direct sunlight above the seventh floor.

I like the idea that he enjoys walking up the stairs outside the building. The dwarf/umbrella idea is silly, because someone who couldn't reach the button for 14 in the elevator would carry a tool with them all the time.

There's another solution- this guy is superstitious. He doesn't want to even go near the button for 13, because he considers it bad luck. But if he gets off on lucky number seven, that clears the bad luck. He also considers rain to be kind of unlucky however, so if it's raining, he isn't bothered about going near button 13, he just presses 14, saying "Oh well, can't get much worse." (It's true that when he climbs the stairs, he still has to pass the 13th floor, but he gets around it by crossing his fingers and walking backwards up the stairs when he is on that floor.)

the stairs must be outside

Because he eats eggs.

a hole in the staircase?