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Poured-Gypsum Underlayments

Poured-gypsum underlayments can provide a satisfactory surface to receive ceramic tile installation systems that are poured at a minimum of 1/2 in. (12 mm) over a structurally sound concrete or precast subfloor, a structurally sound wood subfloor at a minimum of 3/4 in. (19 mm), or a sound mat shall exceed 1 in. (25 mm) in depth and must support design loads with a maximum L/360 deflection. These floors are available in compressive strengths of 1,000 to over 8,000 PSI. Poured-gypsum concrete underlayment shall have a minimum compressive strength of 2,000 PSI for use over wooden substrates, 3,000 PSI for use over a concrete substrates and a minimum density of 105lbs. per cubic foot when tested in accordance with ASTM C472. Poured-gypsum underlayments are suitable for interior substrates only, above grade, and in areas not subjected to water exposure or immersion. There are currently four approved methods the TCNA Handbook uses for poured-gypsum underlayments in tile installations. F200 – Poured-Gypsum over Concrete F180 – Poured-Gypsum Underlayment over Plywood Drying of the gypsum underlayment Poured-gypsum floors are made with a job site mixture of powder and water and require time to dry before they can be primed/ sealed and tiled. Verification that the gypsum underlayment is dry can be determined in accordance with ASTM D4263: Plastic Sheet Method. This test process shall be performed by the gypsum installer, prior to the application of any primers/sealers. Do not proceed with the tile installation until the poured-gypsum is deemed dry and has been primed/sealed. As a general guideline, the following drying times should be observed prior to testing the surface for dryness. Thickness of poured-gypsum Dry time before testing 1/4 in. (6mm) 48 hours 1/2 in. (12mm) 72 hours 3/4 in. (19mm) 5 days 1 in. (25mm) 7 days 2 in. (51mm) 2 weeks Preparation of the poured-gypsum surface In general, ceramic tile is not bonded directly to gypsum underlayments. While each manufacturer of these materials has their own specific requirements, the use of a primer/sealer or a primer and membrane is required. Any exceptions to this recommendation are proprietary in nature and suitability rests solely with the gypsum manufacturer. PRIMER/SEALER Some gypsum manufacturers recommend the use of a primer/sealer over the surface of the dry gypsum before installing any membrane or setting material directly. Also referred to by some as a “sealer” or “overspray,” the use of these primers is intended to prevent the gypsum from absorbing water from the setting material, which can result in poor adhesion. Please note that while you may not have installed the poured-gypsum, you must verify that the primer/sealer was applied in accordance with the manufacturer’s recommendations, after the gypsum floor was deemed dry. MEMBRANES The Tile Council of North America and some gypsum manufacturers recommend the use of membranes in addition to the primer prior to installing the tile over their poured underlayments. For the purposes of these applications, membrane is as defined by ANSI A118.12 for crack-isolation or ANSI A118.10. Check with the manufacturer for their individual requirements. NOTE: The tile contractor shall obtain written documentation verifying that the poured-gypsum floors have met or exceeded the minimum compressive strength of 2,000 psi and minimum density requirements of 115 lbs. per cu. ft. per ASTM C472, have been tested per ASTM D4263 and deemed to be dry, and have been primed/sealed in accordance with the manufacturer’s recommendation. This information shall be provided to the tile contractor by the general contractor, owner, builder or certified poured-gypsum installer. RH111 – Poured-Gypsum over Concrete with Hydronic Heat RH122 – Poured-Gypsum over Wood with Hydronic Heat

NOTE: The requirements of this document exclude patching compounds.

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NTCA Reference Manual | 2024 / 2025

Chapter 3 | Backerboards and Underlayments