diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 0ddb4baa..88a0e188 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -3416,6 +3416,25 @@ def _map_elmhurst_rir_surface(
     if prefix is None:
         return None
     kind = _RIR_KIND_FROM_NAME_PREFIX[prefix]
+    # RdSAP 10 §3.9.1 (PDF p.21) Simplified assessment: the roof-going
+    # surfaces (slope / flat ceiling / stud wall) are NOT measured — the
+    # Summary lodges placeholder Length/Height cells (e.g. a 40 m ceiling
+    # height, a 32 m slope on a 4.65 m-wide gable). The spec instead
+    # derives one timber-framed "remaining area" from the floor area:
+    #   A_RR       = 12.5 × √(A_RR_floor / 1.5)            §3.9.1(d)
+    #   A_RR_final = A_RR − ΣA_RR_gable/other              §3.9.1(e)
+    # The cascade computes A_RR_final itself (heat_transmission.py — the
+    # `12.5 × √(A_RR_floor / 1.5) − rr_walls_in_a_rr_area` residual),
+    # but ONLY when `detailed_surfaces` carries no roof-going kind
+    # (`has_roof_lodgement` gate). Emitting these placeholder rows flips
+    # that gate and bills their raw L×H as explicit roof area (a 7.5×
+    # heat-loss explosion). Drop them for Simplified so the cascade's
+    # residual formula fires — matching how the API path already handles
+    # the same Simplified RR (scalar gable fields, no roof-going
+    # detailed_surfaces; cert 6035) and the gables-only cert 000565.
+    # Detailed (§3.10) assessments DO measure these surfaces — keep them.
+    if is_simplified and kind in ("slope", "flat_ceiling", "stud_wall"):
+        return None
     u_value_override: Optional[float] = None
     if kind == "gable_wall" and surface.gable_type == "Sheltered":
         kind = "gable_wall_external"
diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index 40ba7aff..fbccd757 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -14,10 +14,18 @@ area fraction); SAP 10.3 specification (13-01-2026) Tables 4a/4e/12.
 
 from __future__ import annotations
 
-from typing import Final
+from typing import Final, Optional
 
 import pytest
 
+from datatypes.epc.domain.mapper import (
+    _map_elmhurst_room_in_roof,  # pyright: ignore[reportPrivateUsage]
+)
+from datatypes.epc.surveys.elmhurst_site_notes import (
+    RoomInRoof as ElmhurstRoomInRoof,
+    RoomInRoofSurface as ElmhurstRoomInRoofSurface,
+)
+
 from datatypes.epc.domain.epc_property_data import (
     EpcPropertyData,
     MainHeatingDetail,
@@ -2120,6 +2128,70 @@ def test_elmhurst_main_fuel_to_sap10_maps_bio_liquid_water_heating_labels() -> N
     assert _ELMHURST_MAIN_FUEL_TO_SAP10["Bio-liquid HVO from used cooking oil"] == 71
 
 
+def _placeholder_rir_surfaces() -> "list[ElmhurstRoomInRoofSurface]":
+    # A §3.9.1 Simplified Room-in-Roof lodges the roof-going Length/Height
+    # cells as placeholders (a 40 m flat-ceiling height, a 32 m slope on a
+    # 4.65 m gable) — Elmhurst ignores them and derives the area from the
+    # floor area. Gables ARE measured (4.65 × 2.45 = 11.39).
+    def surf(
+        name: str, length: float, height: float,
+        gable_type: Optional[str] = None, default_u: Optional[float] = None,
+    ) -> "ElmhurstRoomInRoofSurface":
+        return ElmhurstRoomInRoofSurface(
+            name=name, length_m=length, height_m=height, insulation="",
+            insulation_type=None, gable_type=gable_type,
+            default_u_value=default_u, u_value_known=False, u_value=0.0,
+        )
+
+    return [
+        surf("Flat Ceiling 1", 4.00, 40.00),   # placeholder
+        surf("Slope 1", 32.00, 32.00),          # placeholder
+        surf("Gable Wall 1", 4.65, 2.45, gable_type="Exposed", default_u=0.29),
+        surf("Gable Wall 2", 4.65, 2.45, gable_type="Party", default_u=0.25),
+    ]
+
+
+def test_elmhurst_simplified_rir_drops_placeholder_roof_surfaces() -> None:
+    # Arrange — RdSAP 10 §3.9.1 (PDF p.21): a Simplified RR's slope /
+    # flat ceiling / stud wall are not measured; emitting their
+    # placeholder L×H as `detailed_surfaces` makes the cascade bill them
+    # as explicit roof area (7.5× heat-loss explosion) instead of firing
+    # the spec's `A_RR = 12.5√(A_floor/1.5) − Σwalls` residual formula.
+    rir = ElmhurstRoomInRoof(
+        floor_area_m2=29.75, construction_age_band="A",
+        assessment="Simplified Type 1", surfaces=_placeholder_rir_surfaces(),
+    )
+
+    # Act
+    mapped = _map_elmhurst_room_in_roof(rir)
+
+    # Assert — roof-going surfaces dropped, both gables retained.
+    assert mapped is not None
+    kinds = sorted(s.kind for s in (mapped.detailed_surfaces or []))
+    assert "slope" not in kinds
+    assert "flat_ceiling" not in kinds
+    assert kinds == ["gable_wall", "gable_wall_external"]
+
+
+def test_elmhurst_detailed_rir_keeps_roof_surfaces() -> None:
+    # Arrange — a Detailed (§3.10) assessment DOES measure slope / flat
+    # ceiling, so they must be retained (regression guard so the
+    # Simplified drop doesn't bleed into Detailed lodgements).
+    rir = ElmhurstRoomInRoof(
+        floor_area_m2=29.75, construction_age_band="A",
+        assessment="Detailed", surfaces=_placeholder_rir_surfaces(),
+    )
+
+    # Act
+    mapped = _map_elmhurst_room_in_roof(rir)
+
+    # Assert — slope + flat ceiling retained under the Detailed path.
+    assert mapped is not None
+    kinds = sorted(s.kind for s in (mapped.detailed_surfaces or []))
+    assert "slope" in kinds
+    assert "flat_ceiling" in kinds
+
+
 def test_elmhurst_gas_boiler_main_fuel_derives_carrier_from_water_heating() -> None:
     # Arrange — SAP 10.2 Table 4b (PDF p.168) rows 101-119 are "Gas
     # boilers (including mains gas, LPG and biogas)". The code identifies