Greetings .... we have a project going that involves renewing existing white oak floors and adding new flooring in our kitchen area .. I have some questions about some of the seams in the old section and even some of new construction .... there are cracks between the boards ..I would say about a thumb nail wide ..I would say 10-15% ...some areas 6-15" long others longer ...I have asked several times about filling the joints and each time I get an answer about that we should really leave that alone so boards can expand and contract ...are they just avoiding more work? At this point ...2 coats of poly maybe too late to address? Thanks in advance ... Mike.... NE Fla ...

DO NOT APPLY FILLER IN BETWEEN BOARDS

Just wanted to make that clear - they are correct, you need to have space for expansion. If you fill, when the boards expand they will push the filler up and out anyway. If the filler is sealed in, it could create extra pressure on the floor leading to buckling, cracking, edge bonding etc. Just don't do it.

The maximum acceptable gap between boards according to most warranties is 2mm or just a bit thicker than a credit card. If the gaps you see are only the thickness of a fingernail, then you should leave it alone.

Caveat to the above - what is your Relative humidity in the house? Boards will shrink when it falls below 35% and will expand when it gets above 55%. If you see gaps now and the RH is high due to the summer humidity, then in the winter, those gaps will get bigger and you'll have a warranty claim for sure. If the house is currently sitting around 45% or lower, those gaps will be fine and may even close after a full heating and cooling cycle.