Wood's Expansion and Contraction

Expansion and contraction of wood is perfectly normal during changes in the weather. Wood is a natural material that seeks to be in balance with its surroundings. Hardwoods destined for use in home furnishings are carefully kiln-dried for that purpose, and they will take on or give off moisture with extreme changes in relative humidity. When the air is exceptionally warm and humid, solid hardwoods will absorb moisture and expand. Likewise, with much cooler, drier air, the wood will give off moisture and contract. This is completely natural, and craftsman design fine solid hardwood products to accommodate these changes.

Here are a few tips to help you be weather-wise:

• Maintain relative humidity of at least 50 percent; i.e., use a humidifier in winter and an air conditioner in summer.
• When storing table leaves, keep them as close to the table as possible. Keep them in an upstairs closet rather than a damp basement so that the table and leaves are adjusting to the same humidity conditions.
• If you are planning a hardwood floor, cabinet or moulding project be sure to understand hardwoods' special features and how to deal with them.

Remember that hardwoods are natural materials and they will expand and contract with extreme changes in your home's relative humidity. A good contractor will take this into account when installing new hardwood floors, cabinets, mouldings and other decorative millwork.

Follow these tips to make sure the proper steps are taken before and during the project.

Pre-Installation Checklist

1. Your house should be closed in, with all the outside windows and doors in place, before hardwood lumber, millwork or flooring are delivered.
2. The temperature and relative humidity should be maintained at occupancy levels for at least five days.
3. Excess moisture anywhere in the house should be allowed to evaporate.
4. Sheet rock should be allowed to dry for at least two days, and plaster for a week or more, depending on weather conditions.
5. When it's delivered, the lumber, flooring or millwork should be divided into small lots and stored for at least a week in the rooms where they will be installed.
6. Unprotected hardwood products should never be trucked, unloaded or stored in rain, snow or other wet conditions.
7. A contractor who receives a shipment of hardwood paneling, flooring, millwork or cabinetry should pull out several sample boards and use a hand-held moisture meter to measure their moisture content. The figure should closely match the one recommended for your part of the country. If it doesn't, the contractor should let the hardwood adjust to the site before installation.

These important installation tips will help minimize expansion and contraction of hardwood paneling, flooring and millwork during seasonal changes in temperature and relative humidity.

Flooring

1. Moisture passes primarily through end grain, so when wood has been cut or mitered, the ends should be sealed with a transparent sealer before installation.
2. Flooring should never be installed over damp concrete or wet plywood. Even if the wood is dry, it will pick up moisture from the wet subfloor.
3. When laying a solid hardwood floor, a three-quarter inch gap should be left at the wall line for expansion. Baseboards will cover the joint.
4. For wider plank flooring, the back surface should be sealed before installation and after acclimation.

Paneling and Millwork

1. Hardwood paneling should be installed so it moves across the grain. Narrow widths should be used when possible.
2. For large areas, an expansion gap should be left to
   accommodate natural expansion and contraction based on recommendations for your region. The edge should be concealed with baseboards or mouldings.
3. On perimeter walls, a vapor barrier should be installed on the inner face of studs or masonry.

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