Slice S0380.113: H=0 gable lodgement deducts per RdSAP 10 §3.9.2 step (b)

RdSAP 10 §3.9.2 step (b) (PDF p.23) verbatim:

  "Software calculates the area of each gable or adjacent wall by
  using the equation:
         A_RR_gable = L_gable × (0.25 + H_gable) − [(H_gable − H_common_1)² / 2
                                                   + (H_gable − H_common_2)² / 2]"

Step (d):
  A_RR_final = A_RR_wall − (Σ A_common + Σ A_gable + Σ A_party
                            + Σ A_sheltered + Σ A_connected)

The spec equation is signed and applies for all L > 0 — including
H_gable = 0. When the gable is shorter than the common walls the
correction term `(H_gable − H_common)² / 2` exceeds the
L × (0.25 + H_gable) term, producing a negative A_RR_gable.
Elmhurst's worksheet evaluates the equation literally; the negative
value adjusts A_RR_final upward via step (d) without billing a
physical wall area.

Cert 000565 §8.1 lodges Ext3's RR (Simplified Type 2) with an
absent Gable Wall 2:

  Gable Wall 1   L=9.00  H=7.00   Exposed     U=0.45
  Gable Wall 2   L=4.00  H=0.00               U=0.00   ← lodged but H=0
  Common Wall 1  L=5.00  H=1.50               U=0.45
  Common Wall 2  L=7.50  H=0.30               U=0.45

Spec equation for Gable Wall 2:
  A_gable_2 = 4 × (0.25 + 0) − (0 − 1.5)²/2 − (0 − 0.30)²/2
            = 1.0 − 1.125 − 0.045 = −0.17 m²

Worksheet (30) Ext3 residual = 17.35 m² back-solves exactly:
  A_RR_shell = 12.5 × √(32.0 / 1.5)                = 57.7350
  Σ walls (incl. -0.17 absent gable)               = 40.3850
  residual = shell − walls                         = 17.3500  ✓ 4 d.p.

Pre-slice the mapper had two clamps that together dropped the
spec-computed −0.17 m² adjustment:

  mapper.py:3350  `if length_m <= 0 or height_m <= 0: return None`
                  → filtered out any H=0 surface
  mapper.py:3443  `area_m2 = max(0.0, length_m * (0.25 + H) − correction)`
                  → clamped negative gable areas at 0

Combined the cascade computed residual = 17.18 m² (cascade UNDER
by 0.17). Plus a related secondary `if height_m > h` filter on the
correction sum that masked the all-common-walls-taller case.

3-layer fix:

1. `datatypes/epc/domain/mapper.py` `_map_elmhurst_rir_surface`:
   - Split the early-return filter: drop only when L<=0 (no wall),
     OR when H<=0 AND not (Simplified Type 2 with common walls).
   - Apply the spec gable-area formula to BOTH `gable_wall` (party
     default) and `gable_wall_external` kinds in Simplified Type 2
     (the U-value routing differs by kind, but the area equation
     is the same).
   - Remove `max(0.0, ...)` clamp so the signed result reaches the
     cascade.
   - Remove `if height_m > h` correction-sum filter (spec applies
     the full square unconditionally).

2. `domain/sap10_calculator/worksheet/heat_transmission.py` per-
   surface loop:
   - `gable_wall` branch: skip `party += 0.25 × area` when area < 0
     (wall doesn't exist physically) but still add the signed area
     to `rr_walls_in_a_rr_area` so the residual deduction in step (d)
     grows by |area|.
   - `gable_wall_external` branch: same skip pattern for `walls +=
     u × area` and `rr_detailed_area += area`.

Cohort safety: only cert 000565 Ext3 hits this in the corpus. All
other cohort certs are Type 1 RR (no common walls, formula gives
the same answer) or have all gables H > 0. The cascade's per-element
test pins (Ext1's Connected gable + Exposed gable, Ext4's Detailed
RR) unchanged.

Cert 000565 cascade snapshot (HEAD a461b70d → this):
  roof_w_per_k         51.3185 → 51.3768  ✓ EXACT (Δ -0.06 → -0.003)
  total_external_area 857.46  → 857.6323  ✓ EXACT (Δ -0.18 → -0.008)
  thermal_bridging    128.62  → 128.6448  ✓ EXACT (Δ -0.03 → -0.005)
  total_w_per_k       936.97  → 937.0563  ✓ EXACT (Δ -0.09 → -0.004)

  sap_score (int)         29 ✓ EXACT (preserved)
  sap_score_continuous 28.5027 → 28.5007 (Δ -0.0060 → -0.0080)
  ecf                   5.3877 →  5.3876
  total_fuel_cost_gbp  4681.01 → 4680.97
  co2_kg_per_yr        6448.59 → 6448.53
  space_heating_kwh   59019.21 → 59018.52
  main_heating_fuel   34715.31 → 34716.78

**Cert 000565 fabric cascade now essentially exact** (HTC −0.004 W/K
total residual across all 8 fabric components). The remaining
continuous SAP -0.0080 / cost +£0.71 / SH +10 kWh residuals come
from non-fabric upstream (likely ventilation or appliances) —
candidates for a future audit.

Pyright net-zero (57 → 57 errors across touched files).

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

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -2496,22 +2496,85 @@ def test_summary_000565_rooflights_deduct_from_their_own_bp_gross_roof_per_rdsap
         f"{rooflights_by_area[0.50].window_location!r} (expected '4th Extension')"
     )
 
-    # Assert — roof_w_per_k closes to ws (51.38 from §3 per-element rows;
+    # Assert — Ext2 +0.36 W/K roof over-count closes (cascade no longer
-    # Ext3 -0.06 W/K residual from absent Gable Wall 2 is closed in
+    # leaves Ext2's gross roof un-deducted). Combined with S0380.113
-    # S0380.113 — until then, this slice closes the Ext2 +0.36 W/K
+    # (H=0 gable retention) the cascade closes to ws within 1e-2.
-    # over-count, leaving cascade UNDER by 0.06).
+    assert abs(ht.roof_w_per_k - 51.3795) <= 1e-2, (
-    assert abs(ht.roof_w_per_k - 51.32) <= 1e-2, (
         f"cascade roof_w_per_k={ht.roof_w_per_k:.4f}; "
-        f"expected ~51.32 (= ws 51.38 − 0.06 Ext3 residual deferred to "
+        f"ws 51.3795; Δ={ht.roof_w_per_k - 51.3795:+.4f}"
-        f"S0380.113); Δ={ht.roof_w_per_k - 51.32:+.4f}"
     )
-    # Assert — total external area closes the Ext2 +1.20 m² double-count
+    # Assert — Ext2 +1.20 m² rooflight double-count closes.
-    # (remaining residual is Ext3 -0.17 m² from absent Gable Wall 2,
+    assert abs(ht.total_external_element_area_m2 - 857.64) <= 1e-2, (
-    # closed by S0380.113).
-    assert abs(ht.total_external_element_area_m2 - 857.46) <= 1e-2, (
         f"cascade total_external_element_area_m2={ht.total_external_element_area_m2:.4f}; "
-        f"expected ~857.46 (= ws 857.64 − 0.17 Ext3 residual deferred to "
+        f"ws 857.64; Δ={ht.total_external_element_area_m2 - 857.64:+.4f}"
-        f"S0380.113); Δ={ht.total_external_element_area_m2 - 857.46:+.4f}"
+    )
+
+
+def test_summary_000565_ext3_absent_gable_h_zero_lodgement_deducts_per_rdsap_10_section_3_9_2_step_b() -> None:
+    # Arrange — RdSAP 10 §3.9.2 step (b) (PDF p.23) verbatim:
+    #
+    #              ┌                          ┌ (H_gable − H_common_1)²   (H_gable − H_common_2)² ┐ ┐
+    #   A_RR_gable=│ L_gable × (0.25 + H_gable) − │ ─────────────────────── + ─────────────────────── │ │
+    #              └                          └           2                          2            ┘ ┘
+    #
+    # Step (d): A_RR_final = A_RR_shell − Σ(common + gable + party +
+    # sheltered + connected).
+    #
+    # Cert 000565 §8.1 lodges Ext3's Room in Roof as Simplified Type 2:
+    #
+    #   Gable Wall 1   L=9.00  H=7.00   Exposed     U=0.45
+    #   Gable Wall 2   L=4.00  H=0.00               U=0.00   ← lodged but H=0
+    #   Common Wall 1  L=5.00  H=1.50               U=0.45
+    #   Common Wall 2  L=7.50  H=0.30               U=0.45
+    #
+    # Elmhurst's worksheet (30) shows Ext3 remaining area = 17.35 m².
+    # Back-solving via the spec equation with the H=0 Gable Wall 2:
+    #
+    #   A_gable_2 = 4 × (0.25 + 0) − (0 − 1.5)²/2 − (0 − 0.30)²/2
+    #             = 1.0 − 1.125 − 0.045 = −0.17 m²   (negative)
+    #
+    #   A_RR_shell      = 12.5 × √(32.0 / 1.5)                = 57.7350
+    #   Σ walls (incl. -0.17 absent gable)                    = 40.3850
+    #   residual = shell − walls                              = 17.3500 ✓
+    #
+    # Pre-slice the mapper filtered out lodged surfaces with
+    # `height_m <= 0` (mapper.py:3350) and clamped the gable area at 0
+    # via `max(0.0, ...)` (mapper.py:3443). Both clamps prevented the
+    # spec-computed −0.17 m² adjustment from reaching the cascade —
+    # cascade residual landed at 17.18 m² (= 57.735 − 40.555), -0.17
+    # m² under the worksheet.
+    #
+    # Spec-correct path: lodged Type 2 gable walls with H=0 still
+    # contribute via §3.9.2 step (b). The result can go negative when
+    # the common walls are taller than the gable; that signed value
+    # adjusts the residual deduction (step d) without billing a
+    # physical wall area (the wall doesn't exist).
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+    from domain.sap10_calculator.rdsap.cert_to_inputs import heat_transmission_section_from_cert
+
+    # Act
+    ht = heat_transmission_section_from_cert(epc)
+
+    # Assert — total external area closes to worksheet (31) at 1e-2.
+    # Pre-slice it was 857.46 (cascade UNDER by 0.18 after S0380.112);
+    # this slice picks up the +0.17 m² Ext3 residual adjustment, taking
+    # the cascade to ~857.63, within ws 857.64.
+    assert abs(ht.total_external_element_area_m2 - 857.64) <= 1e-2, (
+        f"cascade total_external_element_area_m2={ht.total_external_element_area_m2:.4f}; "
+        f"ws (31)=857.64; Δ={ht.total_external_element_area_m2 - 857.64:+.4f} "
+        f"(expected within 1e-2 after lodged H=0 gable contributes −0.17 "
+        f"m² via the §3.9.2 step (b) spec equation)"
+    )
+    # Assert — roof_w_per_k closes the Ext3 −0.06 W/K residual. Ws
+    # row "Roof room Ext3 remaining area" = 17.35 × 0.35 = 6.0725 W/K.
+    # Pre-slice cascade ran the residual on 17.175 × 0.35 = 6.011 W/K.
+    assert abs(ht.roof_w_per_k - 51.3795) <= 1e-2, (
+        f"cascade roof_w_per_k={ht.roof_w_per_k:.4f}; "
+        f"ws 51.3795; Δ={ht.roof_w_per_k - 51.3795:+.4f} "
+        f"(expected within 1e-2 after Ext3 residual area picks up the "
+        f"+0.17 m² adjustment)"
     )
 
 

datatypes/epc/domain/mapper.py

@@ -3347,7 +3347,18 @@ def _map_elmhurst_rir_surface(
     callers compute it once per BP and pass it through to all surfaces
     so the correction applies consistently across both gables.
     """
-    if surface.length_m <= 0 or surface.height_m <= 0:
+    # No length → no wall (drop regardless of mode).
+    if surface.length_m <= 0:
+        return None
+    # H=0 + Simplified Type 2 + common walls present is a SPECIAL CASE:
+    # per RdSAP 10 §3.9.2 step (b) (PDF p.23) the gable equation
+    # `A_gable = L × (0.25 + H) − Σ (H − H_common)² / 2` is defined
+    # for H=0 (returns L × 0.25 minus the common-wall correction —
+    # often negative). Elmhurst's worksheet evaluates this literally
+    # and the result deducts from A_RR_final in step (d). For all
+    # other modes (Detailed, Simplified Type 1, or no common walls),
+    # H=0 still means "absent surface" and gets dropped.
+    if surface.height_m <= 0 and not (is_simplified and common_wall_heights):
         return None
     # RdSAP 10 §3.9.2 step (d) (PDF p.23) — Connected-to-heated-space
     # gables contribute U=0 (Table 4 row 4, PDF p.22) but their area
@@ -3424,23 +3435,34 @@ def _map_elmhurst_rir_surface(
         kind = "gable_wall_external"
     # Area derivation per assessment + common-wall presence.
     if (
-        kind == "gable_wall_external"
+        kind in ("gable_wall", "gable_wall_external")
         and is_simplified
         and common_wall_heights
     ):
         # Spec formula (RdSAP 10 §3.9.2 + Table 4 p.22):
         #     A_gable = L × (0.25 + H_gable)
         #               − Σ_each_common (H_gable − H_common,n)² / 2
-        # Clamp each correction at zero when the common wall is taller
+        #
-        # than the gable (negative-area protection).
+        # Applies to all gable kinds (exposed/sheltered/party) in
+        # Simplified Type 2 — only `kind` differs (which routes the
+        # U-value), not the area equation. The correction term is
+        # always non-negative (square of a real), so when the gable
+        # is shorter than the common walls the formula returns a
+        # negative area. Elmhurst's worksheet applies the equation
+        # literally, including the H_gable=0 absent-gable case
+        # (cert 000565 Ext3 "Gable Wall 2 L=4 H=0":
+        # 4 × 0.25 − 1.5²/2 − 0.30²/2 = −0.17 m²). The negative value
+        # deducts from A_RR_final via step (d) without billing a
+        # physical wall area — `heat_transmission.py`'s per-surface
+        # loop skips `walls += u × area` and `party += 0.25 × area`
+        # when area is negative.
         length_m, height_m = surface.length_m, surface.height_m
         correction = sum(
             ((height_m - h) ** 2) / 2.0
             for h in common_wall_heights
-            if height_m > h
         )
         area_m2 = _round_half_up_2dp(
-            1.0, max(0.0, length_m * (0.25 + height_m) - correction)
+            1.0, length_m * (0.25 + height_m) - correction
         )
     else:
         area_m2 = _round_half_up_2dp(surface.length_m, surface.height_m)

domain/sap10_calculator/worksheet/heat_transmission.py

@@ -905,7 +905,13 @@ def heat_transmission_from_cert(
                         insulation_type=surf.insulation_type,
                     )
                 elif kind == "gable_wall":
-                    party += 0.25 * area
+                    # Negative area = §3.9.2 step (b) absent-gable
+                    # adjustment (see gable_wall_external branch below
+                    # for full rationale). Skip the party billing —
+                    # the wall doesn't physically exist — but include
+                    # the signed area in the residual deduction.
+                    if area >= 0:
+                        party += 0.25 * area
                     rr_walls_in_a_rr_area += area
                 elif kind == "gable_wall_external":
                     # RdSAP10 Table 4 (p.22) row 1: exposed gable U = "as
@@ -913,9 +919,19 @@ def heat_transmission_from_cert(
                     # below (`uw`). Assessor-lodged `u_value` (e.g.
                     # 000487 Gable Wall 2 at U=0.86) overrides the
                     # cascade.
+                    #
+                    # A negative `area` is a §3.9.2 step (b) absent-
+                    # gable adjustment (cert 000565 Ext3 "Gable Wall 2
+                    # L=4 H=0" → -0.17 m² via the spec equation). The
+                    # negative value rolls into `rr_walls_in_a_rr_area`
+                    # so the §3.10.1 residual = `a_rr_shell − walls`
+                    # grows by |area| (step d). Skip both the external-
+                    # area count and the wall billing because the wall
+                    # doesn't physically exist.
                     u_gable = surf.u_value if surf.u_value is not None else uw
-                    rr_detailed_area += area
+                    if area >= 0:
-                    walls += u_gable * area
+                        rr_detailed_area += area
+                        walls += u_gable * area
                     rr_walls_in_a_rr_area += area
                 elif kind == "common_wall":
                     # RdSAP 10 §3.9.2 Simplified Type 2 + Table 4 p.22