Installing Flooring Over Gypcrete: A Contractor's Guide to Gypsum Underlayment

The problems with gypsum underlayment don't show up during installation. They show up at the final walkthrough when door transitions don't meet ADA compliance — or three months after occupancy when vinyl planks start popping up because moisture testing was skipped.

What is gypsum underlayment?

Gypsum underlayment goes by different names in the field. Most contractors call it gypcrete — Maxxon's brand name — though you'll also hear gypsum concrete or gyp-crete. Whatever you call it, it's a poured, self-leveling floor system that goes over structural concrete or wood subfloors.

The material is gypsum cement mixed with aggregates and additives. Pumped onto the deck, it flows across the surface to level, and sets into a solid substrate. Thickness typically ranges from ¾" to 2" depending on structural conditions and system requirements.

Why multifamily construction uses it

The installation sequence

Why timing matters for flooring contractors

Gypsum underlayment installs after framing, mechanical rough-in, and electrical — but before most interior finishes. That timing creates coordination requirements that directly affect what we do when we arrive with flooring weeks later.

The structural deck is cleaned, primed, and fitted with perimeter isolation strips before the pour. Isolation strips prevent the gypsum from bonding to walls and columns so the floor system moves independently from the structure.

The door frame sequencing problem is the single most common issue we encounter on gypcrete projects. Frames installed before the pour sit low. Frames installed after cure sit high. Both conditions create ADA transition conflicts — and both become our problem to solve, despite having no control over how they are placed.

ADA compliance & threshold transitions

The ADA specifies that floor surface height variance cannot exceed ½ inch without a compliant transition. Patio door assemblies and aluminum thresholds routinely exceed this limit when door frame installation isn't coordinated with the underlayment pour.

Available solutions

How to prep gypsum concrete for flooring installation

Installing flooring over gypsum underlayment requires different substrate preparation than working over concrete. The material characteristics affect our installation methods and material selection.

Understanding gypsum underlayment properties

Gypsum is softer and more friable than concrete. Cured gypsum provides adequate compressive strength for flooring applications (2,000–4,000 psi depending on the product), but it doesn't have concrete's surface hardness. Aggressive mechanical preparation methods used on concrete (grinding, scarifying, shot-blasting) can damage gypsum substrates.

The surface is often dusty and chalky. Gypsum underlayment installation occurs relatively early in the construction sequence, which means the surface accumulates drywall dust, paint overspray, mud from foot traffic, and general construction debris before we arrive.

4-step substrate preparation

1. 01
   Thorough cleaning

   Sweep, vacuum, and damp-mop. By the time we arrive, the surface carries drywall dust, paint overspray, and mud from foot traffic. No adhesive performs on a contaminated substrate.
2. 02
   Primer application

   Drop water on the gypsum — if it absorbs quickly, there's no top coat and we need to prime. For vinyl plank and LVT over gypcrete, we typically use Armstrong S-185 or Mapei Primer L diluted 4:1. These primers seal the surface, control porosity, and improve bond strength.
3. 03
   Patching and leveling

   Use only gypsum-based patching compound — Portland cement products react with gypsum and break down over time. Address divots, joint shadows, and pump-line marks before adhesive goes down.
4. 04
   Moisture testing

   Critical and often skipped. Use a meter calibrated for gypsum, not concrete. Each adhesive manufacturer specifies acceptable moisture levels — verify before mobilizing labor.

Moisture testing & curing requirements

Gypsum underlayment contains significant moisture immediately after installation. The curing process involves evaporation and chemical reactions that gradually reduce moisture content to acceptable levels for flooring installation.

Manufacturers specify minimum curing times before flooring installation, typically ranging from several days to several weeks. Maxxon typically recommends 28 days for full cure, though light foot traffic is possible within 24 hours. Time-based estimates don't account for actual job site conditions — temperature, humidity, and ventilation affect curing rates significantly.

Compatible flooring types over gypcrete

Common gypsum underlayment problems and solutions

What this means for flooring contractors

For flooring contractors, the prevalence of gypsum underlayment in multifamily projects means developing expertise with these systems represents a competitive advantage and a necessity, rather than a specialized capability.

After 40 years installing flooring over gypsum underlayment in multifamily construction, I can tell you this: get the substrate right, or nothing else matters.

Frequently asked questions