Floor Preparation On This Job


Floor Preparation Is A Vital Prerequisite

One of the largest reasons for failure with all wood floors comes from lack of attention to floor prep. Many run of the mill installers often overlook floor prep which comes back to haunt every time. Typically the subfloor is out of tolerance in relation to how flat it should be. Many confuse level with floor preparation. From here forward it's flatness we want to achieve. This section deals with concrete subfloors. Wood subfloor preparation can be found here.

Clean Subfloor

Prior to anything we need to clean the subfloor of any paint, or drywall splatters those Roughing up concrete before gluing hardwoodothers guys love to slop around. Many installers will use a four inch floor scraper and handle this procedure by hand, but other options call for scarifying the concrete with a heavy grit paper attached to the bottom of a flooring buffer (right). Both items can be obtained at many home improvement big box stores. Specify a 12 grit rock paper (seen below).

Failing to properly prep a concrete subfloor for a glue down installation may lead to separation of the hardwood with concrete and glue. Thinking of sealers? It's a good idea to use them but avoid anything that may be slick. Common concrete sealers are good for carpet or floating hardwood floors, but not gluedowns. Today many hardwood glue manufacturers offer their own proven sealers that can be used.

How Flat Should The Subfloor Be?

Manufacturers agree on one thing, subfloor tolerances should not exceed 3/16" of an inch over a 10 foot span. How do we check? In checking flatness, obtain the straightest piece12 grit rock paper of lumber you can find, preferably 8-10 foot in length. In this case we have a piece of leftover white oak plank (shown below) that has proven a valuable source for years. Move the straightedge over all areas of the sub floor in opposing directions. High and low spots should be pencil marked so you know which areas need to be addressed for repair.

Another method used for checking floor flatness is with the use of a string line. Using a helper, grab one end of the line and stretch it over the subfloor while standing on the room perimeter. Lower the line so it touches each side on the surface. By adjusting up or down you will get an idea of areas that may be too high or low that need correction.

Our Findings On This Job

In this case we've found seven areas over the 1200 square foot installation that need help with low areas. The one pictured is the most severe, measuring an approximate 3/8" dip Checking for flat subfloorover a four foot area. Not correcting these types of irregularities before the actual installation results in popping sounds or squishy feelings under foot after the floor is installed. You may not notice it immediately, but it could come to haunt you months later.

High areas should be grinded, otherwise you're likely to get a seesaw feeling after the installation. Grinding creates an enormous amount of dust, but hand held grinders with vacuum attachments can be used. It will not collect all the dust but a major portion of it. In any event be prepared and cover everything you value.

Big Daddy Concrete Grinder

Close off other areas of the home and open windows for good ventilation when grinding. Large heavy duty concrete grinderDust from grinding will also get into kitchen cabinets. Don't expect these hand held concrete grinders to remove a big hump in a few minutes. Time and patience will endure. For major problems one can bring in the Levtec Expander grinder (shown) for flattening concrete subfloors in a flash.

Finding where to rent one of these beauties may be troublesome, but recently a few companies that specialize in floor removal have begun to offer this service.

Floor Leveling Compound?

Now that we've turned your attention from level to flat, we have leveling compound. Don't let it confuse you. Let's just call it floor patch. However it is called leveling compound in many circles. There are many choices out there. Home Depot seems to provide the most accessible source from our last visit around town. A local

flooring supply store is another option, and most sell to the general public. It's important to select a compound that will cure quickly. Ordinary cement mixed products will not cure quickly enough and will cause moisture problems after the installation.

Some names include Mapei and Ardex, with the latter providing a true floor leveling characteristic for those that may have severe problems that require complete work on the entire concrete slab. It's important to note, these fast setting compounds require fast working times. In other words, make sure you're ready, because this stuff will harden before you want it to. It may be wise to play with it first, getting a grasp on the properties.

Preparing The Patching Compound

Follow all manufacturers specifications listed on the product container. Most compounds are mixed with water. In our example we're using Mapei. First add water to an empty fiveMixing floor patching compound gallon bucket and a portion of the mixture. By using a paddle type mixer (attached to a drill) we're looking at getting milk shake consistency. By not adding water first, you will get a clumping mixture and won't be able to get to the compound that will be stuck on the bottom of the bucket.

Cautionary Note: When doing any mixing, it's preferable to do so outdoors or in the garage. This stuff will slop around on floors and walls if you're not familiar in working with it.

Applying The Compound

While using the same straight edge, pour an amount in front of it, moving the material to the front edge with a float trowel. Pull the straight edge back over the low area. This is often termed screeding. It's best to have one person on each side of the straight edge/screed. Chances are you won't get a perfect screed but our area won't be visible after the installation, so looks aren't important.

Again, you won't have much time before the patch begins to harden. Once your first batchConcrete floor leveling with screed is used up, proceed directly to the hose for cleanup. There's nothing worse than dried compound in the bucket and on the tools. Allowing the material to harden for 20-30 minutes some areas can be trowled to a more desired appearance.

Moisture Testing

In our example (pictures below) we have a Tramex concrete moisture meter and a common relative humidity meter. Weather conditions should be noted for all glue downs. If it's raining we recommend putting off the job or close up the house and allow conditions to reach safe relative humidity levels in the range of 40-60%.

Dew point and humidity can adversely affect the bonding of adhesive to all concrete. For drier climates, we understand the above mentioned Rh ranges are not practical and suggest common sense in the event of humid or rainy weather conditions. If in doubt, postpone your project until conditions improve.

While we realize the average consumer won't have the need for sophisticated testing equipment, another effective method of testing for concrete moisture is shown. Duct tape 2' x 2' pieces of 6 mil or similar poly film to the slab in several areas of the layout. Making sure all edges are sealed, allow to sit 24 - 48 hours. If any condensation or color changes take place you have moisture issues that need to be addressed.

Testing humidity and concrete moisture
Plastic used for moisture test
Testing concrete moisture and Rh
Duct Tape Visqueen

More Prep Work. Trimming Door Casings

Quality installations need proper attention to preparation and detail. What to do around the door casings is often asked. They should be trimmed to the same height as the flooring so the hardwood will slide underneath providing a clean professional appearance. Professionals rely on power jamb saws that cost $200-300. If you have a few casings to cut, a hand saw will work just fine. Using a scrap piece of wood, lay it directly on the concrete cutting the casing with the hand saw flat to the scrap wood.

If you have more than a dozen door casings to trim, renting a power jamb saw is an option but hard to locate. Many flooring tools I've mentioned can also be obtained online.

Back To: Glue Down Floors. Introduction Next Page

Related Pages

• Floor Leveling Compound
• Dust Control
• Trim Door Casings
• Hand Held Floor Scraping Tools

Added December 7. 2008

View our own floor leveling video on concrete. It will not win any design awards, but get's the point across.

• Deals with covered topics from above and more.

Video duration: 4:36 minutes
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