Gypsum toppings or underlayments, often referred to “Gypcrete” continue to present problems to the resilient flooring industry. Gypcrete is actually a kind of gypsum underlayment, made by Maxxon Industries. Maxxon Industries makes five different gypsum underlayments for resilient flooring, they also have a line of gypsum self leveling underlayment line which also has five different products. Hacker Industries is also another large manufacturer of gypsum underlayments, they have nine different products in their line.

What good are gypsum underlayments? From a floor covering installer, they are not good. They present more problems than anything good. From an end user, they are less costly, good for sound control, and also enhance the fire resistance of multifamily buildings, which helps those structures meet building code requirements. Gypsum underlayments can be placed at various compressive strengths, depending on the requirements of each job. These strengths range from 1,200 to 5,500 psi. Resilient flooring manufacturers’ sates that floor fills and toppings must have a minimum compressive strength of 3,000-3500 psi, depending on the manufacturer.

So, what are the issues with these gypsum topping products? Generally, it is that the gypsum topping installer did not due the required procedure.
1. These products are to be slump tested, and never are. A slump test is when the gypsum material is placed into a 2″ (inside diameter) x 4″ high cylinder and then the cylinder is removed. The gypsum material should then produce a patty size of 8 inches plus or minus 1 inch. If the patty is larger, they generally have added more water than need, which then lowers the compressive strength.
2. For resilient flooring, the gypsum topping must be sealed with a Top Coat/Over Spray which reduces the porosity of the gypsum underlayment. This step is generally left out and the flooring contractor is responsible.

The gypsum contractor is never responsible for these items, so the flooring contractor has to assume they are up to spec.

When the floor covering installer gets there, these gypsum toppings generally need checked and touched up. Check for the Top Coat/Over Spray by dropping some water on the gypsum topping. If the water gets absorbed rather quickly, there is no Top Coat/Over Spray. If the Top Coat/Over Spray has not been applied (generally it is not), Armstrong S-185 Primer or Mapei Primer L diluted 4 parts water to 1 part Primer L. If the first coat is quickly absorbed, follow by another coat of Armstrong S-185 Primer or Mapei Primer L diluted (3-1 dilution this time).

Patching presents another issue. Portland based cement patching products do not react well with gypsum and will actually break down over time. You will need to do any skim coating or flash patching with a gypsum based patching compound. Mapei has a product in their line called Planitex UNS which is designed for repairing gypsum based toppings. Mapei also makes two gypsum self levelers;

Mapei Planitex SL is a high-performance gypsum-based self-leveler. It is a low-VOC floor-smoothing compound for patching, skimming, smoothing and leveling in dry interior residential or commercial areas. Planitex SL is suitable under normal loading in interiors from featheredge to 1-1/4″.

Mapei Planitex SL 35 is a gypsum-based, self-leveling underlayment with an exceptionally smooth, hard surface for use in dry interior residential and commercial areas. Planitex SL 35 is designed for application at depths of 1/4″ to 3″.

What type of floor prep is required over the gypsum topping? I have broken this down by the flooring product category;

*Vinyl Flooring (Full-Spread or Loose Lay with seam). You will need to ensure that the gypsum toppings are sealed or other wise you will have adhesive and loose lay seam tape issues during installation. The very porous gypsum surface will dry out the adhesive so quickly and the loose lay seam tape will release due to the dusty surface.

Vinyl Flooring (Loose Lay w/ no Seams), no sealer is really needed. Although, sealing the gypsum will reduce dust being kicked up.

Floating Luxury Vinyl Flooring (Konecto/Armstrong LUXE/Starloc). I would ensure that the gypsum toppings are sealed, or the dust that gets kicked up from the gypsum topping will stick to the adhesive strips of these products and reduce the bond strength needed.

Laminate/Floating Cork/Floating Hardwood Flooring. Again, no sealer/overspray is really needed. Although, sealing the gypsum will reduce dust being kicked up.

*Ceramic Tile. When going over gypsum with ceramic tile adhesives, the gypsum must be sealed. As for installing ceramic tile over gypsum, start with sealing the floor with Primer L (may require two coats) and followed either by Mapelastic HPG or Aqua-Defense as water proofing and crack isolation membrane and set tile by standard thin-set or medium bed method. Recommended for dry or limited water exposure areas interiors.

*Hardwood Glue Down. You will need to ensure that the gypsum topping is sealed. After the gypsum is sealed, depending on the hardwood product, you can use either a resin based adhesive or a urethane hardwood adhesive.

*Cork Glue Down. You will need to ensure that the gypsum topping is sealed. After the gypsum is sealed, you can use the recommended adhesive.

*Glue Down Carpet. You will need to ensure that the gypsum toppings are sealed or other wise you will have adhesive issues during installation. Again, it will dry out the adhesive quickly resulting in bonding issues.

Floating Carpet Tile (Interface Stroll). Again, no sealer/overspray is really needed. Although, sealing the gypsum will reduce dust being kicked up and affecting the bond of the adhesive tabs.

When gluing down over gypsum toppings, I strongly recommend doing a 24 hour bond test to check for bond.

Other notes of interest on Gypsum Toppings
Substrates:
Concrete, suspended only. Not made for on or below grade concrete.
Suspended Wood should be agency approved 23/32″ Tongue & Groove subfloor sheathing.

Drying:
Before, during, and after installation of a Gypsum Underlayment, building interior shall be enclosed and maintained at a temperature above 50°F until structure and subfloor temperatures are stabilized. Provide continuous ventilation and adequate heat to remove moisture rapidly from the area until the underlayment is dry. Provide mechanical ventilation if necessary. Under the above conditions, a 3/4″ thick underlayment is usually dry in 5-8 days. Low temperatures, certain sands, poor ventilation and high humidity can lengthen the drying time.

Check for Dryness:
The recommended procedure to test for dryness is to use a moisture meter with a gypsum scale.
Gypsum Toppings recommended moisture meters:
• Delmhorst BD-2100
• Delmhorst G-79
• Tramex CRH