In order to guarantee the success of a wood floor installation the product must be allowed to expand and contract naturally through the four-season cycle. Why? because wood is hygroscopic - which means it will absorb, hold and then release moisture over time depending on moisture levels in the surrounding environment. Changing relative humidity (RH) and temperature levels influence the shape and size of wood flooring on a daily, even hourly basis. Without the ability to expand and contract freely, the risk is that your wood flooring will bind against an obstruction causing squeaking, lifting and joint stress.
Wood flooring products are dried in a kiln at the factory, warehoused, shipped across deserts and oceans by truck, or in steel containers in the heat of the summer and or cold of the winter, eventually arriving at the place of installation. This all takes place with the wood flooring moisture content fluctuating between 6 - 9%. At the wood floors final destination (the home), installation temperature should be approximately 20c (70f) and the RH between 35 - 55%. Once acclimated to correct site conditions, installation can begin. These temperature and relative humidity conditions should be maintained if movement is to be minimized after installation. If these levels don't change the wood flooring won't change. But as we all know, temperature and RH levels do change...a lot...and so care should be taken when planning delivery times and recording moisture and humidity levels on site.
All of this is common knowledge for hardwood flooring installers and yet we still see a high incidence of problems related to inadequate expansion gaps.
Here are a few examples of problems we see:
- Correct expansion gap, wrong moisture level. Excess moisture on site will be absorbed by the wood flooring causing more expansion than the recommended gap size can handle. Small amounts of moisture vapor can cause huge problems. Test the site conditions and record the results for future reference.
A rule of thumb is: no more than a 4% moisture difference between hardwood flooring and sub-floor.
Do not deliver the wood flooring to site until these or manufacturer's recommended site conditions exist.
- The sub-floor is tested dry but the space below ( maybe a crawl space) has excess moisture. 'As below, so above', moisture will be absorbed by the driest material on site, which is often the hardwood flooring, until equilibrium is achieved. Check that all areas around the installation area are dry to specification.
- Spacers (for a floating floor) are designed to hold the starter row of flooring out from the wall, creating and holding the correct expansion gap during the first stage of installation. Often the practice of pinning the first row into place (on wood sub-floors) is used instead of spacers to save time. Pinning the first row into place prevents natural expansion and contraction and voids the manufacturers warranty.
- Installation of fixed items by another trade, such as stair railings, takes place after the floor is installed. Such items are anchored to the sub-floor through the floor covering preventing natural movement.
- Overlap transitions, such as door thresholds and expansion joints are often glued into place with too much adhesive. The excess adhesive squeezes out inadvertently bonding the transition, flooring and sub-floor together. This prevents natural floor expansion and contraction. Care should be taken to only bond the bottom of the overlap transition piece to the sub-floor.
- Glue down floors are often cut tight to tile Schluter strips for a desirable level/flush transition. Even though the wood flooring is glued down, it will still expand and contract and so room for expansion should be built into the floor design. Special, flexible fillers that look like tile grout can be used to fill the expansion gap and provide a finished look.