diff --git a/domain/epc/property_overrides/roof_sloping_ceiling_guard.py b/domain/epc/property_overrides/roof_sloping_ceiling_guard.py
index e9de671cf..6496603e6 100644
--- a/domain/epc/property_overrides/roof_sloping_ceiling_guard.py
+++ b/domain/epc/property_overrides/roof_sloping_ceiling_guard.py
@@ -11,19 +11,65 @@ from domain.epc.property_overrides.roof_type import RoofType
 # family, never the loft ladder (ADR-0066).
 _SLOPING_CEILING_TOKEN = "pitchedwithslopingceiling"
 
+# The measured-depth ladder (Table 17 col (1a) tabulated thicknesses), ascending.
+# A lodged depth snaps to the nearest tabulated depth ≤ N — mirroring the
+# calculator's `_ROOF_BY_THICKNESS` picker — so an off-ladder value never leaks
+# back to the loft LLM route. `400+` / any depth > 400 is the top member.
+_DEPTH_LADDER: list[tuple[int, RoofType]] = [
+    (12, RoofType.PITCHED_SLOPING_CEILING_12MM),
+    (25, RoofType.PITCHED_SLOPING_CEILING_25MM),
+    (50, RoofType.PITCHED_SLOPING_CEILING_50MM),
+    (75, RoofType.PITCHED_SLOPING_CEILING_75MM),
+    (100, RoofType.PITCHED_SLOPING_CEILING_100MM),
+    (125, RoofType.PITCHED_SLOPING_CEILING_125MM),
+    (150, RoofType.PITCHED_SLOPING_CEILING_150MM),
+    (175, RoofType.PITCHED_SLOPING_CEILING_175MM),
+    (200, RoofType.PITCHED_SLOPING_CEILING_200MM),
+    (225, RoofType.PITCHED_SLOPING_CEILING_225MM),
+    (250, RoofType.PITCHED_SLOPING_CEILING_250MM),
+    (270, RoofType.PITCHED_SLOPING_CEILING_270MM),
+    (300, RoofType.PITCHED_SLOPING_CEILING_300MM),
+    (350, RoofType.PITCHED_SLOPING_CEILING_350MM),
+    (400, RoofType.PITCHED_SLOPING_CEILING_400MM),
+]
+_TOP_TABULATED_MM = 400
+
 
 def _normalise_roof_token(description: str) -> str:
     token = description.split(":", 1)[0]
     return re.sub(r"[^a-z0-9]", "", token.lower())
 
 
+def _member_for_depth(insulation: str) -> Optional[RoofType]:
+    """The ladder member for a lodged insulation suffix, or None when it carries
+    no measured depth (`As Built` / `Unknown` / empty) so the caller folds it to
+    `as built`. An explicit `+` or any depth above 400 mm → the 400+ member; an
+    off-ladder depth snaps to the nearest tabulated depth ≤ N; a depth below the
+    ladder (< 12 mm) has no member and also folds to `as built`."""
+    digits = re.search(r"\d+", insulation)
+    if digits is None:
+        return None
+    depth_mm = int(digits.group())
+    if "+" in insulation or depth_mm > _TOP_TABULATED_MM:
+        return RoofType.PITCHED_SLOPING_CEILING_400_PLUS
+    member: Optional[RoofType] = None
+    for tabulated_mm, ladder_member in _DEPTH_LADDER:
+        if depth_mm >= tabulated_mm:
+            member = ladder_member
+    return member
+
+
 def roof_sloping_ceiling_guard(description: str) -> Optional[RoofType]:
     """Deterministically resolve a `PitchedWithSlopingCeiling` roof description to
     its `Pitched, sloping ceiling, …` RoofType member (ADR-0066).
 
-    Returns None for anything that is not a sloping-ceiling marker, so the
+    A measured depth resolves to its ladder member (nearest tabulated depth ≤ N;
+    `400+` / above-400 → the 400+ member); `As Built` / `Unknown` / a sub-ladder
+    depth / no suffix fold to `as built`, which defers the U-value to the age
+    band. Returns None for anything that is not a sloping-ceiling marker, so the
     party-ceiling guard and the LLM classifier still handle the rest.
     """
     if _normalise_roof_token(description) != _SLOPING_CEILING_TOKEN:
         return None
-    return RoofType.PITCHED_SLOPING_CEILING_AS_BUILT
+    _, _, insulation = description.partition(":")
+    return _member_for_depth(insulation) or RoofType.PITCHED_SLOPING_CEILING_AS_BUILT