diff --git a/packages/domain/src/domain/sap/worksheet/internal_gains.py b/packages/domain/src/domain/sap/worksheet/internal_gains.py
index 73f44782..3261f6c9 100644
--- a/packages/domain/src/domain/sap/worksheet/internal_gains.py
+++ b/packages/domain/src/domain/sap/worksheet/internal_gains.py
@@ -493,23 +493,36 @@ def _lighting_capacity_and_efficacy_from_cert(
 def _daylight_factor_from_cert(
     epc: EpcPropertyData,
     overshading: OvershadingCategory,
+    rooflight_total_area_m2: float,
 ) -> float:
-    """Compute C_daylight via L2a + L2b from the cert's windows. Per
-    Table 6d note 3 a single Z_L applies to all glazing in the dwelling.
+    """Compute C_daylight via L2a + L2b from the cert's windows + any
+    rooflights.
+
+    Per Table 6d note 3 a single Z_L applies to all wall glazing. Per
+    Table 6d note 2 rooflights use Z_L = 1.0 regardless of overshading.
+    Rooflights are summed at default g_L = 0.80 (Table 6b DG) × FF = 0.7
+    (Table 6c PVC) — non-default rooflight glazing or framing requires a
+    richer cert-derived representation in a future slice.
 
     When `total_floor_area_m2` is missing or no windows are lodged the
     SAP "no-bonus" default 1.433 is used.
     """
     tfa = float(epc.total_floor_area_m2 or 0.0)
-    if tfa <= 0.0 or not epc.sap_windows:
+    if tfa <= 0.0 or (not epc.sap_windows and rooflight_total_area_m2 <= 0.0):
         return 1.433
     z_l = _Z_L_BY_OVERSHADING[overshading]
-    g_l_numerator = sum(
+    wall_g_l_numerator = sum(
         float(w.window_width) * float(w.window_height)
-        * _g_light(w) * _frame_factor(w)
+        * _g_light(w) * _frame_factor(w) * z_l
         for w in epc.sap_windows
     )
-    g_l = 0.9 * g_l_numerator * z_l / tfa
+    rooflight_g_l_numerator = (
+        rooflight_total_area_m2
+        * _G_LIGHT_DEFAULT
+        * _FRAME_FACTOR_DEFAULT
+        * 1.0  # Z_L = 1.0 for rooflights per Table 6d note 2
+    )
+    g_l = 0.9 * (wall_g_l_numerator + rooflight_g_l_numerator) / tfa
     if g_l > 0.095:
         return 0.96
     return 52.2 * g_l * g_l - 9.94 * g_l + 1.433
@@ -537,6 +550,7 @@ def internal_gains_from_cert(
     dwelling_volume_m3: float,
     heat_gains_from_water_heating_monthly_kwh: tuple[float, ...],
     overshading: OvershadingCategory = OvershadingCategory.AVERAGE,
+    rooflight_total_area_m2: float = 0.0,
 ) -> InternalGainsResult:
     """SAP 10.2 §5 orchestrator — chain every line ref (66)..(73) for the
     dwelling identified by `epc`.
@@ -565,7 +579,9 @@ def internal_gains_from_cert(
     appliances = appliances_monthly_w(total_floor_area_m2=tfa, n_occupants=n)
 
     c_l_fixed, eff_fixed = _lighting_capacity_and_efficacy_from_cert(epc)
-    c_daylight = _daylight_factor_from_cert(epc, overshading)
+    c_daylight = _daylight_factor_from_cert(
+        epc, overshading, rooflight_total_area_m2=rooflight_total_area_m2
+    )
     lighting = lighting_monthly_w(
         total_floor_area_m2=tfa,
         n_occupants=n,
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
index 78377857..3e237527 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
@@ -246,8 +246,9 @@ COMBI_LOSS_OVERRIDE: Optional[tuple[float, ...]] = LINE_61_M_COMBI_LOSS_KWH
 # §12-1 default 80 lm/W × 15 W = 1200 lm each.
 SECTION_5_BULB_COUNT_LEL: int = 8
 # Window areas per worksheet §6 (5 windows: East 3.74 ×2, NW 1.76 + 1.98,
-# SE 0.5). All DG air-filled (g_L=0.80) on PVC frames (FF=0.7).
+# SE 0.5). All DG air-filled (g_L=0.80) on PVC frames (FF=0.7). No rooflights.
 SECTION_5_WINDOW_AREAS_M2: tuple[float, ...] = (3.74, 3.74, 1.76, 1.98, 0.5)
+SECTION_5_ROOFLIGHT_AREAS_M2: tuple[float, ...] = ()
 # Vaillant ecoTEC pro 28 combi, pump in heated space, unknown install date
 # → Table 5a 7 W heating-season-only row.
 SECTION_5_PUMP_AGE_STR: str = "Unknown"
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
index 045ec96b..a9f030ea 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
@@ -185,8 +185,9 @@ COMBI_LOSS_OVERRIDE: Optional[tuple[float, ...]] = LINE_61_M_COMBI_LOSS_KWH
 # §5 Internal gains — cert-derived inputs + expected outputs
 # ============================================================================
 SECTION_5_BULB_COUNT_LEL: int = 9
-# 3 windows: East 1.28, West 1.17 + 6.76. All DG / PVC.
+# 3 windows: East 1.28, West 1.17 + 6.76. All DG / PVC. No rooflights.
 SECTION_5_WINDOW_AREAS_M2: tuple[float, ...] = (1.28, 1.17, 6.76)
+SECTION_5_ROOFLIGHT_AREAS_M2: tuple[float, ...] = ()
 SECTION_5_PUMP_AGE_STR: str = "Unknown"
 
 LINE_66_M_METABOLIC_W: tuple[float, ...] = (144.9204,) * 12
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py
index 08246823..dd14e6ae 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py
@@ -217,8 +217,9 @@ COMBI_LOSS_OVERRIDE: Optional[tuple[float, ...]] = LINE_61_M_COMBI_LOSS_KWH
 # §5 Internal gains — cert-derived inputs + expected outputs
 # ============================================================================
 SECTION_5_BULB_COUNT_LEL: int = 10
-# 2 windows: NE 8.74, SW 1.8. All DG / PVC.
+# 2 windows: NE 8.74, SW 1.8. All DG / PVC. No rooflights.
 SECTION_5_WINDOW_AREAS_M2: tuple[float, ...] = (8.74, 1.8)
+SECTION_5_ROOFLIGHT_AREAS_M2: tuple[float, ...] = ()
 SECTION_5_PUMP_AGE_STR: str = "Unknown"
 
 LINE_66_M_METABOLIC_W: tuple[float, ...] = (152.4740,) * 12
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
index f3789d83..deb6a15e 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
@@ -232,8 +232,9 @@ ELECTRIC_SHOWER_OVERRIDE: Optional[tuple[float, ...]] = LINE_64A_M_ELECTRIC_SHOW
 # §5 Internal gains — cert-derived inputs + expected outputs
 # ============================================================================
 SECTION_5_BULB_COUNT_LEL: int = 7
-# 2 windows: South 0.77, South 6.69. All DG / PVC.
+# 2 windows: South 0.77, South 6.69. All DG / PVC. No rooflights.
 SECTION_5_WINDOW_AREAS_M2: tuple[float, ...] = (0.77, 6.69)
+SECTION_5_ROOFLIGHT_AREAS_M2: tuple[float, ...] = ()
 SECTION_5_PUMP_AGE_STR: str = "Unknown"
 
 LINE_66_M_METABOLIC_W: tuple[float, ...] = (149.5185,) * 12
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
index be98b226..721447c0 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
@@ -228,8 +228,9 @@ LINE_65_M_HEAT_GAINS_FROM_WH_KWH: tuple[float, ...] = (
 # §5 Internal gains — cert-derived inputs + expected outputs
 # ============================================================================
 SECTION_5_BULB_COUNT_LEL: int = 8
-# 3 windows: NE 0.81, NW 2.7, SE 5.52. All DG / PVC.
+# 3 windows: NE 0.81, NW 2.7, SE 5.52. All DG / PVC. No rooflights.
 SECTION_5_WINDOW_AREAS_M2: tuple[float, ...] = (0.81, 2.7, 5.52)
+SECTION_5_ROOFLIGHT_AREAS_M2: tuple[float, ...] = ()
 SECTION_5_PUMP_AGE_STR: str = "Unknown"
 
 LINE_66_M_METABOLIC_W: tuple[float, ...] = (128.8087,) * 12
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py
index d9a8d1b3..05c460a3 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py
@@ -194,8 +194,10 @@ COMBI_LOSS_OVERRIDE: Optional[tuple[float, ...]] = LINE_61_M_COMBI_LOSS_KWH
 # §5 Internal gains — cert-derived inputs + expected outputs
 # ============================================================================
 SECTION_5_BULB_COUNT_LEL: int = 9
-# 3 windows: NE 1.18 + 3.88, SW 4.43. All DG / PVC.
-SECTION_5_WINDOW_AREAS_M2: tuple[float, ...] = (1.18, 3.88, 4.43)
+# Wall windows: NE 3.88, SW 4.43. The 1.18 m² NE entry is a rooflight
+# (Z=1.0 in the worksheet §6 column, per Table 6d note 2).
+SECTION_5_WINDOW_AREAS_M2: tuple[float, ...] = (3.88, 4.43)
+SECTION_5_ROOFLIGHT_AREAS_M2: tuple[float, ...] = (1.18,)
 SECTION_5_PUMP_AGE_STR: str = "Unknown"
 
 LINE_66_M_METABOLIC_W: tuple[float, ...] = (157.9824,) * 12
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_internal_gains.py b/packages/domain/src/domain/sap/worksheet/tests/test_internal_gains.py
index 0e00cac6..f819652d 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_internal_gains.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_internal_gains.py
@@ -601,6 +601,7 @@ def test_internal_gains_from_cert_matches_elmhurst_worksheet_all_fixtures(
         dwelling_volume_m3=fixture.LINE_5_VOLUME_M3,
         heat_gains_from_water_heating_monthly_kwh=fixture.LINE_65_M_HEAT_GAINS_FROM_WH_KWH,
         overshading=OvershadingCategory.AVERAGE,
+        rooflight_total_area_m2=sum(fixture.SECTION_5_ROOFLIGHT_AREAS_M2),
     )
 
     # Assert
@@ -609,7 +610,7 @@ def test_internal_gains_from_cert_matches_elmhurst_worksheet_all_fixtures(
             fixture.LINE_66_M_METABOLIC_W[m], abs=1e-3
         ), f"(66) month {m+1}"
         assert result.lighting_monthly_w[m] == pytest.approx(
-            fixture.LINE_67_M_LIGHTING_W[m], abs=2e-1
+            fixture.LINE_67_M_LIGHTING_W[m], abs=5e-3
         ), f"(67) month {m+1}"
         assert result.appliances_monthly_w[m] == pytest.approx(
             fixture.LINE_68_M_APPLIANCES_W[m], abs=5e-2
@@ -627,5 +628,5 @@ def test_internal_gains_from_cert_matches_elmhurst_worksheet_all_fixtures(
             fixture.LINE_72_M_WATER_HEATING_GAINS_W[m], abs=1e-3
         ), f"(72) month {m+1}"
         assert result.total_internal_gains_monthly_w[m] == pytest.approx(
-            fixture.LINE_73_M_TOTAL_INTERNAL_GAINS_W[m], abs=3e-1
+            fixture.LINE_73_M_TOTAL_INTERNAL_GAINS_W[m], abs=5e-3
         ), f"(73) month {m+1}"