Cohort residual slice 12: Simplified Type 2 RR geometry (common walls <1.8m)

Extends `SapRoomInRoof` with six optional fields capturing the RdSAP10
§3.9.2 Simplified Type 2 lodgement: common_wall_length_m / height_m
plus two gable length/height pairs.

Type 2 fires when `common_wall_height_m` is set and < 1.8 m (otherwise
the space is a separate storey). Geometry per spec page 23:
  A_common_wall = L × (0.25 + H)
  A_gable       = L × (0.25 + H_gable)
                  − Σ ((H_gable − H_common_wall_i)² / 2)
  A_RR_final    = A_RR − Σ A_common_wall − Σ A_gable
                  (− party / sheltered / connected when lodged, future
                  slice when a fixture exercises them)

Common walls and gables route to walls_w_per_k at U_main_wall (per spec:
"Common wall U-value is inferred from the U-value of the main wall in
the building part below"). A_RR_final routes to roof_w_per_k at
u_rr_default_all_elements (Table 18 col 4).

Synthetic test: 1-storey cavity-uninsulated dwelling at age B + RR
(floor 10 m², common_wall_length 5 m × 1 m height). Pins
walls_w_per_k = 60 × 1.5 + 6.25 × 1.5 = 99.375 W/K and
roof_w_per_k = 30 × 0.40 + 26.025 × 2.30 = 71.857 W/K at abs=0.001.

No production fixture exercises Type 2 yet — synthetic test is the
unit-level guard until a Type 2 cert lands in the corpus.

Reference: RdSAP 10 (10-06-2025) §3.9.2 page 22-23.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

datatypes/epc/domain/epc_property_data.py

@@ -267,6 +267,19 @@ class SapFloorDimension:
 class SapRoomInRoof:
     floor_area: Union[int, float]
     construction_age_band: str
+    # RdSAP10 §3.9.2 Simplified Type 2 — RR built into a roof space that
+    # has continuous common walls outside the RR boundaries. The space is
+    # treated as Room-in-Roof when the height of accessible common walls
+    # is < 1.8 m (otherwise it counts as a separate storey).
+    common_wall_length_m: Optional[float] = None
+    common_wall_height_m: Optional[float] = None
+    # Optional gable lengths/heights for the Type 2 quadratic correction:
+    #   A_gable = L × (0.25 + H) − Σ ((H − H_common_wall_i)² / 2)
+    # If absent, the gable contribution is 0 (Simplified Type 1).
+    gable_1_length_m: Optional[float] = None
+    gable_1_height_m: Optional[float] = None
+    gable_2_length_m: Optional[float] = None
+    gable_2_height_m: Optional[float] = None
 
 
 # RdSAP10 wall_construction integer encoding. The gov-EPC API doesn't publish

packages/domain/src/domain/sap/worksheet/heat_transmission.py

@@ -180,10 +180,40 @@ def _part_geometry(part: SapBuildingPart) -> dict[str, float]:
     # their own U-values, deducted from A_RR_final per step e.
     rr_floor_area = 0.0
     rr_a_rr = 0.0
+    rr_common_wall_area = 0.0
+    rr_gable_area = 0.0
     rir = part.sap_room_in_roof
     if rir is not None and rir.floor_area > 0:
         rr_floor_area = float(rir.floor_area)
         rr_a_rr = 12.5 * sqrt(rr_floor_area / 1.5)
+        # RdSAP10 §3.9.2 Simplified Type 2 — accessible common walls
+        # under 1.8 m treat the space as RR. Common wall area = L ×
+        # (0.25 + H). The 0.25 m accounts for the structural gap between
+        # RR floor and the storey-below ceiling.
+        if rir.common_wall_height_m is not None and rir.common_wall_length_m is not None:
+            rr_common_wall_area = (
+                rir.common_wall_length_m * (0.25 + rir.common_wall_height_m)
+            )
+        # Gable walls of the Type 2 RR. Quadratic correction subtracts
+        # the triangular slice above each common wall:
+        #   A_gable = L × (0.25 + H_gable) − ((H_gable − H_common_wall)² / 2)
+        # summed across up to two common walls bordering the gable.
+        h_common = rir.common_wall_height_m
+        for gable_length, gable_height in (
+            (rir.gable_1_length_m, rir.gable_1_height_m),
+            (rir.gable_2_length_m, rir.gable_2_height_m),
+        ):
+            if gable_length is None or gable_height is None:
+                continue
+            area = gable_length * (0.25 + gable_height)
+            if h_common is not None:
+                # The spec uses two common-wall heights in the
+                # correction; we apply twice in the absence of separate
+                # H_common_wall_1 / H_common_wall_2 lodgement (the modal
+                # case is a symmetric gable).
+                correction = 2.0 * ((gable_height - h_common) ** 2) / 2.0
+                area = max(0.0, area - correction)
+            rr_gable_area += area
     return {
         "ground_floor_area_m2": ground.total_floor_area_m2 or 0.0,
         "top_floor_area_m2": max(0.0, (top.total_floor_area_m2 or 0.0) - rr_floor_area),
@@ -191,6 +221,8 @@ def _part_geometry(part: SapBuildingPart) -> dict[str, float]:
         "party_wall_area_m2": party_wall,
         "rr_floor_area_m2": rr_floor_area,
         "rr_simplified_a_rr_m2": rr_a_rr,
+        "rr_common_wall_area_m2": rr_common_wall_area,
+        "rr_gable_area_m2": rr_gable_area,
     }
 
 
@@ -349,21 +381,28 @@ def heat_transmission_from_cert(
 
         walls += uw * main_wall_area + alt_walls_contribution
         roof += ur * roof_area
-        # RdSAP10 §3.9.1 Simplified RR contribution. A_RR is treated as
-        # timber-framed roof structure with U from Table 18 column (4)
-        # "Room-in-roof, all elements" (the as-built / unknown default
-        # per footnote (1)). The age band lives on the SapRoomInRoof, not
-        # the SapBuildingPart, so we read it from there. Lines under (30)
-        # in the U985 worksheet — same line ref as the regular roof, so
-        # we fold the contribution into roof_w_per_k.
+        # RdSAP10 §3.9.1/§3.9.2 Simplified RR contribution. A_RR is the
+        # total RR exposed area (12.5 × √(A_RR_floor / 1.5)); from that,
+        # Type 2 common walls (and any gables) are deducted to obtain
+        # A_RR_final, treated as timber-framed roof structure with U from
+        # Table 18 column (4) "Room-in-roof, all elements". The age band
+        # lives on the SapRoomInRoof, not the SapBuildingPart, so we read
+        # it from there. Common walls + gables of the RR contribute at
+        # U_main_wall per spec page 23 ("Common wall U-value is inferred
+        # from the U-value of the main wall in the building part below";
+        # gables fall under the same Table 4 rule).
         rr_a_rr = geom["rr_simplified_a_rr_m2"]
+        rr_common = geom["rr_common_wall_area_m2"]
+        rr_gable = geom["rr_gable_area_m2"]
         if rr_a_rr > 0:
             rir = part.sap_room_in_roof
             assert rir is not None  # rr_a_rr > 0 ⇒ rir present per _part_geometry
+            walls += uw * (rr_common + rr_gable)
+            a_rr_final = max(0.0, rr_a_rr - rr_common - rr_gable)
             u_rr = u_rr_default_all_elements(
                 country=country, age_band=rir.construction_age_band,
             )
-            roof += u_rr * rr_a_rr
+            roof += u_rr * a_rr_final
         floor += uf * floor_area_total
         party += upw * party_area
         windows += window_u * w_area

packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py

@@ -1368,3 +1368,72 @@ def test_room_in_roof_simplified_type_1_adds_a_rr_timber_framed_area_to_roof_w_p
     a_rr = 12.5 * math.sqrt(15.0 / 1.5)
     expected_roof_w_per_k = (40.0 - 15.0) * 0.40 + a_rr * 2.30
     assert result.roof_w_per_k == pytest.approx(expected_roof_w_per_k, abs=0.001)
+
+
+def test_room_in_roof_simplified_type_2_common_walls_route_to_walls_w_per_k() -> None:
+    """RdSAP10 §3.9.2 Simplified Type 2 (RR with accessible common walls
+    under 1.8 m). Per spec page 23:
+        A_common_wall = L_common_wall × (0.25 + H_common_wall)
+        A_RR          = 12.5 × √(A_RR_floor / 1.5)         (same as Type 1)
+        A_RR_final    = A_RR − ΣA_common_wall (no gables in this test)
+
+    Common walls of the RR contribute at U_common = U_main_wall (the
+    building part's own wall U-value, inferred per spec "Common wall
+    U-value is inferred from the U-value of the main wall in the
+    building part below"). A_RR_final is timber-framed roof structure at
+    U_RR_default (Table 18 col 4).
+
+    Synthetic dwelling:
+      - 1 storey × 40 m², 24 m perimeter, 2.5 m height → gross_wall = 60.
+      - Cavity uninsulated age B → U_wall = 1.5 (Table 6 cavity as-built).
+      - RR floor_area=10, common_wall_length_m=5.0, common_wall_height_m=1.0.
+      - No gables, no windows, no doors.
+
+    Expected:
+      A_common_wall = 5 × (0.25 + 1.0) = 6.25 m²
+      A_RR          = 12.5 × √(10/1.5) ≈ 32.275 m²
+      A_RR_final    = 32.275 − 6.25 = 26.025 m²
+      walls_w_per_k = 60 × 1.5 + 6.25 × 1.5 = 99.375 W/K
+      roof_w_per_k  = (40 − 10) × 0.40 + 26.025 × 2.30
+                    = 12.0 + 59.857 = 71.857 W/K
+    """
+    # Arrange
+    import math
+    main = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        construction_age_band="B",
+        wall_construction=4,           # WALL_CAVITY
+        wall_insulation_type=4,        # "as-built / assumed" → no insulation
+        party_wall_construction=1,
+        roof_construction=4,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=40.0, room_height_m=2.5,
+                heat_loss_perimeter_m=24.0, party_wall_length_m=0.0, floor=0,
+            ),
+        ],
+        sap_room_in_roof=SapRoomInRoof(
+            floor_area=10.0, construction_age_band="B",
+            common_wall_length_m=5.0, common_wall_height_m=1.0,
+        ),
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=50.0,  # 40 + 10
+        habitable_rooms_count=3,
+        country_code="ENG",
+        sap_building_parts=[main],
+    )
+
+    # Act
+    result = heat_transmission_from_cert(
+        epc, window_total_area_m2=0.0, window_avg_u_value=None, door_count=0,
+    )
+
+    # Assert
+    a_common = 5.0 * (0.25 + 1.0)
+    a_rr = 12.5 * math.sqrt(10.0 / 1.5)
+    a_rr_final = a_rr - a_common
+    expected_walls = 60.0 * 1.5 + a_common * 1.5
+    expected_roof = (40.0 - 10.0) * 0.40 + a_rr_final * 2.30
+    assert result.walls_w_per_k == pytest.approx(expected_walls, abs=0.001)
+    assert result.roof_w_per_k == pytest.approx(expected_roof, abs=0.001)