Try a 'floating' floor over concrete

Home work

April 20, 1996|By Karol V. Menzie and Randy Johnson

Even if your unfinished basement is a dirt-floored dungeon, you've probably considered turning it into usable living space. That is, after all, what the IRS considers it, when instructing you about how to figure the square-footage of your house for tax purposes.

But unless it was built to be finished later, you may run into problems making the basement comfortable enough that anyone wants to spend time there.

A reader from Sykesville e-mailed us a question about turning his basement into living space. "What is the preferred method of subflooring over concrete?" he wrote. "Is it furring strips nailed into the concrete? Or can I glue them down? Then put foam insulation between the strips and plywood over the whole thing? How about particle board instead of plywood? I've heard of plywood separating and creating 'pockets.' How about just gluing the plywood (or particle board) directly to the concrete if overhead space is a problem?

"I'm not too excited about nailing into concrete; I feel that any such puncture into the concrete could be an invitation for water to enter (though there is no water problem now)."

Randy has installed several floors over concrete lately. The method he prefers is to use pressure-treated 2-by-4s laid flat and fastened into the concrete.

There are two ways to fasten the 2-by-4s to the concrete. Neither is difficult, but both require a tool that few homeowners are likely to own. One technique is to nail the 2-by-4s with a power nailer, a device that uses .22 caliber shells to drive 2 1/2 -inch nails into the concrete. (Yes, it's loud.) However, some concrete is so hard the power nailer won't penetrate.

The other tool is a hammer drill, which drills holes for expansion bolts. The hammer drill will probably penetrate any concrete, so if you're sure you want to fasten the 2-by-4s to the concrete and you have to rent a tool to do it, the hammer drill is the best bet.

However, it isn't absolutely necessary to fasten the new floor to the concrete. You can install a "floating" floor -- a grid of 2-by-4s laid over the concrete, reinforced with spacers and fastened together with flat metal plates. Framing the walls on top of the grid holds it down.

The difference between fastening the 2-by-4s into the concrete and using a floating grid is a bit of rigidity -- that is, there's always the possibility that the grid could move slightly. The movement is not likely to be significant, but you wouldn't want to TC use a floor covering that requires absolute rigidity, such as ceramic tile. Vinyl sheet goods or carpeting would work fine.

Here's the procedure for installing a floating floor: Start by

covering the concrete with plastic sheeting, then install the 2-by-4s (nailed or not). To insulate, use 1 1/2 -inch-thick foil-covered foam board, which can easily be cut with a utility knife to fit between the 2-by-4s and spacers. (Make it fit tightly for the most insulation value.) Use 3/4 -inch tongue-and-groove exterior-grade plywood for the subfloor, nailed or screwed to the grid. Particle board is less desirable than plywood, because if it does get wet, it will swell. Marine-grade plywood will withstand repeated soaking -- but it's expensive.

We don't recommend gluing plywood directly to the concrete -- it's not likely to stick.

Putting a hole in the concrete doesn't necessarily lead to water problems. Basement water problems come from the presence of subsurface water or poor yard grading. If your house is less than 15 years old, you may already have a drain tile-sump pump system to keep water out of the basement. If your house is older, or doesn't have such a system, you should consider installing a ** sump pump, at least, in a well with the outflow pipe running to the exterior. You should also make sure the yard is properly graded so water flows away from the house.

Mr. Johnson is a Baltimore construction manager. Ms. Menzie is a feature writer for The Sun.

Pub Date: 4/20/96

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