Basement structure — forensic map

The basement is the house’s only hard-measured level, so it’s the datum everything else re-registers to (see HISTORY.md). But it is not a uniform box — it grew in phases, so framing, edge conditions, and the post/beam lines stagger by zone. Companion to basement-joists.md (the joist grid + the E–W width).

Working strategy — interior-first

Duct routing and air-handler clearances care about interior clear space, not the exterior envelope. So: build the model from the cinder inner faces inward, nail the interior N–S dimension (measured to the raw cinder face where exposed — the east/utility side has the most bare block), and leave the exterior/edge detailing (sill/rim/parge nuances, exact outer faces) for later or the scan. That’s enough to place ducts + size the air-handler space, and to firm up the Main floor and above.

Three zones (west → east)

  N ↑         west/den        │stairs│      east/utility
   ┌────────────────────────────────────────────────────────┐
   │ ●────────●────────●  north girder ─────────●NE ↑turns N │  3 N posts, W-wall→NE
   │                                  chimney●══════●E(pocket)│  cross-beam, E side
   │     ●SW─────●s-mid  (→ stairs)                           │  SW beam, further S
   └────────────────────────────────────────────────────────┘
      W wall              ‖jst-12       ‖jst-15         E wall
                        (N-S zone-edge beams = stair sides)

• West / den (carpet): garage to the west, kitchen crawlspace to the north. Rim joist alone — no visible sill (paneling hides it; “doesn’t feel like wood below”).
• Center / stairs: bathroom (≈stair-width) at the north end, stairs to the south, bar under/behind mid. A void with the main drain stack behind the bathroom (NE, at the east end of the kitchen crawlspace); drain + supply penetrate the rim into the extended kitchen. No sill plank, but the rim is ~2× thick (likely doubled when the extension was added).
• East / utility: bare foundation floor; electrical panel + sump in the SE corner; air handler, water heater, dead furnace, laundry. Heavy timber sill + rim. Bedroom crawlspace to the south (tight). An interior wall runs under the east zone-edge beam.

Perimeter framing (varies by wall / zone)

• East/utility: a heavy timber sill (~5.5″ — a 6× or built-up member, “beyond a 4×4”) on a ~2″ sloped parge bed, with the rim (~1.5″) on its outer edge and a small lip inboard. Heavy timber sills are normal in the original section of an old foundation.
• West/den: rim alone, no visible sill (paneling).
• Center: no sill plank; rim ~2× thick (doubled at the extension).
• Consequence: there is no single block-face ↔ joist-end offset. Measure the interior clear span per zone / to the cinder face directly. The earlier interior = L − 13″ shortcut only holds where a plain 1.5″ rim/sill applies.

Beams & posts (staggered — the phased-build signature)

• North girder: 3 posts, W wall → NE post, then turns north (does not reach the E wall).
• East cross-beam: chimney → E wall, seated in a cinder pocket; the east zone-edge beam (joist-15) passes behind the chimney without bearing on it, and the utility “middle joist” dead-ends against this cross-beam.
• SW beam: SW + south-middle posts, stairs → W wall, sitting further south than the east cross-beam — the two do not form one line.

• Zone-edge beams (N–S, ~3.5″): joist-12 = den	stairs edge; joist-15 =
  stairs	utility edge (interior wall beneath it).

• 6 posts total; exact beam-line latitudes and the 6th post still to pin.

Open questions / scan targets

• North-center corner — bathroom + drain-stack void + doubled rim (pipes penetrate the rim into the extended kitchen). The odd part; leave loose for now.
• Bedroom crawlspace (S of utility) — un-enterable, peephole only; infer from the south-extension footprint + the single AC duct route until scanned.
• Confirm actual joist / rim thickness (old lumber may exceed 1.5″).
• Exact beam-line latitudes and the identity of the 6th post.
• Whether the west/den hides a sill under the paneling.