Lack Of Humidity Causes Gaps In Birch Hardwood Floors



I bought a brand new house in August. Birch hardwood floor was installed. It’s site-finished. After a month, the floor starts to crack. Some of the wood strips started to contract.

Now it’s Feb, gaps become much bigger. They range from 1 mm to 4 mm. Is this something within the normal range? The builder is going to fix it by putting in filler. But I think 4 mm is too wide for filler. Am I right? Should I ask the builder to replace the pieces that significantly contracted?




Dear Frank

Here we go again the amazing winter shrinking floors. This is especially a problem with prefinished floors, and if the floor was stapled down on OSB as the subfloor, there will be all kinds of shifting.

But wait. Have you been doing your part in stabilizing the environment this solid wood is in ? You should maintain an indoor humidity level of about 40-60%. And I mean all year round. So, if this was a particularly hot and humid summer (a long one at that) in Ottawa, you may have been tempted to throw the doors and windows open, rather than waste energy using the air conditioner. Prefinished floor are not finished on site, so they are susceptible to these high humid conditions, and will swell up quite a bit. The boards will push against each other, and move into the expansion gaps at the long edges. You will not notice this. The indoor humidity can easily reach 80% for weeks if not months.

But at some point in the Fall we turn the heat on and bake these floors to desert dryness. An un-humidified house can easily reach a 25% level in the middle of winter. These extremes on solid wood will cause the wood to reach an equilibrium moisture content of as high as 14% in the summer and as little as 5% in the dead of winter. And this movement will easily cause the gaps you speak of and worse. The immediate goal is for you to bring your indoor humidity up to at least 35-40% right now. If you are uncertain of your indoor humidity get a wet-dry bulb thermometer at

This is the proper instrument to measure indoor humidity. The cheap metal ones are not at all accurate, and may mislead you. You can use a furnace powered humidifier, or the portable ones. And in the summer be sure to de-humidify your basement, constantly. and when the indoor humidity level reach past 60 % use the air conditioner. Do all this if you expect your solid wood floors to stay tight.

The gaps that you have now are probably permanent. Don’t put any filler into prefinished floors, it never looks good. In any case wait until spring before you do any filling, and do it sparingly. Don’t even think about repairs until the spring, the floors amy have corrected themselves by then, if you have been diligent in raising your indoor humidity.

This sort of thing almost never happens to my site finished floors, the wood is so well sealed that the humidity changes affects it very little. A thick plywood subfloor helps also, and floor nails must be used, never staples. I bet you were never informed of any of this when you bought this floor.