STUMPERS CCXIX

The blanks below can be filled in to make two common words of near opposite meaning and that differ by only a signal letter - L. The common letters are not in the same order in the two words.

_ _ _ _
_ L _ _ _

HINTS:
#1: The first word appears in reverse in the second word.
#2:The two words name things that are found in dwelling places.

Answer:

ROOF
FLOOR