ASTM F-710, Does the Subfloor Need to be THAT Flat?
Installation instructions, from most major flooring manufacturers, reference ASTM Standard F-710 for hard surface resilient flooring. The Standard simply defines the required flatness of a concrete subfloor. The subfloor, or ‘underfloor’ as I like to call it, “shall not deviate more than 3/16 of an inch in ten feet.” Photo 1 shows a ten straight edge being used to gauge the undulation of the floor. Photo 2 demonstrates the maximum allowable deviation- 3/16 of an inch within 10 lineal feet- with a mark on the shim shingle below the straight edge.
ASTM Standard F710 is a straight forward rule however a quick audit of hard-surface flooring installations indicates that most installers do not provide provisions to accommodate this ASTM standard during installation. The result can be seen in Photos 3, 4 and 5.
In order to simulate the outcome of resilient flooring tiles installed over a flat subfloor that complies with ASTM F710, I set up, labeled and outlined in pencil, a group of four tiles over a flat subfloor surface as shown in photo 6. To demonstrate what happens when there is a hump in the concrete sub floor, I placed a 3-and-one-half inch bump under the 2×2 foot floor tiles. Photo 7 shows that the joints are no longer tight and photo 8 reveals a deviation from a net fit where the uneven subfloor caused the tile pattern to pull away from square. When this happens across the span of a room, cutting tiles and increasing or skewing joint lines is necessary to counteract the runoff caused by an unprepared underfloor. This is often an eyesore and results in claims and complaints from customers to the company that makes the flooring.
As you can see in Photo 9, when the floor meets the Standard set by ASTM F710, the modular flooring is not defective as it visually provides an acceptable finished appearance -that the manufacturer had envisioned when designing, engineering and manufacturing the flooring- with tight joints and straight lines between flooring tiles.
Photo 10 discloses the immediate problems created when a floor does not comply with this Standard due to a severe hump in the underfloor. Close inspection of the photograph reveals modular components that veer to the left and to the right making it impossible to navigate the hump without gaps and misaligned corners. Even the best flooring installer cannot overcome severe undulation unless they even out the problem areas of the subfloor prior to laying the floor tiles.
The bottom line…installed floors that do not conform to ASTM Standard F 710 are not a manufacturing defect, they are the direct result of poor workmanship or failure to follow the flooring manufacturer’s printed installation instructions. It is important to know that the smaller the format of the modular flooring, the more difficult and challenging it will become to install the flooring with tight net joints in both directions and without runoff. Often in severe situations, a flooring installer may be tempted to make field cuts to modify the floor tiles, so that it is brought into what is perceived as alignment, but soon after cutting the challenge of installing with a net fit at the corners and joints is still impossible.
Replacing a floor can be expensive…especially when the flooring manufacturer is not participating in the cost so take the time to resolve sub floor issues before flooring is installed or you’ll be left with a choice- rip up the just-installed floor in order to level the underfloor or live with an unattactive cut and paste job intended to conceal subfloor undulation that your flooring installer did not fix -it’s your call.
Printable PDF file for ASTM F-710, Does the Subfloor Need to be THAT Flat?