When the gap is large enough for bees to get through it could be an issue if the colony is weak(robbers). If the gap is not large enough for bees to pass it is no worry. They will fill the gap with propolis if they don't like it.
For years I kept square hives with no gaps or openings except the front entrance. Now a little more knowledgeable, the bottom is open and screened, I drill holes is the front of summer honey supers, and I made screened covers to let all the moisture out during honey production. My wife got me Langstroth's Hive and the Honeybee for my birthday. Would you believe he prefered gaps and ventilation 150 years ago.
I made some migratory covers last summer and it turned out the wood was not as seasoned as I thought it was. As the top boards shrank a gap 1/4 inch opened up between them, not exactly water tight. I make an aluminum cover to fit over the whole thing from roof flashing, the bees can move through the cracks into the one inch 'attic' space. The aluminum is not sealed real well against the sides allowing a lot of air movement. That hive wintered better than std. cover setups. Or was it just the colony?
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