diff --git a/datatypes/epc/domain/epc_property_data.py b/datatypes/epc/domain/epc_property_data.py
index d18b2341..c75e730f 100644
--- a/datatypes/epc/domain/epc_property_data.py
+++ b/datatypes/epc/domain/epc_property_data.py
@@ -427,6 +427,7 @@ class SapBuildingPart:
     floor_u_value_known: Optional[bool] = None
 
     roof_construction: Optional[int] = None
+    roof_construction_type: Optional[str] = None  # str from site notes e.g. "PS Pitched, sloping ceiling"
     roof_insulation_location: Optional[Union[int, str]] = (
         None  # TODO: make enum/mapping?
     )
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 937100fc..d6d44462 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2178,6 +2178,7 @@ def _map_elmhurst_building_part(
         party_wall_construction=_elmhurst_party_wall_construction_int(walls.party_wall_type),
         sap_floor_dimensions=floor_dims,
         wall_thickness_mm=walls.thickness_mm,
+        roof_construction_type=_strip_code(roof.roof_type),
         roof_insulation_location=_strip_code(roof.insulation),
         roof_insulation_thickness=_resolve_sloping_ceiling_thickness(
             roof, age_code,
diff --git a/packages/domain/src/domain/sap/worksheet/heat_transmission.py b/packages/domain/src/domain/sap/worksheet/heat_transmission.py
index 1c302c61..01d0be61 100644
--- a/packages/domain/src/domain/sap/worksheet/heat_transmission.py
+++ b/packages/domain/src/domain/sap/worksheet/heat_transmission.py
@@ -68,7 +68,7 @@ from domain.ml.rdsap_uvalues import (
     u_wall,
     u_window,
 )
-from math import floor, sqrt
+from math import cos, floor, radians, sqrt
 
 
 def _round_half_up(value: float, dp: int) -> float:
@@ -96,6 +96,10 @@ _WINDOW_CURTAIN_RESISTANCE_M2K_PER_W: Final[float] = 0.04
 # rounding policy — applied to gross wall / roof / floor / party / window
 # / door / alt-wall / RR sub-area inputs to the §3 cascade.
 _AREA_ROUND_DP: Final[int] = 2
+# RdSAP 10 §3.8 "Roof area" — pitched-sloping-ceiling roofs use the
+# inclined surface area (floor area divided by cos(30°)) rather than
+# the horizontal projection.
+_COS_30_DEG: Final[float] = cos(radians(30.0))
 
 
 @dataclass(frozen=True)
@@ -538,10 +542,15 @@ def heat_transmission_from_cert(
         rw_area_part = (
             _round_half_up(roof_windows_area_total, _AREA_ROUND_DP) if i == 0 else 0.0
         )
-        gross_roof_area = _round_half_up(
-            geom["top_floor_area_m2"] if exposure.has_exposed_roof else 0.0,
-            _AREA_ROUND_DP,
-        )
+        # RdSAP 10 §3.8 "Roof area": roof area is the greatest of the
+        # floor areas on each level. For a pitched roof with a sloping
+        # ceiling, divide that area by cos(30°) — the worksheet enters
+        # the inclined surface area, not the horizontal projection.
+        top_floor_area = geom["top_floor_area_m2"] if exposure.has_exposed_roof else 0.0
+        roof_type = (part.roof_construction_type or "").lower()
+        if "sloping ceiling" in roof_type:
+            top_floor_area = top_floor_area / _COS_30_DEG
+        gross_roof_area = _round_half_up(top_floor_area, _AREA_ROUND_DP)
         roof_area = max(0.0, gross_roof_area - rw_area_part)
         floor_area_total = _round_half_up(
             geom["ground_floor_area_m2"] if exposure.has_exposed_floor else 0.0,