If every board is nailed down on one side of each board, then gaps would open between each board when they shrink. Dust and debris would fall into cracks and any liquid spills would get between the boards, dripping down into the "tongue and groove" seam, then down to the sub floor (or at least into the boards themselves). If every board is nailed on both sides, the boards would either crack or buckle when expanding or contracting, because they would not be allowed to move. The only way I can see to allow expansion and also have no gaps would be to glue every single board together into a massive room filling panel and then nail the panel only on one side of the room, allowing the entire panel to expand as a whole. But this is not how floors are done. I see many solid wood floors that are nailed everywhere, with no gaps, and seem to have no problems. How is this possible?
I see many solid wood floors that are nailed everywhere, with no gaps, and seem to have no problems. How is this possible?
Solid wood floors do experience seasonal movement, but you may not notice it because many floors are made up of boards that are only 2 1/4" to 3" wide, so any shrinkage between summer and winter is distributed over many joints.
For example, a 2 1/2"-wide flat sawn oak board going from 12% to 7% moisture content shrinks by around 0.04", or just over 1/32". The difference is half that for quarter sawn boards, and since floors tend to have some flat sawn and some quarter sawn planks, the average value is probably around 0.03". Also, the one face of the board that's most exposed to the air is usually given a couple coats of polyurethane; that won't stop seasonal movement, but it may attenuate it somewhat.
The reason that wood floors fastened with nails, cleats, or staples is that those fasteners allow for seasonal movement. In contrast, engineered flooring is so stable that it's often glued in place.
If you look closely at a solid wood floor in the winter, you'll see that there definitely are gaps there. You may not find them at every joint, but you'll probably see a gap every few boards. Look at the same section again in the summer and those gaps will have tightened up.
Note: Please don't take the numbers in my example too literally. I chose the range 7-12% because that's about the range of seasonal moisture changes of the hardwood in my shop; the numbers may be different for you if you're in a different climate, and the particular facts of flooring (wood species, only one face exposed to the air, finish applied, etc.) would likely affect the numbers as well. The important point is that using narrow boards is one way to minimize the size of gaps by distributing the seasonal movement over many joints; many tiny gaps is often more desirable than a few larger ones.
So there is no way to avoid gaps in solid wood floor?
Seasonal movement is a fact of life. Relative humidity levels change with the seasons, solid wood expands and contracts with moisture content, and wood moisture content is affected by humidity. The main question is how to deal with that movement.
You could glue the floorboards together and float the entire floor, meaning that you don't attach it to the subfloor. That might work for a while, especially while the moisture content is increasing and the floor is expanding. But when the moisture content starts dropping, all those edge joints will be in tension as the floor is contracting, and chances are some of them will fail. At that point you'll get several large gaps instead of many tiny ones. Also, this method means that you have to allow for all the movement at the perimeter of the floor, so you need baseboard molding that provides sufficient space for the floor's movement.
You could put the floor together with tight joints and quickly seal the whole thing so thoroughly that no moisture gets in or out. But it's hard to prevent any change — even gymnasium floors, which typically have a lot more finish than residential floors, undergo seasonal movement. And most people who want a wood floor aren't thinking of something that looks like a basketball court.
You could carefully control the temperature and humidity in the room so that they stay constant and avoid the factors that cause seasonal movement in the first place. This really isn't a way to design the floor specifically, but rather a separate mitigation strategy.