diff --git a/CONTEXT.md b/CONTEXT.md
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--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -105,6 +105,10 @@ _Avoid_: patches (deprecated), corrections, manual EPC, edits
 Resolving a **Landlord Description** (unbounded free-text a landlord supplies for one component — "CWI" / "Cav filled" / "cavity insulated" all name one thing) onto a **Recognised Internal Description** via an LLM classifier, persisted in the `landlord_*_overrides` table (`source=classifier`) as a reviewed cache. Four vocabularies are kept **distinct** and must not be conflated: a **Landlord Description** (unbounded input); a **Recognised Internal Description** (the closed target taxonomy — e.g. a `MainHeatingSystemType` archetype — each binding to a Simulation Overlay); a **Lodged Description** (the gov-EPC `main_heating[].description` rendering, e.g. "Room heaters, electric" — only an example of which system *types* occur, never a map key); and the **SAP main heating code** (Table 4a/4b, what the calculator consumes). The classifier maps Landlord → Recognised Internal → SAP code. When it cannot confidently place the text it emits **`None`** (no overlay → the lodged EPC stands, surfaced to the user as "no suitable match"), **never the nearest wrong archetype** — the target taxonomy must be complete enough that a real system always has a correct home, so the classifier never overflows into a garbage-drawer archetype (ADR-0041).
 _Avoid_: "the LLM mapper is unreliable" (the failure mode is a too-small target taxonomy, not LLM language ability); conflating the landlord input vocabulary with the gov-EPC lodged rendering or the RdSAP entry-tool catalogue; treating a deterministic dict as a *replacement* for the LLM rather than a reviewed cache of its output
 
+**Lodgement Sentinels (ND / NI / AB)**:
+The RdSAP string sentinels a cert may lodge in place of a numeric value (e.g. `roof_insulation_thickness`). They are data, not noise, and are **not interchangeable**: **`ND`** (Not Defined) — no data; resolve to the construction age band's **as-built** state (roof as-built: A–D uninsulated, E–F limited insulation, G+ insulated). **`NI`** — insulation **present** but thickness unknown (RdSAP 10 §5.11.4: score as 50 mm); never "no insulation". **`AB`** — As Built; resolve by age band like ND. The mapper deliberately passes roof sentinels through raw — the calculator's resolution depends on distinguishing them (ADR-0047). Scoring and eligibility may legitimately resolve the same sentinel differently (`NI`: 50 mm to the U-cascade, not-eligible to the roof generator).
+_Avoid_: reading `ND` as "no insulation"; reading `NI` as "no insulation" (it means the opposite); normalising sentinels at the mapper boundary; comparing a sentinel string against an int (`"ND" != 0` is always True)
+
 ### Modelling
 
 **Effective EPC**:
diff --git a/docs/adr/0047-roof-eligibility-resolves-sentinel-thickness-by-age-band.md b/docs/adr/0047-roof-eligibility-resolves-sentinel-thickness-by-age-band.md
new file mode 100644
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+++ b/docs/adr/0047-roof-eligibility-resolves-sentinel-thickness-by-age-band.md
@@ -0,0 +1,109 @@
+# Roof Eligibility Resolves Sentinel Thickness by Age Band (vaulted joins sloping; ND ≠ NI)
+
+Property 742265 (uprn 10033526327, cert 2780-3922-3202-6042-9200) lodges a
+`Pitched (vaulted ceiling)` roof (gov-API `roof_construction` 5) with
+`roof_insulation_thickness = "ND"` and got **no roof Recommendation**, leaving
+its plan short of the scenario's goal band on an unlimited budget. Two
+generator gaps compound (ADR-0021's dispatch table has no vaulted row, and its
+uninsulated trigger `!= 0` compares the `"ND"` *string* against int 0, so it
+never fires), while the SAP10 Calculation already handles both: it treats
+vaulted as a slope-following ceiling and resolves the `"ND"` sentinel via the
+roof description / age-band defaults (`heat_transmission.py` — the
+`is_sloping_ceiling` disjunction and the raw-sentinel inspection). The
+generator diverged from the calculator — the exact split ADR-0021 set out to
+prevent.
+
+## Decision
+
+**Amends ADR-0021.** The roof Recommendation Generator changes in two ways;
+the mapper and the calculator change in none.
+
+**1. Vaulted is a sloping ceiling.** The dispatch matches the same substring
+disjunction the calculator uses (`"sloping ceiling" in roof_type or "vaulted"
+in roof_type`): a vaulted ceiling has no loft void, so its one applicable
+Measure is `sloping_ceiling_insulation` (rafters, 100 mm) — never the loft
+fallback. ADR-0021's dispatch table gains the row:
+
+| Roof type (`roof_construction_type` substring) | Measure | Uninsulated trigger | Overlay |
+|---|---|---|---|
+| `sloping ceiling` **or `vaulted`** | `sloping_ceiling_insulation` | resolved thickness 0 (below) | → 100 mm |
+
+**2. Sentinel-aware uninsulated trigger, resolved by age band — codes, not
+descriptions.** `roof_insulation_thickness` is `Optional[Union[str, int]]`; a
+lodged string sentinel is data, not noise. The generator resolves eligibility
+for the two pitched branches (sloping/vaulted and loft) as:
+
+| Lodged value | Meaning (RdSAP 10) | Eligibility |
+|---|---|---|
+| int `0` | explicitly uninsulated | eligible (unchanged) |
+| int `> 0` | measured thickness | not eligible (unchanged) |
+| `"NI"` | insulation **present**, thickness unknown (§5.11.4: assume 50 mm) | **not eligible** |
+| `"ND"` / `None` | not defined — resolve to the age band's **as-built** state | eligible iff as-built uninsulated (bands **A–D**) |
+
+Age-band as-built states (construction age code): **A–D uninsulated · E–F
+limited insulation · G+ insulated.** Only A–D resolves to eligible. This
+extends the codebase's existing precedent (`_PRE_1950_AGE_CODES = {A,B,C,D}`
+in the mapper's code-8 rule; pinned by cert 001479 Ext2 at U=2.30) from the
+scoring path to eligibility.
+
+**Coherence rule (load-bearing):** sentinel resolution must never make an
+undefined cert *more* eligible than an explicitly-lodged one. An explicit
+50 mm lodgement is not eligible today, so `"ND"` on a band-E/F ("limited
+insulation") dwelling must also resolve to not-eligible — recommending a
+top-up to the dwelling whose insulation we *don't* know while withholding it
+from the identical dwelling whose thin insulation we *do* know would be
+incoherent. Whether limited-insulation **top-up** should be a Measure at all
+is a separate product question that applies equally to explicit thin
+lodgements; it is out of scope here (see Consequences).
+
+**The mapper stays untouched — the `"ND"` passthrough is load-bearing.** The
+calculator inspects the *raw* lodgement to distinguish explicit `int(0)`
+(drop the description; uninsulated) from `"NI"` (keep the description; §5.11.4
+retrofit-50 mm) from `"ND"` (defer to description / age default). Normalising
+sentinels at the mapper boundary would change scoring inputs for every ND-roof
+cert in the corpus for zero eligibility benefit.
+
+**Two homes for sentinel resolution, deliberately.** Scoring asks "what
+thickness should the U-cascade see" (`NI` → 50 mm); eligibility asks "is this
+roof a candidate" (`NI` → no). Same sentinel, different questions, different
+answers — a single shared resolver would just relocate the split into its call
+sites. Each home carries a regression test pinning the *disagreement* as
+intentional. Consolidate only if a third consumer appears (rule of three).
+
+## Considered options
+
+- **Normalise `ND` → 0 in the mapper** (extend the code-8 rule to code 5 and
+  to the `"ND"` string). Rejected: changes calculator inputs corpus-wide —
+  `roof_thickness_explicitly_zero` would fire and drop the roof description
+  the §5.11 resolution depends on — risking the pinned Elmhurst accuracy
+  corpus to fix an eligibility bug.
+- **Defer to `roofs[].description`** ("no insulation (assumed)") for ND
+  resolution, mirroring the calculator exactly. Rejected: eligibility should
+  key on codes, not free-text description parsing; descriptions drift by
+  lodgement software and the age-band code carries the same as-built signal
+  deterministically.
+- **Gate eligibility on the calculator's effective roof U-value.** Rejected:
+  no other generator gates on scored output (walls/floors gate on lodged
+  attributes), and it re-couples eligibility to scoring against ADR-0016's
+  separation.
+- **Treat `"ND"` alone as uninsulated** (sentinel-literal). Rejected: ND means
+  *not defined*, not *none* — a band-G ND roof is almost certainly insulated
+  as built and would get a junk Recommendation.
+
+## Consequences
+
+- 742265 (band B, vaulted, ND) becomes eligible: `sloping_ceiling_insulation`
+  over 55.82 m² (~£2.2k, verified by local re-model). Any pitched-loft cert
+  lodging `"ND"` with age band A–D is likewise un-withheld.
+- The **flat branch is deliberately untouched**: its inverted convention
+  (`None` = uninsulated, per the Elmhurst mapping) means an `"ND"` flat roof
+  is *not eligible* today and stays so — a separately-verifiable follow-up,
+  not silently folded into this change.
+- **Open follow-up (product):** should limited-insulation dwellings (explicit
+  thin lodgements *and* ND + bands E–F alike) be offered an insulation
+  top-up? Today neither is; changing that is a trigger redesign, not a
+  sentinel question.
+- ADR-0021's "one dispatching generator, one Measure per roof" and its
+  dispatch order (sloping → flat → no-access → loft) are unchanged; the
+  sloping test simply matches one more substring, ahead of the fallbacks as
+  before.