Unfortunately, most concrete basement floor slabs are very uneven. So if you are looking to finish your basement and install flooring, you will likely need to consider methods for leveling your concrete basement floor. One of the most common ways to level uneven concrete basements floors is to apply a high quality self-leveling underlayment.
To level an uneven concrete floor with a self-leveling underlayment, start by thoroughly cleaning the concrete floor and removing all dust and residue. Next, apply a self leveling underlayment latex primer to the areas of your concrete floor that need to be leveled. After allowing the primer to dry, mix up the self-leveling underlayment according to the manufacturer’s instructions. Pour the underlayment on the uneven floor and spread it around as required. The underlayment will seek its own level within minutes. Allow the self-leveling underlayment to cure, leaving a perfectly level concrete slab.
This article will show you how to level an uneven concrete basement floor, step by step.
1. Concrete Floor Surface Preparation
Like most projects, a successful application of underlayment to uneven concrete basement floor begins with surface preparation. Start by using a putty knife to remove any large chunks or concrete or contaminants such as like gum, stickers, etc. from the concrete floor.
While you are scraping off any large surface impurities, check for areas that have extreme “high” or “points”. although low points in the concrete floor will be easily filled in using the concrete underlayment, it is recommended that significant “high points” or lumps in the concrete basement slab be removed.
In order to grind down the significant high points on the concrete floor, it is recommended that you rent a concrete grinder from your local box store. These grinders work to grind down concrete high spots while also removing any coating that might inhibit adhesion of the new underlayment.
After removing the surface contaminants with a putty knife and grinding down any significant high spots with a concrete grinder, give the entire basement concrete subfloor a good vacuum.
Once your uneven basement concrete slab has been properly prepared, you can proceed to the next step in the floor leveling process.
2. Apply Latex Primer for Optimal adhesion
After cleaning the surface of the concrete basement floor, you will need to apply an acrylic/latex primer to facilitate adhesion of the new underlayment layer to the existing concrete floor. Select a suitable primer based on the type of underlayment you will be using. Most self-leveling concrete underlayment manufacturer’s have a specific primer that they recommend.
Apply acrylic/latex primer to the concrete basement floor using a paint roller or a stiff push broom. Ensure that you apply a thin, even coat of the primer, avoiding puddling.
After applying the primer to the application area, allow it dry to a clear film. The primer will usually dry within 1-3 hours, depending on the air and temperature conditions. If your concrete basement floor is highly porous, a second thin coat of primer is recommended.
After allowing the primer to dry, you can proceed to the next step.
3. Mix up the Self-Leveling Concrete Underlayment
Once the acrylic/latex primer has dried to clear film, you can begin mixing up the self-leveling concrete underlayment according to the manufacturer’s instructions.
To mix up the underlayment, add clean, cool water to a bucket (the amount of water will depend on underlayment manufacturer’s instructions) and slowly introduce the concrete underlayment powder to the bucket while mixing with a drill mixer.
Continue to mix the underlayment for approximately two minutes (or as long as the manufacturer recommends) until a smooth, lump free consistency is achieved. Do not overmix. Overmixing the underlayment can introduce air – shortening the pot-life of the mixture and potentially resulting in “pinholing” during the application and curing process.
Once the underlayment has been mixed in accordance with the manufacturer’s instructions, you can apply the self-leveling underlayment to the concrete basement floor slab.
4. Apply Self-Leveling Underlayment
At this stage, begin applying the self-leveling concrete underlayment by pouring it out from the bucket and unto the uneven concrete basement floor. After pouring an appropriate amount of underlayment onto the concrete floor, begin to spread out the mixture by using a trowel, squeegee, or long-handled gauge spreader.
Spread out an appropriate amount of underlayment based on the size of your application area. Most underlayments can be applied from a feathered edge all the way up to a 1.5″ thickness.
After spreading around the underlayment, the product will seek its own level within 20 minutes. After you are satisfied with your product application, clean off your tools immediately and allow the self-leveling underlayment to cure.
5. Allow Self-Leveling Underlayment to Cure
After applying the self-leveling underlayment to the concrete basement floor, allow the product to cure in accordance with the manufacturer’s recommendations. Typically, the underlayment will cure to a walkable hardness within 2-4 hours after application.
If a second coat of underlayment is required, it is recommended that the second coat be applied as soon as the first coat has cured to a walkable hardness.
If more than 12 hours passes before you apply a second coat, it is recommended that you apply another layer of acrylic/latex primer before applying a second coat of underlayment.
Most floor coverings (laminate, LVP, etc.) can be applied over the self-leveling underlayment after 16 hours of cure time.
By following the steps outlined in this article, you will be left with a level concrete basement subfloor that is ready for nearly any flooring installation.
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