7 April

Leveling Over Existing Underlayment


Leveling over existing underlayments presents unique challenges for contractors.  The risk increases when dealing with an unidentifiable underlayment, where the base substrate remains covered.  However, following these simple steps can help reduce risk, and properly ensure a safe and proper installation of a new floor over existing patches & self-leveling underlayments.


Step 1: Identify the Underlayment


If possible, it is always best to determine the specific underlayment you are installing over.  This will aid in product selection and determine best prep solutions.  If you can identify the underlayment remember Best practices require installing gypsum underlayments & patch over gypsum substrates, and cementitious underlayments & patch over cementitious substrates.


If the underlayment cannot be identified, consult Dependable Technical Services for suggestions.


Step 2: Check the Bond


It is frequently said that, “you are as good as what you are bonded to.”  Regardless of underlayment type, ensure the underlayment is properly bonded to the base substrate.  Sound the floor and check for hollow spots.  Hollow spots indicate a void between the underlayment and the base substrate; if the underlayment is not bonded to the base substrate, the underlayment must be removed.  You can identify hollow spots by tapping the floor with a broomstick or using a steel ball bearing.


Be aware; in many cases, the hollow sounds that imply a lack of bond may stem from a void in the original substrate.  If sounding detects an apparent void, further examination is required.  Cracks, dust emanating from the cracks upon contact, and/or vibration of the underlayment indicate the underlayment (or patch) is likely unbonded.  In these cases, remove the unbonded underlayment mechanically, and properly prepare the substrate for the new underlayment per manufacturers’ instructions.



Underlayments that suffer from significant map-cracking need close examination.  In many cases, significant map-cracking indicates differential drying (weaker material on the surface), compromised bond, and a lack of volume stability in the existing underlayment; it may continue to shrink over time.  For these reasons, Dependable recommends mechanical removal of underlayments suffering from significant map-cracking unless a tenacious bond between the underlayment and the substrate can be determined.


Ultimately the objective is to identify the cause of the failure and address it, prior to installing the new underlayment.  It is much more economical to solve the problem correctly before installing another underlayment.  In many cases it is appropriate to move forward with installation of the underlayment if the base layer is tenaciously bonded to the substrate and the hollow sounds are emanating from within the slab.  However, “when in doubt – take it out.”


Step 3:  Seal and Prime the Floor


Once all unbonded or weak, damaged areas of the underlayment and the base substrate are addressed, you can proceed with floor preparation.  All floors must be clean and free of bond breakers, sweep and vacuum the floor prior to any further preparation.  Prime the underlayment prior to leveling or repairing.  Most underlayments are porous and require more than one coat of primer to properly seal them.  Follow primer manufacturers’ recommendations for primer and dilution ratios for the substrate in question.


Step 4: Install Your Chosen Underlayment


Follow the installation instructions on our latest technical data sheet, and level your floor!


The long term success of any floor consistently falls on a couple of factors:


  1. Correct assessment of the existing conditions

  2. Appropriate material selection for application and end use

  3. Proper preparation of the existing substrate (concrete, underlayment, patch or other)

  4. Follow manufacturer’s instructions for SLU/patch installation. strives to empower contractors with the most robust materials and systems to ensure a sound, durable base for any finished floor.