diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 7660a45b..d8754292 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -3061,19 +3061,26 @@ _RDSAP20_GLAZED_AREA_BAND_MULTIPLIER: dict[int, float] = {
 _RDSAP20_SYNTH_ORIENTATIONS: tuple[int, ...] = (1, 3, 5, 7)
 
 
-def _synthesise_20_0_0_sap_windows(schema: RdSapSchema20_0_0) -> List[SapWindow]:
-    """ADR-0027 Reduced-Field Synthesis of `sap_windows` for a 20.0.0 cert.
+def _synthesise_reduced_field_windows(
+    glazed_area: int,
+    total_floor_area: float,
+    glazing_type: int,
+) -> List[SapWindow]:
+    """ADR-0028 shared core for pre-SAP10 reduced-field glazing synthesis.
 
-    993/1000 certs carry no per-window array, only a glazed_area band + floor
-    area; synthesise total glazing as `ratio x TFA`, split 4-way across N/E/S/W
-    with `window_width = area/4, window_height = 1.0` (width x height is the only
-    quantity the calculator reads, so height=1 makes width carry the area
-    exactly — matching the existing Elmhurst precedent).
+    Certs with no per-window array carry only a glazed_area band + floor area;
+    synthesise total glazing as `ratio x TFA x band_multiplier`, split 4-way
+    across N/E/S/W with `window_width = area/4, window_height = 1.0` (width x
+    height is the only quantity the calculator reads, so height=1 makes width
+    carry the area exactly — the Elmhurst precedent).
+
+    This is the single home of the inherited 20.0.0 coefficients across the
+    pre-SAP10 RdSAP family (20.0.0 / 18.0 / 17.1). `glazing_type` is pre-resolved
+    by each spec's seam (cascade + that spec's own "ND" handling), keeping spec
+    vocabulary out of the numeric core.
     """
-    band_multiplier = _RDSAP20_GLAZED_AREA_BAND_MULTIPLIER.get(schema.glazed_area, 1.0)
-    total_area = (
-        _RDSAP20_GLAZING_RATIO * float(schema.total_floor_area) * band_multiplier
-    )
+    band_multiplier = _RDSAP20_GLAZED_AREA_BAND_MULTIPLIER.get(glazed_area, 1.0)
+    total_area = _RDSAP20_GLAZING_RATIO * float(total_floor_area) * band_multiplier
     per_window_area = total_area / len(_RDSAP20_SYNTH_ORIENTATIONS)
     return [
         SapWindow(
@@ -3081,11 +3088,7 @@ def _synthesise_20_0_0_sap_windows(schema: RdSapSchema20_0_0) -> List[SapWindow]
             glazing_gap=0,
             orientation=orientation,
             window_type=0,
-            # ADR-0027: 20.0.0 glazed_type codes 1-8+ND are identical to 21.0.1's,
-            # so reuse the verified 21.0.1 cascade (fixes code 1 "DG pre-2002"
-            # being mis-read as single). g⊥ comes from window_transmission_details
-            # (slice 6), so glazing_type only feeds the daylight g_L lookup.
-            glazing_type=_api_cascade_glazing_type(schema.multiple_glazing_type),
+            glazing_type=glazing_type,
             window_width=per_window_area,
             window_height=1.0,
             draught_proofed=False,
@@ -3097,6 +3100,17 @@ def _synthesise_20_0_0_sap_windows(schema: RdSapSchema20_0_0) -> List[SapWindow]
     ]
 
 
+def _synthesise_20_0_0_sap_windows(schema: RdSapSchema20_0_0) -> List[SapWindow]:
+    """ADR-0027/0028 seam: 20.0.0 glazed_type codes 1-8+ND are identical to
+    21.0.1's, so route multiple_glazing_type through the verified cascade (fixes
+    code 1 "DG pre-2002" read as single), then call the shared core."""
+    return _synthesise_reduced_field_windows(
+        schema.glazed_area,
+        schema.total_floor_area,
+        _api_cascade_glazing_type(schema.multiple_glazing_type),
+    )
+
+
 # ADR-0028: multiple_glazing_type "ND" (Not Defined, 69/1000 18.0 certs) has no
 # cascade mapping — treat as the DG-modal default (cascade code 2 → daylight g_L
 # 0.80, matching the calculator's `_G_LIGHT_DEFAULT` for unknown glazing).
@@ -3104,46 +3118,21 @@ _RDSAP18_ND_GLAZING_TYPE: int = 2
 
 
 def _synthesise_18_0_sap_windows(schema: RdSapSchema18_0) -> List[SapWindow]:
-    """ADR-0028 Reduced-Field Synthesis of `sap_windows` for an 18.0 cert.
-
-    A separate seam from the 20.0.0 helper so 18.0 can diverge as we learn more,
-    but it reuses the inherited 20.0.0 coefficients unchanged (ADR-0028: the
-    18.0 corpus can't self-fit — 958/1000 band-1 with no measured band-1 windows
-    — and its band-4 rich certs reproduce `0.148 x 1.51 = 0.223`). 990/1000 certs
-    carry no per-window array, only a glazed_area band + floor area; synthesise
-    total glazing as `ratio x TFA`, split 4-way across N/E/S/W with
-    `window_width = area/4, window_height = 1.0`.
-    """
-    band_multiplier = _RDSAP20_GLAZED_AREA_BAND_MULTIPLIER.get(schema.glazed_area, 1.0)
-    total_area = (
-        _RDSAP20_GLAZING_RATIO * float(schema.total_floor_area) * band_multiplier
-    )
-    per_window_area = total_area / len(_RDSAP20_SYNTH_ORIENTATIONS)
-    # ADR-0028: 18.0 glazed_type codes 1-8 are identical to 20.0.0's (verified
-    # against epc_codes.csv), so reuse the verified cascade for integer codes;
-    # the "ND" string falls back to the DG-modal default.
+    """ADR-0028 seam: reuses the inherited 20.0.0 coefficients via the shared
+    core (the 18.0 corpus can't self-fit — 958/1000 band-1 with no measured
+    band-1 windows — and its band-4 rich certs reproduce `0.148 x 1.51 = 0.223`).
+    18.0 glazed_type codes 1-8 are identical to 20.0.0's (verified vs
+    epc_codes.csv): route integer codes through the verified cascade; the "ND"
+    string falls back to the DG-modal default. Own seam so 18.0 can diverge."""
     mgt = schema.multiple_glazing_type
     glazing_type = (
         _api_cascade_glazing_type(mgt)
         if isinstance(mgt, int)
         else _RDSAP18_ND_GLAZING_TYPE
     )
-    return [
-        SapWindow(
-            frame_material=None,
-            glazing_gap=0,
-            orientation=orientation,
-            window_type=0,
-            glazing_type=glazing_type,
-            window_width=per_window_area,
-            window_height=1.0,
-            draught_proofed=False,
-            window_location=0,
-            window_wall_type=0,
-            permanent_shutters_present=False,
-        )
-        for orientation in _RDSAP20_SYNTH_ORIENTATIONS
-    ]
+    return _synthesise_reduced_field_windows(
+        schema.glazed_area, schema.total_floor_area, glazing_type
+    )
 
 
 # ADR-0028: multiple_glazing_type "ND" (Not Defined, 56/1000 17.1 certs) → the
@@ -3152,43 +3141,20 @@ _RDSAP17_1_ND_GLAZING_TYPE: int = 2
 
 
 def _synthesise_17_1_sap_windows(schema: RdSapSchema17_1) -> List[SapWindow]:
-    """ADR-0028 Reduced-Field Synthesis of `sap_windows` for a 17.1 cert.
-
-    A separate seam from the 18.0/20.0.0 helpers so 17.1 can diverge, but it
-    reuses the inherited 20.0.0 coefficients unchanged (ADR-0028: 969/1000 band-1
-    with no measured band-1 windows; its band-4 rich certs reproduce the model).
-    990/1000 certs carry no per-window array — synthesise total glazing as
-    `ratio x TFA`, split 4-way across N/E/S/W with height=1.0.
-    """
-    band_multiplier = _RDSAP20_GLAZED_AREA_BAND_MULTIPLIER.get(schema.glazed_area, 1.0)
-    total_area = (
-        _RDSAP20_GLAZING_RATIO * float(schema.total_floor_area) * band_multiplier
-    )
-    per_window_area = total_area / len(_RDSAP20_SYNTH_ORIENTATIONS)
-    # ADR-0028: 17.1 glazed_type codes 1-8 are identical to 20.0.0's (verified
-    # against epc_codes.csv); the "ND" string falls back to the DG-modal default.
+    """ADR-0028 seam: reuses the inherited 20.0.0 coefficients via the shared
+    core (969/1000 band-1 with no measured band-1 windows; band-4 rich certs
+    reproduce the model). 17.1 glazed_type codes 1-8 are identical to 20.0.0's:
+    route integer codes through the verified cascade; the "ND" string falls back
+    to the DG-modal default. Own seam so 17.1 can diverge."""
     mgt = schema.multiple_glazing_type
     glazing_type = (
         _api_cascade_glazing_type(mgt)
         if isinstance(mgt, int)
         else _RDSAP17_1_ND_GLAZING_TYPE
     )
-    return [
-        SapWindow(
-            frame_material=None,
-            glazing_gap=0,
-            orientation=orientation,
-            window_type=0,
-            glazing_type=glazing_type,
-            window_width=per_window_area,
-            window_height=1.0,
-            draught_proofed=False,
-            window_location=0,
-            window_wall_type=0,
-            permanent_shutters_present=False,
-        )
-        for orientation in _RDSAP20_SYNTH_ORIENTATIONS
-    ]
+    return _synthesise_reduced_field_windows(
+        schema.glazed_area, schema.total_floor_area, glazing_type
+    )
 
 
 # GOV.UK API `glazing_type` integer → (u_value W/m²K, g_perpendicular,