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How to use self-leveling underlayment to fix your uneven floors.

Estimated Reading Time: 4 Minutes

Before you put down your new floors, underlayment is a great way to ensure your subfloor will be level. Level floors help with flooring installation as well as avoiding damage in the future.

Step 1 : Clean up the floor surface

To prep your floor for the underlayment you want to get as much residue up as possible.  If your flooring is concrete, an easy way to do this is with a razor scraper. For this job we opted to mostly use a Floor Scraper since it has some flexibility and it was perfect for this job.  Scrapers that have a flexible blade make it easy to use and will absorb the shock and save your energy.

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Step 2 : Seal up cracks and holes in the floor and joints.

Using spray foam around the wall lines is a great way to ensure that all cracks are filled, and your underlayment won’t go where it isn’t intended.  This will take some time to apply and trim off excess, but it is well worth the effort. The last thing you want is to find you’ve wasted product because it’s running into your basement or under the wall.  If you are pouring onto cracked concrete, we recommend using caulk in those cracks. There is no need to overfill them.  You  can consider using a self-leveling caulk. Then you will need to wait about 24 hours for it to dry before moving onto step 3.  If caulk is not necessary, then once you’ve trimmed the excess foam you can move onto step three.

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Step 3: Determine your high and low spots.

We used a laser level system to determine our high spot. Then we used Tapcon screws and created level reference points on the rest of the floor. You can rent a laser level at most rental stores.  The screws can also help you to figure out how much underlayment you will need. The manufacturer instructions should say how many square feet each bag will cover. If you need to make up a half inch in one corner of your room vs an eighth of inch that will greatly change how much underlayment you need.   If a laser level is not available, you have a few options. You can use a very long level or essentially make your own. To make your own you will need to get a very straight 2”x4” board and set a smaller level on the board’s edge.  Or, if you already know where your high spot is, then begin pouring in the lowest area and continue with the rest of the floor area until the leveler fills in and reaches to your high spot.

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Step 4: Apply primer to your floor

Primer gives the self-leveling underlayment something to stick to. Follow the manufacturer’s instructions for the dilution needed. We recommend using an extension pole to save yourself time and back fatigue. Be sure to apply in the direction of an exit and do the entire room at once.

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Step 5: Mix the underlayment

It’s best to have 3 buckets ready to mix up, you may want more depending on the size of your room and how many people you have helping. The mixture should be slightly thicker than water so it can run to the low spots, again follow the exact manufacturer's instructions.  You will not need to trowel the product, but we do suggest having a squeegee to pull any excess, so you keep to the top of your level screws.  We also have Gunite Shoes so you won’t feel like you can’t get into those corners. Start applying your underlayment at the lowest point and work towards an exit.  As you continue mixing buckets be sure to maintain your product consistency.  Make sure that you always clean your bucket in between batches because if there is any buildup in the bucket, it will absorb the moisture of the new batch and the product will not spread properly.

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The underlayment needs 12 hours to dry but after that you can install your new flooring. There is no need to pull your screws back out, but if any happen to be sticking up, you should be able to just unscrew them back out.  Then you can be confident you have a level surface to work on that will look great for years.


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