diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index 5729fd9a..f90593ec 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -344,6 +344,32 @@ def _joined_descriptions(elements: list[Any]) -> Optional[str]:
     return " | ".join(parts)
 
 
+def _joined_main_roof_descriptions(roofs: list[Any]) -> Optional[str]:
+    """Join roof descriptions for the MAIN (non-RR) roof U-value, dropping
+    "Roof room(s)" entries.
+
+    A room-in-roof carries its own §3.9/§3.10 shell area + U-value cascade
+    (Table 17 / Table 18 col 4), so a "Roof room(s), no insulation
+    (assumed)" lodgement must NOT leak into the main pitched/flat roof's
+    `u_roof`. Without this filter a multi-roof cert like "Pitched,
+    insulated (assumed) | Roof room(s), no insulation (assumed)" applies
+    the RR's "no insulation" 2.30 to the WHOLE main roof, ~3x over-stating
+    its heat loss (the 4700-family regular-roof-U leak).
+
+    Falls back to the unfiltered join when every roof entry is a Room-in-
+    Roof (pure-RR dwelling) so that case keeps its prior behaviour."""
+    if not roofs:
+        return None
+    parts = [
+        d
+        for e in roofs
+        if (d := getattr(e, "description", "")) and "roof room" not in d.lower()
+    ]
+    if not parts:
+        return _joined_descriptions(roofs)
+    return " | ".join(parts)
+
+
 def _part_geometry(part: SapBuildingPart) -> dict[str, float]:
     if not part.sap_floor_dimensions:
         # A part with no floor dimensions has no derivable RR shell or
@@ -559,7 +585,7 @@ def heat_transmission_from_cert(
         return HeatTransmission(0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0)
 
     country = Country.from_code(epc.country_code)
-    roof_description = _joined_descriptions(epc.roofs)
+    roof_description = _joined_main_roof_descriptions(epc.roofs)
     wall_description = _joined_descriptions(epc.walls)
     floor_description = _joined_descriptions(epc.floors)
 
diff --git a/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py b/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
index bb55b94a..71dd4197 100644
--- a/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
+++ b/tests/domain/sap10_calculator/worksheet/test_heat_transmission.py
@@ -36,12 +36,51 @@ from domain.sap10_calculator.worksheet.heat_transmission import (
     heat_transmission_from_cert,
 )
 from domain.sap10_calculator.worksheet.heat_transmission import (
+    _joined_main_roof_descriptions,  # pyright: ignore[reportPrivateUsage]
     _part_geometry,  # pyright: ignore[reportPrivateUsage]
     _round_half_up,  # pyright: ignore[reportPrivateUsage]
     _window_bp_index,  # pyright: ignore[reportPrivateUsage]
 )
 
 
+class _Desc:
+    """Minimal stand-in for a roof element carrying a `description`."""
+
+    def __init__(self, description: str) -> None:
+        self.description = description
+
+
+def test_joined_main_roof_descriptions_drops_room_in_roof_entries() -> None:
+    # Arrange — a multi-roof cert: main pitched roof (insulated) plus a
+    # Room-in-Roof lodged uninsulated. The RR has its own shell U cascade,
+    # so the main-roof U-value description must NOT inherit the RR's
+    # "no insulation" marker (which would force the whole main roof to
+    # U=2.30). Cert 8536-0624-4600-0934-1292.
+    roofs = [
+        _Desc("Pitched, insulated (assumed)"),
+        _Desc("Roof room(s), no insulation (assumed)"),
+    ]
+
+    # Act
+    result = _joined_main_roof_descriptions(roofs)
+
+    # Assert — only the non-RR primary roof remains.
+    assert result == "Pitched, insulated (assumed)"
+
+
+def test_joined_main_roof_descriptions_keeps_pure_rr_fallback() -> None:
+    # Arrange — a pure room-in-roof dwelling (every roof entry is an RR):
+    # filtering would leave nothing, so preserve prior behaviour by
+    # falling back to the unfiltered join.
+    roofs = [_Desc("Roof room(s), no insulation (assumed)")]
+
+    # Act
+    result = _joined_main_roof_descriptions(roofs)
+
+    # Assert
+    assert result == "Roof room(s), no insulation (assumed)"
+
+
 def test_part_geometry_floorless_part_honours_full_key_contract() -> None:
     # Arrange — a building part lodged with NO sap_floor_dimensions (e.g.
     # a party-wall-only or RR-only extension; observed on 5 certs in a