Apply a brush or roller waterproofing membrane evenly across the entire gypcrete surface before installing large format tile.

Start by testing the gypcrete moisture content with a calcium chloride test or a relative humidity probe to confirm it is within the membrane manufacturer recommended limits. Clean the surface by vacuuming and wiping with a damp cloth to remove dust and loose particles since even fine gypsum dust will prevent good adhesion. Prime the gypcrete if the waterproofing system requires it, using a thin, even coat and allowing the specified dry time so the primer forms a tacky but not wet surface. Mix the brush or roller membrane thoroughly and work in manageable sections so you can maintain a wet edge for consistent coverage and avoid lap marks. Apply at the manufacturer specified mil thickness or coverage rate, typically two coats applied perpendicular to each other, and always allow the full cure time between coats and before setting mortar. For seams, corners and penetrations, embed a reinforcing fabric into the first wet coat, then roll smooth and follow with the second coat to ensure continuous, overlapping coverage that prevents any direct mortar to gypcrete contact.

Common issues include insufficient surface preparation which leads to poor adhesion, and the fix is to re-clean and prime the area and replace any membrane that did not bond properly. A second issue is applying the membrane too thin or unevenly which lets moisture wick into the gypcrete and weakens the mortar bond; remedy this by confirming coverage with a wet film gauge or by following the coverage rate and applying a second perpendicular coat. A third concern is neglecting corners, joints or penetrations where failures start; prevent that by reinforcing those areas with fabric while the first coat is wet and ensuring overlaps of at least two inches beyond the trouble spots so the membrane is continuous under the tile.

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