Thinset contact coverage — why combed ridges must be collapsed (80% / 95% rule)

Combing the mortar isn’t the goal — contact under the tile is. Ridges left standing leave voids (<80%); sliding the tile to collapse them plus back-buttering gives the ≥95% contact ANSI requires for wet and large-format tile.

Two cross-sections of a tile set in thinset. On the left, the combed mortar ridges are left standing, so the tile rests only on the ridge tips and leaves big voids underneath — well under 80 percent contact, which causes hollow spots, cracks, and lippage. On the right, the ridges have been collapsed by sliding the tile across them and the tile is back-buttered, giving a nearly full mortar bed with no voids. ANSI A108.5 requires at least 80 percent mortar contact for interior dry installations and at least 95 percent for exterior, wet, and large-format tile. Comb straight lines, set the tile, then slide it across the ridges to flatten them, and back-butter large tile. — Source: ANSI A108.5 §2.2.3 (mortar contact); NTCA trowel-and-error method

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Two cross-sections of a tile set in thinset. On the left, the combed mortar ridges are left standing, so the tile rests only on the ridge tips and leaves big voids underneath — well under 80 percent contact, which causes hollow spots, cracks, and lippage. On the right, the ridges have been collapsed by sliding the tile across them and the tile is back-buttered, giving a nearly full mortar bed with no voids. ANSI A108.5 requires at least 80 percent mortar contact for interior dry installations and at least 95 percent for exterior, wet, and large-format tile. Comb straight lines, set the tile, then slide it across the ridges to flatten them, and back-butter large tile.

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    <img src="https://remodelcalculators.com/api/figure-embed/thinset-coverage-rule" alt="Two cross-sections of a tile set in thinset. On the left, the combed mortar ridges are left standing, so the tile rests only on the ridge tips and leaves big voids underneath — well under 80 percent contact, which causes hollow spots, cracks, and lippage. On the right, the ridges have been collapsed by sliding the tile across them and the tile is back-buttered, giving a nearly full mortar bed with no voids. ANSI A108.5 requires at least 80 percent mortar contact for interior dry installations and at least 95 percent for exterior, wet, and large-format tile. Comb straight lines, set the tile, then slide it across the ridges to flatten them, and back-butter large tile." width="800" height="440" style="width:100%;height:auto;border:0" loading="lazy">
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    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/thinset-coverage-rule" target="_blank" rel="noopener">Thinset contact coverage — why combed ridges must be collapsed (80% / 95% rule)</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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