Q: I just read a column of yours on do-it-yourself installation of hardwood flooring and have a question.

I noticed there was nothing in the article about making sure the subfloor was level beforehand, just that the homeowners chose a nail-down stick floor and they stapled a layer of building felt to the subfloor.

So is it just floating floors that need to have an absolutely level subfloor? Or does the building felt "absorb" any difference in level, so to speak?

A: The subfloor doesn’t have to be dead level, but it must be flat. That means no bumps, ridges, nooks or crannies. These imperfections must be sanded flat or filled. This holds true whether the flooring material is solid wood or laminate.

Wes Clark and his wife Hui Ping live in a 20-year-old house in a hilly area of Castro Valley, Calif. Bill helped with the installation. Although they didn’t put a level on the floor, we’re certain there had been some settling over the years, resulting in a slight slope one way or another.

Q: I just read a column of yours on do-it-yourself installation of hardwood flooring and have a question.

I noticed there was nothing in the article about making sure the subfloor was level beforehand, just that the homeowners chose a nail-down stick floor and they stapled a layer of building felt to the subfloor.

So is it just floating floors that need to have an absolutely level subfloor? Or does the building felt "absorb" any difference in level, so to speak?

A: The subfloor doesn’t have to be dead level, but it must be flat. That means no bumps, ridges, nooks or crannies. These imperfections must be sanded flat or filled. This holds true whether the flooring material is solid wood or laminate.

Wes Clark and his wife Hui Ping live in a 20-year-old house in a hilly area of Castro Valley, Calif. Bill helped with the installation. Although they didn’t put a level on the floor, we’re certain there had been some settling over the years, resulting in a slight slope one way or another.

After all, the house sits just uphill from a major earthquake fault. But whatever slope exists did not present a problem when laying the floor.

On the other hand, Kevin built his home in Idaho over a 10-month period — from October to the following September, with time off for Christmas. There was a fair amount of snow that winter. When it melted, the moisture caused the subfloor to swell, creating ridges at the joints.

The next summer, Kevin installed hardwood flooring in the first-floor living area. To get it to lie correctly he had to take a belt sander to the joints to smooth them down. It was quick work, taking maybe an hour for the 700-square-foot area.

The building felt installed by the Clarks was simply a vapor barrier. It was not intended to be a floor-leveling product. The 15-pound felt they used is tar-impregnated paper. It’s very thin and does not cushion the floor or attenuate sound to any great degree.

Kevin did not use building felt between his flooring and the subfloor. Rather, he sent Bill under the house to install batt insulation with a tar-impregnated paper face. This acted as a vapor barrier between the unconditioned air of the crawl space and the conditioned interior air.

Floating floors are installed either over a separate thin spongy underlayment or have a cork or sponge-like underlayment attached to the bottom of the flooring. They don’t require a dead-level floor, but the floor should be flat and free of bumps and ridges.

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