Why do they happen?:
Loss of Moisture. Wood accumulates moisture and expands respectively, later it dries and shrinks, creating a gap. Wood moves significantly more side to side than along its length. Typically a large butt end gap is caused by an unfastened board drifting, or the pulling force created from a settling foundation. Sometimes you can simply drop a smidge of glue and kick it back in place. When wood grows it pushes itself into surroundings often producing a cupped board. This force also crushes the wood like a dent from a hammer and when it dries it may not close back up all the way which is why some floor are happy year round. If mild, everything may simply settle back into its previous place when conditions allow. I am reminded of a time we gouged a stair tread with the buffer. The gentleman working with me dripped water into the dent, the dent rose a little higher, and higher as he repeated the process until it was flush again! Small seasonal gaps are considered a normal part of hardwood flooring which most people accept without agitation and maybe even admire as character.
Let's start our conversation at the beginning, installation.
Acclimation:
There are places with rainy seasons, starkly contrasted dry seasons. For others the humidity is stable all year. Regardless flooring to-be installed needs to be acclimated to the subfloor, and the subfloor needs to be dried into a state of normal living conditions beforehand. It is a notorious issue with new construction that the subfloor is not ready but the GC is! Now, now, now! The hvac has not been ran yet, there is radiant heating in a new slab (slabs take a long time to naturally dry), the subfloor might of even been installed before the roof, and rained on. To exasperate the complications the whole place is steamy from layers of latex paint evaporating water into the atmosphere. You want to please him, but rest assured if there is a failure, your to blame. Put your foot down and explain the importance of preparing the environment. The time that it takes wood to adjust is related to how big of a moisture content gap your trying to close, the species of wood, and the airflow provided. For that reason there is no set rule on acclimation timing. Flooring contractors learn the baselines of a normal homes, and the baseline of lumber sources to create a starting point which typically works out. Take your measurements from different locations around the home. I once had half a room reading at 13% while the rest of house was 7%. It turned out there was a small plumbing leak underneath the floor which was promptly corrected, but it still took me 2 weeks to get the subfloor acceptably dry.
Moisture Barrier:
Key word, barrier, not blocker. Either a sheet or roll on barrier is recommended (and required by industry standards) over any space which is situated over a space lacking climate control. This slows down the exchange of moisture providing an averaging moisture to the wood over a longer period of time, which gives homeowners time to correct moisture problems. If the moisture is blocked on a wooden subfloor condensation could become trapped, leading to mold. In the case of concrete slabs, you can and should block by using 2 coats of a roll on liquid moisture barrier.
Species & Cut:
Its a VG (quarter sawn) douglas fir porch, it is 100 years old. It has seen drought and Flood, wind and rain. It's directly nailed to joists with no subfloor, no insulation, and the only thing separating it from the earth is 3 feet of air. But, alas, there are hardly any gaps at all. That is good stability which is granted by the species of douglas fir, and complimented further by the angle of original milling. Compare that to a white pine floor of the same age, protected inside of the home. It has been through history too, the basement flooded one time, the homeowner didn't have AC back in 1924 and he loved the sound of rain through an open window. It's an absolutely gorgeous original floor and part of the charm is the dark quarter inch gaps framing and highlighting each board. We will get into how to solve the textural issue of excessive gaps without loosing character later. These are two extremes of high and low stability, but most common hardwoods are somewhere in between.
Paradoxes of Expectations:
Modern HVAC is such that a new home is wrapped in plastic, and "breathing" takes place from the inside through air exchange and humidity control. This probably works great in certain climates, but here in Minnesota where some winters can be extremely cold, the heat may stay on constantly, stripping the air of moisture. We need that humidity, not just for the house but for our bodies as well. The problem is the humidity gets misted out of vents which are situated next to windows and icicles develop on the inside of windows, which is not a good thing! So it may be unrealistic to expect a home owner to always maintain a set humidity. If you are installing in a dry season, consider creating some gaps with spacers every few rows or introduce humidity to avoid damage from excessive swelling during damper seasons.
It's not just wood, most synthetics and composites move as much if not more. "Waterproof" synthetic flooring is a salesman's game.
Understanding now why the gaps are happening. In the next article we will discuss the variety of techniques and products at our disposal for addressing existing gaps.
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