How do hardwood flooring expansion gaps work?

Question

I know that when you install hardwood flooring, you need to leave a small expansion gap along the walls. I understand that this is to allow the wood some space to move when the wood expands due to humidity and temperature fluctuations.

My question is why the expansion gap works, especially with floors that are nailed or stapled to the subfloor. If each board is nailed down, how is an expansion gap along the edges of the room going to help when the wood expands? Wouldn't each board just collide into the ones alongside it?

Plus, even if you have a floating floor where most of the boards are not nailed down, the ones along the edge of the room typically would be nailed...so wouldn't they be immobile and prevent any movement into the expansion gaps?

Answer 1

It's more accurate to say that the wood fibers swell rather than the wood moves. A board which is nailed down will still swell and contract around the nails, generally leading to a larger hole and squeaky floor. The nails hold the floor down and prevent gross movements, such as the floorboards sliding around underfoot.

Another key point to grasp is that boards swell along the growth rings in an amount proportional to their size. For practical purposes that means they really only have noticeable changes in width and it's a couple of percent of the width of the board. Adding an extra 1/64" to any individual board isn't a big deal, but it can add up across a lot of boards.

The gap is really about averting disaster. You're a board soaking up humidity. The tongue and groove joint gets a bit tighter, taking up some of the slack. Nail holes enlarge or nails bend slightly as you shove your neighbor aside an imperceptible amount. The floor remains flat and all is well, unless you're tight against the wall to begin with. In that case boards swell, and they press on their neighbors. They can't move sideways or down, so they go up. Boards cup, edges rise, the floor is warped.

Answer 2

Since I cannot comment right now (not enough reputation), I will write my understanding and clarify the OP question here.

As pointed out by Matthew Gauthier, even if the hardwood floor is nailed down, it still swell and contract according to its environment. Let's suppose the subfloor on which the hardwood floor is nailed down has very negligibly small expansion coefficient (this is the initial simple model, more complications can be considered later). During the expansion or contraction of hardwood floor, since the subfloor is not changing, there must exist at least one point on the hardwood floor that is not moving relative to the subfloor. Let's assume this stationary point to be in the center of the room (the conclusion will not be affected if this point is near the room wall). The further away from this point, the more expansion or contraction movement at the other points. So here is the conclusion: the gap near the wall will indeed help to avert the disaster, but the hardwood floor needs to overcome the holding force from all those many nails first before the possible disaster near the room wall. This would also lead to larger nail hole when the nail is further away from the stationary point. The hole could be near the size of the expansion gap which is of ~ 1/4 inch.

Please point out any flaws in the above logic.