Wood & laminate
Expansion gaps: how wide, where, and why floors buckle
Every floor moves. The gap at the walls is the room it moves into, and a floor without one buckles, cracks or lifts. Here are the gaps for each type and the arithmetic behind them.
Why floors move
Wood takes up moisture from the air and swells, and gives it back and shrinks. A wood floor is a little wider in a humid summer than in a dry winter. Laminate’s fibreboard core does the same, less. Vinyl plank barely reacts to humidity but grows and shrinks with temperature, which matters in sunrooms and by tall windows. Tile does not move enough to need a gap at the walls, though it does need movement joints in large areas and at the perimeter, which are filled with sealant rather than grout.
The movement is small per board and large across a room. If the floor has nowhere to go, it pushes against the walls and lifts: a floating floor peaks along a joint, a nailed floor cups or crowns, and in the worst case the boards break free of their fasteners.
How wide the gap should be
| Flooring | Gap at each wall | Unbroken run before a break |
|---|---|---|
| Solid hardwood, nailed | equal to its thickness, 3/4 in for standard boards | no limit for the run |
| Engineered wood, floating | about 1/2 in | about 40 ft |
| Laminate, floating | about 3/8 in | about 40 ft |
| Vinyl plank, floating | about 1/4 in | about 50 ft |
These are the common figures; the instructions in the carton always override them. The National Wood Flooring Association’s rule for solid wood is a gap equal to the board’s thickness. For floating floors, makers give both a gap and a maximum length the floor may run in any direction before it must be broken with a transition strip, because a long floating floor moves as one sheet and the total movement grows with its length.
The run limit and transition strips
A floating floor that runs 58 ft along a combined living and dining area exceeds the usual 50 ft limit for vinyl plank. It needs a break somewhere in that length, usually a T-moulding at a doorway or a natural division, with a gap under the moulding so each section can move on its own.
A long open-plan vinyl floor
58 × 24 ft of floating vinyl plank.
- Gap at every wall: 1/4 in
- Run limit: 50 ft, so the 58 ft length needs 1 transition strip
- The 24 ft width is within the limit
The expansion gap calculator gives the gap and the strips for each floor type and room, and the thresholds and transitions calculator counts the strips you need at doorways and changes of floor.
How much does a wood floor really move?
The movement of wood across its width is about 0.37% for every percentage point of moisture content it gains, for flatsawn red oak, according to the US Forest Products Laboratory’s Wood Handbook. Across a whole room the figure is surprising.
A solid oak floor in a house with big seasonal swings
A 22 × 16 ft room, boards running along the 22 ft length, in a house closed and unheated for part of the year, with the wood moving through 8 points of moisture content.
- Width across the boards: 16 ft = 192 in
- Change: 192 in × 0.00369 × 8 = 5.67 in if every board moved freely
That is the worst case. In practice, nails restrain each board and the movement is shared between small gaps between boards and the gaps at the two walls parallel to the boards. But it explains why solid wood needs a full 3/4 inch gap, why it gaps in winter, and why holding the house between 35% and 55% relative humidity matters more than any gap.
For solid wood, most of the movement is across the boards, so the gaps on the two long walls parallel to the boards do most of the work; the end walls see almost none. A floating floor moves in both directions and needs the full gap all round.
Where the gap goes
The gap goes at every wall and every fixed object: door frames, pipes, kitchen islands, stair nosings, fireplace hearths and the walls of closets. Undercut door casings with a jamb saw so the floor slides under them. Drill pipe holes a gap larger than the pipe. Do not set cabinets or a kitchen island on a floating floor; install them first and float the floor up to them with a gap.
Hiding the gap
The baseboard covers most gaps up to about 1/2 inch. For larger gaps, or when the baseboard stays on the wall and the floor is laid against it, add shoe moulding or quarter round, which is 3/4 inch deep. The baseboard and quarter round calculator turns the wall length into pieces.
The most important rule about trim: it is fixed to the wall, never down through the floor. A quarter round nailed into a floating floor pins its edge, and the floor then buckles away from the pinned edge. Transition strips sit on a track fixed to the subfloor in the gap, never through the planks.
The gap and the layout
The gap takes width away from the planks. A 3/8 inch gap at each wall takes 3/4 inch off the width the rows must fill, which changes the width of the last row. The plank layout calculator includes the gap, and the plank layout guide explains the arithmetic.
When floors buckle anyway
A floor with the right gap can still buckle if water reaches it or if the room’s humidity goes far outside the range. The usual causes are a leak, a basement or slab without a vapour barrier, a house left without heating or cooling, or flooring laid before it acclimated. The comparison of wood floors covers how each type responds to moisture, and the vinyl plank guide covers the temperature limits vinyl makers set for sunrooms.