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Understanding Flooring Underlayment

Understanding Flooring Underlayment

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October 28, 2009

Underlayment may be the least understood component of a beautiful floor. Nationally syndicated newspaper columnist Tim Carter, author of the "Ask the Builder" column, says, "Most flooring failures or problems are traced to inadequate or poorly installed flooring underlayments."

From the bottom up, floors consist of these components:

  • Joists: the long supports running from wall to wall
  • Subfloor: sheets of material nailed to the joists to provide a structural foundation for the floor
  • Underlayment: sheets of material fastened to the subfloor to provide a smooth surface for flooring
  • Finished flooring: carpet, various types of tile, or resilient flooring, which is often made of cork, vinyl, linoleum, or rubber

Resilient flooring is popular because it's relatively inexpensive, water-and-damage-resistant, and springy and comfortable to stand on. Because of its very springiness and flexibility, resilient flooring requires a perfectly smooth underlayment so that imperfections don't show through the flooring.

You can install underlayment yourself, but to ensure that it's done perfectly you may want to visit with a professional flooring and installation company.

Choosing the Underlayment

Underlayment comes in several sizes, thicknesses, and materials, including various wood products as well as cement and gypsum backer board. To avoid headaches and heartbreak, it's important to get written specifications from both the underlayment manufacturer and the finished flooring manufacturer regarding:

  • What type of underlayment the finished flooring will need
  • What type of fastener to use in securing the underlayment to the subflooring (underlayment staples, ringshank flooring nails, or drywall screws), and how far apart to space the fasteners
  • And what adhesive will work best between the underlayment and the finished flooring

Manufacturers may have different requirements for a commercial building than for a residence.

Installing Underlayment

  1. Use a moisture meter to make sure your subfloor has less than 15 percent moisture.
  2. Lay out the underlayment sheets, beginning in the middle of the room, so that you can drive fasteners into the joists. Stagger the boards to ensure that four corners never meet.
  3. Fasten the underlayment to the subflooring as recommended by the underlayment manufacturer. Make sure that all fastener heads are slightly indented into the underlayment.
  4. Cut and install any necessary underlayment strips around the edges of the room.
  5. Trowel flooring patch into all fastener dimples and joints. Let the patch dry completely; then sand the floor smooth. Don't over-sand or you'll create a dimple that will show through resilient flooring.

Installing perfectly smooth underlayment is a painstaking, precise job. If it's not a task you want to try doing yourself, get advice and help from a professional flooring and installation company.

About the author: Jane Greer writes and teaches writing at a lovely community college.

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