Before we look at some reasons for this it is important to understand that in order to guarantee success of a wood floor it must be allowed 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 temperatures 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 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 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.
This is common knowledge for hardwood 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 vapour can cause huge problems. Test the site conditions and record the results for future reference.
Do not deliver the wood flooring to site until these or manufacturers recommended site conditions exist.
- The subfloor 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 subfloors) is used instead of spacers to save time. Pinning the first row into place prevents natural expansion and contraction and leads to problems.
- Installation of fixed items by another trade, such as stair railings, takes place after the floor is installed. Such items are anchored to the subfloor through the floor covering preventing natural movement.
- Over lap transitions, such as door thresholds and expansion joints are often glued into place with too much adhesive. The excess adhesive squeezes and spreads out inadvertently bonding the transition, flooring and subfloor together. This prevents natural floor expansion and contraction. Care should be taken to only bond the bottom of the overlap transition piece to the subfloor.
- 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.