The Importance of Flatness

Reasons for gaps and peaks in floor finishes

Sometimes the final “look” of a flooring is not as it appears in the promotional literature and advertising.


Here we have to realise that the flooring is perfectly laid in the imagery on a “perfectly” flat  and smooth surface.

It shows an “ideal result”.

It has to for marketing purposes!

In “reality” an “ideal result” is not achieved- the problem not being with the floor covering but with the actual underlying surface that  through minute variance can cause a “domino” effect across the surface.

It may look smooth and level but don’t just go by a visual look or feel. Prove it the old-fashioned way by using straight edge rulers and levels over a certain distance.

Where bumps and ridges lie below the top layer -and it can be seen on the surface- we call this “telegraphing”.

Most rubber products manufactured in the world today are made to within very high tolerances of less than 1% surface variance.

To blame the product for an uneven appearance is not acceptable- but often the first and easiest way to point the blame for the failure of an entire project.

That is where “expertise” in laying flooring- especially rubber is required. The way the flooring looks can almost certainly be co-related to the field experience over the years but an installer that specialises in this field. So an “experienced” installer helps.

Designers and architects need to understand this concept rather than being dismissive and saying we no longer will specify this or that product because of the way it looks after install. Naivety is rife with all due respect when this attitude is taken.

Things needed to be looked at in perspective rather than giving sweeping generalisations about the product and its performance.

So here’s an interesting article to explain things in a more un-emotional and reasoned fashion!

ASTM F-710, Does the Subfloor Need to be THAT Flat?

Read more at – Reggie’s Floor Stories™Resilient FloorsTrouble ShootingFlooring Installation

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 1 (10-foot Straight-edge used to gauge flatness of 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.

Photo 2 (Plywood shim  replicates maximum acceptable 3/16” deviation in height.)

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.

Photo 3 VCT run off and gaps at joints
Photo 4 Rubber floor tile run-off, gapping at seams.
Photo 5 Flooring tile with severe runoff.

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.

Photo 6: Set up of 4 tiles laid on a flat subfloor with tight joints and the perimeter traced.

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.

Photo 7: After simulating bump in subfloor, evident opening at joints.
Photo 8: With 3-1/2” subfloor deviation, tile alignment shifts drastically.

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 9: LVT installation meets ASTM-710 Standard for flat subfloor.

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.

Photo 10: Result of severe underfloor humps.

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 unattractive cut and paste job intended to conceal subfloor undulation that your flooring installer did not fix -it’s your call.