Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
---
title: Cementitious Self-Leveling Underlayment
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## Self-leveling underlayment is a flowable, polymer-modified cementitious mortar that seeks its own level under gravity, correcting local depressions, slab curl, trowel ridges, and out-of-flat conditions that would telegraph through thin or large-format finish flooring. It is a preparation layer, not a wear surface or a finish in its own right. {note}
−## The work shall include primer, the underlayment, moisture testing of the substrate, substrate preparation to the required surface profile, and any topical moisture mitigation, primer, and edge or joint treatment necessary to deliver a sound, bonded, flat surface to the finish-flooring installer. {note}
+## The work shall include primer, the underlayment, moisture testing of the substrate, substrate preparation to the required surface profile, and any topical moisture mitigation, primer, and edge or joint treatment necessary to deliver a sound, bonded, flat surface to the finish-flooring installer.
## SLU is the appropriate remedy when measured substrate flatness is coarser than the finish flooring can tolerate. Resilient and large-format finishes commonly require FF 35 or better, which existing slabs and renovation substrates rarely meet without correction. {note}
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## The primer shall be the type specified by the underlayment manufacturer for the measured substrate condition.
−## A latex bonding primer shall be used over sound, dry concrete within moisture limits.
−
−## An epoxy primer shall be used where the slab is porous, dusty, or moisture-compromised within the system's published tolerance.
−
−## The underlayment shall be placed within the primer's open window; if the primer dries beyond the manufacturer's reuse limit, it shall be re-primed.
−
−## Primer dry time and open window vary by primer type and ambient conditions; a primer placed too wet or left open too long both defeat the bond. {note}
−
```datasheet
label: Primer type
…6 unchanged lines
```
+## A latex bonding primer shall be used over sound, dry concrete within moisture limits.
+
+## An epoxy primer shall be used where the slab is porous, dusty, or moisture-compromised within the system's published tolerance.
+
+## The underlayment shall be placed within the primer's open window; if the primer dries beyond the manufacturer's reuse limit, it shall be re-primed.
+
```datasheet
label: Maximum elapsed time between priming and underlayment placement
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```
+## Primer dry time and open window vary by primer type and ambient conditions; a primer placed too wet or left open too long both defeat the bond. {note}
+
# Material Properties {toc}
−## The compressive strength of the underlayment shall be matched to the load and the finish system above it. A product acceptable under carpet is undersized under large-format tile or a warehouse epoxy topping. {note}
+## The compressive strength of the underlayment shall be matched to the load and the finish system above it. A product acceptable under carpet is undersized under large-format tile or a warehouse epoxy topping.
## Compressive strength shall be selected for the service load and finish system and shall be verified per ASTM C109 at 28 days.
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## The underlayment shall be placed within the specified thickness range; both a minimum and a maximum single-pour thickness shall be stated.
−## Where required depth exceeds the product's single-pour maximum, the manufacturer's aggregate-extension method or successive bonded pours shall be used.
−
−## The feather-edge minimum and the deep-fill maximum are product-specific; the default specification range is 1/8 in. to 1 in., with feather-edge fills to 1/16 in. permitted for local repairs. {note}
−
−## The finished underlayment surface shall meet the specified flatness tolerance, verified with a 10 ft straightedge or per ASTM E1155.
−
−## Sensitive finishes such as LVT and large-format tile require FF 35 or better; FF 25 (3/16 in. in 10 ft) is the minimum acceptable result of the leveling work. {note}
−
```datasheet
label: Minimum single-pour thickness
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```
+## Where required depth exceeds the product's single-pour maximum, the manufacturer's aggregate-extension method or successive bonded pours shall be used.
+
+## The feather-edge minimum and the deep-fill maximum are product-specific; the default specification range is 1/8 in. to 1 in., with feather-edge fills to 1/16 in. permitted for local repairs. {note}
+
+## The finished underlayment surface shall meet the specified flatness tolerance, verified with a 10 ft straightedge or per ASTM E1155.
+
```datasheet
label: Required finished flatness tolerance
…6 unchanged lines
```
+## Sensitive finishes such as LVT and large-format tile require FF 35 or better; FF 25 (3/16 in. in 10 ft) is the minimum acceptable result of the leveling work. {note}
+
# Radiant Heating Encapsulation {toc}
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## The underlayment shall provide a minimum cover of 3/4 in. (19 mm) over the top of any hydronic tube or electric heating element.
−## The radiant heating system shall be off for 48 hr before placement and shall not be energized for 72 hr after placement.
−
−## The heating-element layout is shown on the drawings; coordinate cover depth against the as-installed element elevation. [[drawing: radiant element layout]]
−
−## Because the radiant-heat spec usually lives in a separate division, the off/on schedule has no enforcement unless this underlayment work cross-references it; the pre-installation conference is where that coordination is fixed. {note}
−
```datasheet
label: Minimum underlayment cover over radiant element
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```
+## The radiant heating system shall be off for 48 hr before placement and shall not be energized for 72 hr after placement.
+
+## The heating-element layout is shown on the drawings; coordinate cover depth against the as-installed element elevation. [[drawing: radiant element layout]]
+
+## Because the radiant-heat spec usually lives in a separate division, the off/on schedule has no enforcement unless this underlayment work cross-references it; the pre-installation conference is where that coordination is fixed. {note}
+
+## Whether radiant heating is present beneath the underlayment, and its type, shall be specified so the cover depth and placement schedule are cross-referenced to the radiant heating specification.
+
```datasheet
label: Radiant heating present
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default: 1
```