At the Site: Why Flat Matters
Flat. A simple four-letter word with so many connotations. If you are water skiing, a flat lake or body of water makes it easier and safer. Flat roads are easier to pedal a bike on. On the other hand, flat beer, flat line, and flat tire all have negative connotations. In our trade, we need a flat subfloor; the flatter the better.
One strives to meet the industry standard of flat to within 3/16” in a 10’ radius, or 1/8” in a 6’ radius. When nailing a wood floor, the subfloor should be flat to ¼” in 10’, or 3/16” in 6’. The science or geometry behind the numbers is simple—the shortest distance between two points is a straight line. Let’s say we are measuring a line 5’ long on an acceptable flat surface, and the distance from point A to point B is 5’. Now, let’s say the surface between point A and point B has a hump in it. That means the distance is longer from points A and B, because the hump (or valley) adds to the distance. So, one run is on a flatter surface than the next run; this is when the install becomes a nightmare.
While we’re discussing flatness, let’s examine the term level. Level and flat are not the same. They are not interchangeable. You can have a wheelchair ramp where the floor on the ramp is flat, but obviously, the ramp is not level. I like to demonstrate this fact by using a coffee table. The tabletop is dead flat. When I lift one end of the table, it is no longer level, but it is still flat.
A subfloor that is not flat can create insurmountable problems for a contractor doing a decorative floor due to the inherent runoff and the gain. When doing ornamental floors, a flat subfloor is critical, as once a herringbone or chevron pattern starts to run off due to the subfloor, you are in a real world of hurt. This is obviously true of other pattern floors, border work, inlays, etc., all of which require a very flat subfloor.
Another type of wood floor that requires a very flat subfloor is sports floors. The spec for them is 1/8” in 10’. Undulations in a sports floor can cause injury to participants, while undulations in an ornamental floor can cause YOU mental injury, not to mention financial.
The remedy is to evaluate any subfloor for flatness using a straight edge and carefully marking the highs and lows. When working with a concrete subfloor, you grind off the high areas and fill the low ones. Using a quality self-leveling Portland-based product works best. Small dips, holes, or cracks can be trowel-filled. The National Wood Flooring Association (NWFA), along with many manufacturers, offers training on the proper use of self-leveling underlayment.
The bottom line is that if you want to elevate your business and offer ornamental floors, first pay close attention to the subfloor flatness, and then choose a manufacturer you trust to provide quality products. Although it may cost a bit more, it will ultimately save you time, money, and reputation in the long run. If you are bidding on a decorative floor, don’t let yourself be beaten down on price for your labor or materials. Doing this type of work requires more time and attention to detail, as well as better milling. Trust me, if you screw it up, it will follow you for years to come. Remember, sometimes the floor you didn’t do is the best floor you never did!
Bob Goldstein handles technical training and sales for Vermont Natural Coatings. He is an NWFA Certified Wood Flooring Inspector, Certified Installation Professional, and Certified Sand and Finish Professional