I am currently installing 850 sq ft of hardwood floor in my house. I have
3/4 inch ply subfloors and for the most part they are very flat - except one
area. It looks like a single joist at the bottom of a stairway is about
1/4-3/8 inch higher than the rest of the floor resulting in something of a
little hill. It appears the builder (house is 10 years old) noticed this
because he sanded away probably about 1/8 of an inch already.

The hardwood boards will run perpendicular to this joist. My question is,
what affect would it be to leave it as it is and install the hardwood over
it? Is there a way to handle this - short boards or longer boards in the
area? Should I break out a belt sander and continue working away on it?

Thanks!