Sunday, November 12, 2017

Is your concrete flat?

How to get a flat concrete garage floor? I posted the information below on a forum when I was researching how to specify a flat garage floor and what standard of work could be expected:

What does flat mean? First of all, flat does NOT mean level.

Flat: smooth and even; without marked lumps or indentations. No concave or convex areas. A vertical wall, sloped surface or level floor can be flat (a flat surface can be sloped).

Level: a perfectly horizontal plane with respect to the distance above the center of the earth. A bubble level will show this to be level in all directions

On the high end of concrete floors, you can have "super flat" concrete installed, they do this particularly for warehouses that have high lifts and robots that run in the isles. However, this requires laser guided machines than smooth and flatten the concrete. I don't think many of us are going to do this for our relatively small backyard garages or shops.

There are some standards for flatness and levelness and these are measured in FF (flatness) and FL (levelness) numbers. And these are not easy things to measure. The sell special gizmos to do the measuring. Essentially, measuring with a long straight edge isn't really acceptable, because each person who measures will measure differently. The other obvious problem with using a straight edge is that what you really want to do is hold that edge just above the plane of the floor and then measure below it, you don't want to just lay it on the floor. Companies make cool gizmos to actually measure FF and FL, and they are pretty sophisticated (see

I called a bunch of concrete guys. Essentially, none were willing to commit to a standard like "less than an 1/8" dip below 6' straight edge anywhere on the floor". After my floor was poured and I wanted to make it flatter, I also spoke to a couple grinding and polishing companies that sounded interested in "flattening my floor and they said "oh we can definitely make it flat", but as soon as I mentioned my 6 foot straight edge they lost interest. No one had a plan or a method to make the floor flat. No one said anything about flooding the slab and marking high spots, or anything like that. No one owned an electronic dipstick to measure flatness. I suspect this stuff just doesn't happen in standard residential construction.

I did find one company that seemed pretty competent at flattening concrete floors and their info mentioned they do "slab correction". This guy does lots of work including warehouses that use high lifts that really need flat floors. Essentially, he said my floor was "normal" for work these days (sadly). He could make it better and wasn't wildly expensive, he'd charge just somewhat more than a regular floor polishing for the extra time and effort. But he questioned making an effort to get it really within 1/8".

I found this chart online:
"Although there are no direct equivalents between F-numbers and straightedge tolerances, ACI 302, "Construction of Concrete Floors and Slabs," gives the following table of approximate values:

F-number Gap under an unleveled 10-foot straight edge
(fraction of an inch)
FF12 1/2
FF20 5/16
FF25 1/4
FF32 3/16
FF50 1/8

Apparently typical concrete floors are in the FF20 to FF25 range. I did find this statement: "Although "1/8th inch in ten feet" has been used to specify billions of square feet of concrete, it was seldom, if ever, achieved. The typical industrial floor, for example, is closer to a 5/8th inch deep envelope, rather than a 1/8th inch deep envelope."

A big issue is that typical concrete guys never check their work. How many visit the site 30 days after the pour and measure the surface with a proper dipstick style gauge according to the standard? So essentially, your residential concrete guys do not really know how good or bad their work comes out!

I'm a geek, so when my concrete cured enough to walk on, I went out and tried a 6' straight edge all around the floor in a grid pattern. I really tried about 40 placements. Only one area was out of whack. Mine may not be that great, but I also suspect very few people have garages that would really meet the FF50 (1/8") standard.

On contracts. I've been around the track a couple times and I understand these things. It sounds great to have everything specified in a contract and to have it be enforceable. In my case, I didn't know enough to ask the details about flatness, let alone specify something. But at least around here, it wouldn't be easy to even find a residential contractor with a dipstick gauge who would commit to an FF number. The other problem is that the residential finisher is usually a subcontractor who makes a modest wage. Even the head contractor , unless he is a big outfit, isn't in a financial position to "replace your foundation" if it doesn't work out. Certainly, the finisher, who is maybe making $250 to $1000 is going to disappear before buying you a new slab. I personally think you are just far better off trying to understand what you can get, checking references, and looking at similar work than expecting the contract to make it "all right". In my case, yes, I wish the floor was better in one spot, but I can live with it and I'm not going to make others pay for something I didn't understand well enough to demand better.

So, in conclusion:
I'm no expert on concrete, but having done this once, and read some others advice; here are my recommendations:
- You aren't going to get a "perfectly flat" floor, especially with high PSI concrete poured on a warm dry day.
- It seems harder to make a floor with a curb (stem wall) around the edge flat (especially with just one pour) than a slab without a curb, as the finisher cannot use the top of the form as a guide when there is a curb. If you are going to have a curb/stem wall, perhaps look into doing the floor as a separate pour.
- Talk with the actual finisher, not only your general contractor, about what he can do. Make sure he knows this is going to be your "pride and joy", not just another lousy garage. Make sure he has a wide bull float and will run it both ways, North-South and East-West. Is there enough room to run the float both ways or are other buildings in the way? Go see similar work he has done and if it matters bring a dipstick or at least a straight edge. Does he own a dipstick? Has he ever returned to a job after it has cured and tested the floor's flatness?
- During the job, have him show you it is flat, while it is wet. Maybe they can go out with skis and show you a straight edge on the surface.
- Don't let any of the finishers hang out on their skis on the floor. If they stay in one spot too long they will make a depression. (My low spot just happens to be were a finisher hung out playing with his phone while he waited for the concrete to get a bit dryer.)
- Lastly, If you can get it close to a 1/8" gap under a 10 foot straight edge, you are doing amazingly well. 1/4" is probably more reasonable.
- Be present during the pour and finishing, ask questions, and make them do it right.

PS: On sloping the garage floor vs making it level: 
Most people slope the garage floor so liquids will drain toward the car door opening. This can be particularly important in places where it snows or rains significantly, and it maybe part of the building code.
Often, concrete finishers who slope the concrete significantly care much less about flatness. The more the floor slopes, the less likely water will puddle! Typically, builders who slope floors do at least 1/8 inch per foot (1 inch in 8 feet). If the floor is sloped sloped significantly, many people will want to level things like workbenches, shelves, and floor mounted tools with leveling feet. And if you must have leveling feet for the sloped floor anyway, flatness may not matter as much to you.
But be careful with floor slope; some builders will slope the floor as much as 1/4 inch per foot or more. That would be 1 inch of slope in 4 feet. It's also 4 inches in in 16 feet. Many would consider that too steep for many garage uses.

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