Question: Our company has been hired to place a large interior, steel-troweled floor in a custom home that’s scheduled to have a polished finish. We are the concrete flatwork contractor, not the concrete polishing contractor. The floor will be located in an area that has many windows so there will be lots of natural sunlight on the floor. While we do excellent work, we are worried that polishing may show some high or low spots that may distract from the overall appearance of our floor. Our contract is fairly general and only specifies the strength and thickness of the concrete.

What about the flatness of the floor? How flat should this floor be for polishing? We typically strikeoff, bull float, and float the floor using hand tools before trowelling with a walk-behind power trowel. Will this finishing procedure produce the flatness required? If not, what finishing procedure should we use?

Answer: In general, flatter floors will improve the overall look of the polished floor. Bumpy floors or floors with high and low spots can appear wavy and have nonuniform surface appearances. So, your concerns are valid.

High and low spot problems

Polished floors have excellent light reflection. But if they’re not flat, they may appear wavy because of how the light is reflected from the varying angles of the uneven surface. The law of reflection states that when a light ray reflects off of a surface, the “angle of incidence” (incoming light or incident ray) equals the “angle of reflection” (reflected ray) with respect to the surface. What this means: High and low spots essentially act like convex and concave mirrors, reflecting light in various directions.

An example of a polished high spot with exposed coarse aggregates.
Mike Murray/Decorative Concrete Supply Inc. An example of a polished high spot with exposed coarse aggregates.

More importantly, bumpy floors may have nonuniform surface appearance due to more exposure of fine and coarse aggregate particles on the top of high spots compared to the flatter surfaces, or low spots, surrounding the bumps. This is because the tops of high spots are ground more aggressively than low spots. The grinder may even bridge over a low spot or floor depression, leaving it untouched, because the diamonds simply cannot reach the surface. These patches of exposed fine and coarse aggregate particles combined with unground surfaces may not be acceptable, especially if the architect or owner was not expecting the inconsistences.

When using dyes or stains, high and low spots may also cause color variations across the floor. Dyes and stains can pool in low spots and run off high spots, causing low spots to be darker and high spots to be lighter in color than the surrounding surfaces. Again, depending on the expectations of the architect or owner, these dark and light areas may not be acceptable.