Self Leveling Underlayments – Achieving Level
[This article was originally published in 2008. I have aggregated all of the articles from our Self Leveling project into a new, more complete article with links to each of the individual articles, including the one below. Complete article: How to Pour Self Leveler Over a Large Area]
Our basement radiant heated flooring install required us to pour a self-leveling underlayment (aka self-leveling mortar or self-leveling cement) over our ThermoTile radiant heating wire to achieve a level surface for tiling. I’ll say up front, our results were not perfect, but I don’t think it is the fault of the product. Read further for details.
Our Choice: LevelQuik ES
We chose Custom’s LevelQuik ES for the job. LevelQuik ES is an extended-setting SLU that provides 15 minutes of “free flow” time and allows another 15 minutes of “working” time if you rough the surface of the pour. It can be poured up to 2” thick and feather-edged to less than 1/16”, making it ideal for smoothing uneven surfaces.
If you’ve never seen an SLU in action, it is truly amazing. SLUs go on with the consistency of pancake batter, and harden to bear as much as 4000 lbs. per sq. inch. They are truly a marvel of modern technology. That said, they don’t always come out perfectly level — at least, not if you don’t–or can’t–follow the instructions exactly.
How Can You Ensure a Level Pour?
There is a dearth of information available about how to achieve a perfectly flat, level pour with an SLU, particularly if you are pouring over a fragile surface like one covered with heating wire. I was able to find a little bit digging around on the net and I also met with a local concrete contractor to discuss the job. Here’s what I learned:
1. Always follow the directions on the bag. I cannot stress this enough. SLUs that are improperly mixed or installed can delaminate from the surface, cure to an unlevel/uneven surface, or lead to weak bonds with the flooring installed on top of them. (E.g. LevelQuik RS & ES require the use of a latex primer that is essential to a strong bond).
2. Use a gauge spreader. Achieving a very level floor requires dragging a gauge spreader across the pour. Since we are pouring SLU over an electric heating element, a gauge spreader would present risks of either pulling up the wire or accidentally cutting it — both of which are unacceptable in this application.
[Update on 6/28/08: Instead, we tried a few ideas and eventually landed on this idea to modify a squeegee to spread the mortar evenly. The results in subsequent pours were considerably better than what you'll read below.]
3. Pour at least 3/8″, and preferably 1/2″ or more. Pours that are less than 1/2” tend to not level as well as pours greater than 1/2”. Pouring SLU into a floor area is similar to pouring pancake batter into a frying pan. Before the batter completely fills the pan, it will be unlevel unless you purposefully spread it out. As the batter fills up the pan completely, the entire surface tends to level out.
Our Method & Pictures
Due to the heating wire and cost constraints, we couldn’t meet #s 2-3 above, but we still decided to use LevelQuik ES for the job. We poured the SLU into the framed area and spread it out with a large squeegee. The results were good, but not perfect, with as much as 5/16” difference over a 4 foot area.
The next two pictures show a 3/16″ gap on the far left of the 4 ft. level. This is primarily caused by a crown in the floor near the middle/right of the level.
These two pictures show a 5/16″ gap in the middle of the level, caused by a depression near the middle of the level and a crown on the left side. Click on the pictures to see larger versions.
What do you think? Do you have any experience with self-leveling underlayments? Did you have better results?
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