Main Floor Concrete

We were very happy with how the basement concrete slab turned out. Tyco Concrete had come through for us on short notice and they had done a really nice job. So one week later we had them come back in to do a second pour, this time for the main floor. We had really debated about how we would like to finish the main floor concrete though despite months of reading and looking.

I should digress for a moment and simply state our reasons behind the concrete floor in the first place:

  1. Thermal mass – thermal mass is a the ability of a material to absorb and store energy (heat in particular). For passive solar heating in the winter months, the sun shining on the concrete will act like a battery, gaining heat during the day, and allowing it to release the heat in the evening. You could use a tile or brick to similar effect. A large brick or stone wall would also work, but you need the sun shining on it. Conversely, in the summer and “shoulder” (April and October) months you really don’t want the sun shining on the thermal mass as this can lead to overheating (thus the importance of passive shading and overhangs).
  2. In-floor heating – we still need a heat system. It is true that our thermal mass is not quite as good as if it had no in-floor heat (a colder mass will heat MORE than a mass that is already pre-heated) – however who wants to walk around on a cold concrete floor in the morning, honestly?
  3. Concrete is sexy.

Okay so now that that is cleared up, we had to decide on how we would like to eventually finish the floors. We had already decided that acid staining and dyeing the concrete was really not our thing – much too fancy-pants for us. That basically left us with two options: power trowel (same as the basement) or grind and polish. Both looks we really like.

Grind and polished concrete – not our place

The grind and polish look is something I really like. You need a concrete grinder machine with diamond discs starting with very rough grits of 80 and 120, which grind the top layer of concrete off exposing the pea gravel aggregate that sinks to the bottom and progressing up to finer and finer grits. Eventually getting up to 800, 1200, 2000 grit discs that give a highly polished look to the floor. You get a lot of interesting variation and different colors of the pea gravel coming through (although some people specify all grey or black pea rock if they want something more consistent). There is a couple downsides with this for us though. Firstly the concrete topper they were going to pour was only going to be 1.5″ thick, which is pretty darn thin. Although you are only taking about 1/8″ or so off the top, we had 1/2″ PEX in-floor piping and metal concrete mesh overtop – grinding too much off could be a horrible thing. We had seen this first hand – a good friend had built an eco-house in town and wanted a ground and polished concrete floor. Unfortunately the contractor ground off about 1/4″ too much. It looked great initially, but the layer of concrete over the in-floor heat was so thin that in the next few weeks the concrete started to crack badly following the pattern of the in-floor lines… It looked so bad. On a thicker floor you’d have nothing to worry about, mind you. But needless to say I was a bit paranoid of that risk. The second consideration is that you need to grind and polish before drywall as it makes a crazy mess. And you can’t grind and polish until it has cured for one month. That would mean that we would have to put the interior on hold for a month which we really did not want to do.

The other option was to simply power trowel the main floor, same as the basement. We have seen this look a lot in some more modern homes and I really like the simplicity of it. It is not complicated at all and is in fact the simplest, cheapest and easiest way to go (pour and trowel is about $2.50/sq.ft completed while the the grinding and polishing cost would be an additional $5-6/sq.ft above and beyond). You pour floor, power trowel the crap out of it and call it a day (in 28 days you can seal it, buff it, wax it, whatever). As I said we liked how the basement floor turned out, particular the very “swirly” areas, as my wife calls them. I hoped that we could make the floors slightly different then the basement floor still though. I looked into the possibility of adding a bit of black pigment to darken the grey slightly – however I abandoned this idea after I was told the pigment dries the concrete faster and can lead to an uneven finish.

Eventually the decision came down to, what is the simplest option? Through this process we have found ourselves periodically down a rabbit hole wondering how we got here and how everything became so complicated. Our answer in those situations, or when we’ve debated about two or three different things is – simple is always better. The more complicated, the more things can go wrong.

So I told the concrete guy, “finish the concrete just like the basement – only, more swirly please.” (He told us that the metal blades of the power troweled as what make it swirled and darker, but troweling longer and on a higher speed for the blades, they can darken the concrete more).

The morning of the pour was crazy again, our builder did not realize they were coming so early with the concrete truck and he’d left a bunch of stuff around the house. I received a text at 6:30am from the concrete guy – “someone has to get over here and move all this shit – truck is here.”

IMG_2982Fortunately we are living very close right now so I threw on some clothes and was out the door. We frantically (concrete starts to cure as soon as it leaves the plant – being 30 minutes from the city, every extra moment counts) moved a trailer, two big garbage bins, scrap wood, plywood and all sorts of junk. Meanwhile the rest of the concrete crew was even more frantically throwing down the concrete mesh (which provides structural support, like rebar, in thinly poured floors like ours). This stuff was crazy heavy and looked so cumbersome to work with, but these guys were pros, they had the whole floor laid and secured in about 20 minutes.

And so the pour began again. I could not stay and watch and truthfully, I did not want to see it. Seeing that grey/brown sludge of mud being rolled in and dumped on the floor simply made me nervous. I just wanted to see it pretty at the end.

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When we got home all was quiet again. We went to the back door and peaked our heads in.

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So swirly!

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So very swirly!

6 thoughts on “Main Floor Concrete

  1. Love the blog!! Question: Why determined the depth you poured on the main floor? I’ve read differing amounts. Just interested in how you made your decision.

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    1. Hi Phil, the depth of the concrete was simply made by the thickness of a 2×4. All of the framing we did using a double bottom plate (two 2x4s) rather than just one in order to accommodate the 1.5″ of concrete topper. This was the depth that our architect designer recommended. She has done a lot of concrete floors and this is the typical depth she uses. That’s really all the thought we spent on it. I did read “Concrete at Home” by Fu-tung Cheng. He used 2″ depth for his pours as he said this caused less cracking. I suppose you have to add a 1/2″ strip of plywood or something like that to get the extra depth.

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