Concrete Floors: Flatness vs. smoothness

September 11, 2015

Guildford Mall 4[1]
Photos courtesy Apex Granite and Tile

By Keith Robinson, FCSC, RSW, LEED AP
There is a great deal of confusion in the world of flat concrete floor surfaces. On one side of the discussion are the people who place concrete floors—the concrete floor finishers. As a Work Results-based component of the final construction, they obtain guidance about what is or is not ‘good’ concrete floor finishing from reference standards such as Canadian Standards Association (CSA) A23.1, Concrete Materials and Methods of Concrete Construction, and CSA A23.2, Test Methods and Standard Practices for Concrete, and from the American Concrete Institute (ACI) Manual of Practice documents ACI 117, Specifications for Tolerances of Concrete Construction and Materials, and ACI 302.1R, Guide for Floor and Slab Construction.

In the world of concrete floor finishers, the results of their work are easily quantified through measurement criteria contained in ASTM E1155, Standard Test Method for Determining Floor Flatness (FF) and Floor Levelness (FL) Numbers, and ASTM E1486, Standard Test Method for Determining Floor Tolerances Using Waviness, Wheel Path, and Levelling Criteria. The referenced standards and measurement criteria are supported by the Concrete Floor Contractors Association of Canada (CFCA). The group holds its members accountable for providing concrete finishing to the CSA A23.1 requirements for overall F-number tolerances for flatness and levelness. Based solely on these requirements, everything in the world of concrete floor finishers appears perfectly controlled and achievable.

On the other side of this discussion, however, there are those who install floor finishes. These flooring installers include a wide range of trades ranging from soft surfaces such as carpeting to hard surfaces such as tile. This group is as diligent about its Work Results as the concrete floor finishers, and also has a set of binding standards, provided from the various associations and manufacturers governing the components of the work for which they are responsible.

Flooring installers are governed by the same CSA and ACI reference standards and ASTM testing criteria listed for the concrete floor finishers, but are usually asked to meet more restrictive measurement requirements. This difference in standards is where a problem starts to become apparent. It raises the question, who is responsible for achieving required floor tolerances for installed finishes? This author has been around for a few years, and remembers a time when construction practices differed from those commonly accepted today as being standard.


To understand what is different now compared to then, we need to explore applicable tolerances, and perhaps become more tolerant of the differences encountered between the level of concrete finishing provided by the concrete floor finishers, and the expectations of the flooring installer. However, we also need to understand the changes to floor finish products that are altering the expectations for reasonable floor flatness and levelness used in the past.

The structural tolerance
Table 12 of CSA A23.1 (shown in Figure 1) states clearly what we can expect for slab and floor finishes. It includes an exception on levelness for elevated slabs—tolerances for FL do not apply for that installation condition.

Canadian Standards Association (CSA) A23.1, Concrete Materials and Methods of Concrete Construction, states clearly what can be expected for slab and floor finishes. It includes an exception on levelness for elevated slabs.
Guildford Mall 2 cropped[3]
A successful finished tile project requires an understanding of who is responsible for what when it comes to tolerances in concrete slabs and floor finishes.

CFCA supports the tolerance requirements of CSA A23.1 within its guide specifications. However, do specifiers and flooring installers understand what constitutes an FF20 or FF25 floor finish? As explanation, FF indicates how well the concrete floor finisher worked the surface—more effort usually results in better overall flatness. The FL metric indicates how skillfully the side forms were set by the contractor and where the concrete was struck-off. The FL number has nothing to do with the concrete floor finisher’s workmanship or skill level in producing the specified FF number—in other words, placing concrete and finishing concrete are different Work Results.

There is one more sticking point: FF and FL are measured within days of concrete placement, and before removal of shoring for suspended slabs, and concrete is a natural material that continues to change as it cures. Most changes occur within the first 30 days, although they continue for four to six months or longer depending on humidity and temperature conditions during the curing period.

Drying shrinkage and curling modifies the surface profile to a much greater extent than the concrete floor finishers can account for within their Work Result. Curling has become more common with the use of high perm rating under slab moisture mitigation membranes where the concrete mix design and reinforcing have not been modified to account for the improved floor flatness requirements for finish materials. The esthetic and functional tolerances listed in this article are not integral to the structural design, and as such will not be accounted for unless specifically identified by the architect or interior designer.

The esthetic tolerance
Floor finishing materials manufacturers establish tolerances for flatness based in part on what makes their products look good. A typical manufacturer’s requirement will state a required tolerance of a 3-mm (1/8-in.) gap measured under a 3-m (10-ft) long straightedge.

The tolerance does not have a direct correlation to the FF and FL tolerances established to meet structural requirements, with the additional concern the FL does not (and should not) apply to the finishes. The materials are not affected by the levelness component of the structural tolerance. Essentially, the straightedge measurement is not a practical tolerance guide for concrete floor construction.

When it comes to thin and flexible floor materials, the concern is more about the actual waviness of the substrate—more peaks means a less esthetic appearance for the applied floor finishes. The flooring manufacturers are describing the smoothness and evenness of the floor, not the flatness or levelness.

The tolerances for flatness and levelness (i.e. FF and FL) are used improperly by specifiers in an attempt to describe an expectation for the concrete floor finisher to which they can relate; specifiers unintentionally create unrealistic tolerances for conventional concrete floor finishes. As an example, based on the manufacturers’ 3-mm straightedge gap measurement criteria, it is possible to achieve an FF20, FF25, FF50, or even an FF150.

The floor flatness number is a statistical measurement that takes into account the relative waviness of the concrete floor. Waviness in concrete is omnidirectional, meaning the peaks and valleys do not align in parallel rows as they would if you were watching waves crashing against a beach. Based on the statistical model supported by the FF approach; the 3-mm gap measurement counts the numbers of peaks and troughs measured along the 3-m straightedge, and it is the number of observed gaps that count toward the floor flatness rating along several repeatable lines of measurement established by ASTM 1155.

Fewer valleys mean a higher FF measurement. Four troughs under the 3-m straightedge is equivalent to FF20 whereas one trough provides FF150. By the same statistical measurement, a floor having two 8-mm (5/16 in.) troughs under the 3-m straightedge would also achieve the same FF20 rating, but probably would not be acceptable as an esthetic tolerance.

The FF result is important, but does not tell the whole story without the understanding of the 3-mm gap limitation. Specifying a higher FF number results in a more esthetically pleasing expectation. The closer to true planar perfection the specifier sets as a project requirement, the higher the cost will be to achieve that level of perfection. If true perfection is specified, the expectation may not be practically achievable using concrete placing methods only.

Application of Overlay on Prepared Floor Surfaces[4]
Application of overlay on prepared floor surfaces. Photo courtesy Mapei

The functional tolerance
Functional tolerances for resilient floor coverings on concrete floor substrates are established by ASTM F710, Preparing Concrete Floors to Receive Resilient Flooring. This standard requires the concrete to be smooth enough to prevent irregularities, roughness, or other defects from telegraphing through resilient flooring products. It also requires a flatness to within an equivalent of a 4-mm (5/32 in.) gap measured along a 3-m (10-ft) straightedge with no gap measurement greater than 0.8 mm (1/32 in.) within 300 mm (12 in.). This requirement fits within the expectations of FF20 to FF25, and the esthetic tolerances expected by many flooring products manufacturers.

The primary difference between ASTM F710 and ASTM E1155 is the functional tolerance is measuring the relative waviness of the floor. Using the omnidirectional short-span measurement addresses the typical concerns of using FF20 or FF25 where the installed gap tolerance is greater than the 3-mm (1/8-in.) required by flooring manufacturers—a floor that is acceptable by the statistical structural tolerance may not meet the esthetic or functional tolerance required by the flooring installers.

Hard-surface flooring materials require a higher degree of flatness, often approaching true planarity to avoid installation problems such as lippage between large tile products. The larger the hard surface material is, the more restrictive the F number becomes. The Terrazzo, Tile, and Marble Association of Canada (TTMAC) has found stone and ceramic tile products with any dimension larger than 400 mm (16 in.), or any tile installation requiring joints 3 mm (1/8 in.) and less, are particularly problematic when installed on conventionally achieved FF20 and FF25 floor substrates, and recommend FF50 to FF60 as a more appropriate tolerance.

In addition to the floor flatness criteria, there is also a structural deflection concern wrongly associated with floor levelness. Suspended floors deflect and all slabs can change shape significantly in the first few days of placement, which is the reason flatness is measured within 72 hours and before shoring is removed from suspended slabs.

The deflection occurring after shoring is removed can be significant enough to produce a sharp drop-off at the structural lines of support and the field area of the floor space. This drop-off can lead to significant lippage concerns with large-bodied tile installations as the tiles are placed across the crest of the deflection. To counteract this concern, specifiers often set requirements for L/600 to L/720 slab deflection restrictions that are not economically feasible to attain from a structural costing point of view. Changes to the reinforcing can help limit the sharpness of the drop-off; however, in reality, the flooring installers will be dealing with L/360 and tiling installation details need to account for the change in floor slopes caused by deflections.

Identification of the problem
The root of the problem is the differences between the three types of tolerances; we use the structural tolerance to define the esthetic tolerance, but are looking for a different measurable functional tolerance result. A smooth and even concrete finish satisfies the requirement for the esthetic concern of not telegraphing concrete imperfections through flexible floorcoverings. This is achievable using the FF20 and FF25 floor substrates.

Too often, defects attributable to the concrete floor finisher through (and appropriate references to) patching and levelling within the Work Results specified in Division 03−Concrete for work done by the concrete floor finishers are not being addressed. Corrections and repairs made by the concrete floor finishers address only the structural tolerances, and not the esthetic or functional tolerances. Defects can also occur as a result of weather conditions—for example, rainwater dilution of the surface paste can lead to powdery residues, exposed aggregates, pitted surfaces, or loosely bonded surfaces.

Additionally, there are consequences of multi-floor construction that leave remnants of previously installed temporary construction such as crane block-outs and screed points. As a result, flooring installers are often made responsible for repairing or correcting these concrete fossils, which is different than the spot patching and smoothing they are required to do as a part of their Work Result covered by Division 09−Finishes.

Finding a solution
In the historical sense to which this author previously alluded, floor preparation on suspended slabs often entailed installation of a topping material. This was called a two-course pour and covered a multitude of floor problems including crane block-outs, overworked surfaces, rain damage, and other types of imperfections leading to flooring installation issues.

CFCA addresses the requirements for limitations of floor flatness established by CSA A23.1, and states tolerances greater than FF35 are only practical through the use of improved mix design and reinforcing, by the use of deferred application of cementitious floor toppings, or incorporating advanced (i.e. non-standard) methods of construction. Installation of cementitious overlays is not the responsibility of the concrete floor finisher. The logical conclusion is the flooring installers have this responsibility since the increased tolerances are a requirement associated with the products they install.

As a practical solution to most of the improved floor flatness problems, toppings are the ideal solution. They are also a good idea for most suspended slabs in multi-storey buildings to maintain a consistent, smooth, and even finish as required by most flooring manufacturers.

The above image shows rain damage to a slab. Photo courtesy DIALOG

Incorporating floor topping into the specification
The different types of toppings for specific installation requirements can be recommended by a number of manufacturers of cementitious and non-cementitious products. The goal for the specifier is to identify a range of products that capture any compatibility issues inherent between toppings and the substrates that they are applied to, and the finishing materials placed over them. Trusting product representatives and using them as intelligent resources is important; this way, the specifier can focus on co-ordinating the construction documentation.

MasterFormat has a placeholder at Section 09 05 61–Common Work Results for Flooring Preparation. The specification starts by indicating the expected floor flatness as either FF20 or FF25. Keeping in mind concrete floors will continue to change shape between the time of placement and the time of finishing, there needs to be a mechanism that allows for changed site conditions, either by cash allowance or unit price adjustment.

Having money put to one side can greatly improve the co-ordination between the structural tolerance and the esthetic or practical tolerances. One could specify a fixed price to bring the floors from the initial structural requirement of FF25 to the functional requirement of FF50, and use the cash allowance or unit prices to adjust if the actual conditions encountered are different than those expected.

Putting the responsibility for improved floor smoothness and evenness to the flooring installers takes a lot of the guesswork out of the equation with regards to the end result required for the project, and places contract risk for this component into the scope of those best able to address and correct the work. Multiple Work Results in Division 09 can make reference to the Common Work Results specification, meaning each flooring installer has control over his or her own work and is in a more responsible position for co-ordinating the ‘gap conditions’ where different works abut one another.

This is a new approach to an old idea. Change is necessary, but in this case a return to the way things used to be done is an improvement to the entirety of concrete flooring community.

Prez_LR[6]Keith Robinson, FCSC, RSW, LEED AP, has worked as a specifications writer since 1981, and is currently an associate at DIALOG in Edmonton. The immediate past-president of Construction Specifications Canada’s (CSC’s) executive council, he sits on several standards review committees for ASTM and the National Fire Protection Association (NFPA). Robinson is also a contributor to the Terrazzo, Tile. and Marble Association of Canada’s TTMAC Specification Guide 09 30 00 Tile Installation Manual and works closely with the Concrete Floor Contractors Association (CFCA) to address specification requirements for floor flatness and levelness. He can be reached at[7].

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