diff --git a/tools/site_audit/gardenfly.js b/tools/site_audit/gardenfly.js
index 67890c7..a73193d 100644
--- a/tools/site_audit/gardenfly.js
+++ b/tools/site_audit/gardenfly.js
@@ -89,9 +89,17 @@ export function hardwareByName(name) {
  * @param {Array}  [o.ids]     4 anchor ids; null/omitted = bare bed (the control)
  * @param {Array|object} [o.hw] hardware per pick (aligned with ids), or one tier for all 4
  * @param {number} [o.tension]
+ * @param {number} [o.porosity] fabric flown. DEFAULT 0.30 (shade cloth — the
+ *                             fabric a competent player flies, same as sweep.js,
+ *                             and every pinned number to date was measured on
+ *                             it); pass 0 for membrane. SPRINT16: the F key is
+ *                             live and storm_06's whole bet is membrane-vs-
+ *                             cloth, so the audit engine must be able to fly
+ *                             both — C's gate-3.1 pin and B's membrane-balance
+ *                             tables ride this parameter.
  * @returns {{ hp, state, byHail, byRain, cornersLost, peaks, marginal, cost }}
  */
-export function flyGarden({ anchors, bed, stormDef, siteDef = null, use = null, ids = null, hw = null, tension = 1.0 }) {
+export function flyGarden({ anchors, bed, stormDef, siteDef = null, use = null, ids = null, hw = null, tension = 1.0, porosity = 0.30 }) {
   const wind = windForSite(stormDef, siteDef, anchors);
   use?.(wind);
 
@@ -99,8 +107,7 @@ export function flyGarden({ anchors, bed, stormDef, siteDef = null, use = null,
   if (ids) {
     const hwArr = Array.isArray(hw) ? hw : Array(4).fill(hw ?? hardwareByName('rated shackle'));
     cost = hwArr.reduce((s, h) => s + (h.cost || 0), 0);
-    // shade cloth (porosity 0.30): the fabric a competent player flies — same as sweep.js
-    rig = new SailRig({ anchors, gridN: 10, porosity: 0.30 }).attach(ids, hwArr, tension);
+    rig = new SailRig({ anchors, gridN: 10, porosity }).attach(ids, hwArr, tension);
     // D's skipped-attach trap: a rig without its shadow mesh scores as a bare
     // bed and the number LOOKS plausible. Refuse to fly it.
     if (!rig.rigged || !rig.pos || !rig.tris || !rig.tris.length) {
diff --git a/tools/site_audit/gardenfly.selftest.js b/tools/site_audit/gardenfly.selftest.js
index b819372..5b5b248 100644
--- a/tools/site_audit/gardenfly.selftest.js
+++ b/tools/site_audit/gardenfly.selftest.js
@@ -122,6 +122,22 @@ export async function buildGardenflyTests() {
     venRun = { bare: fly([]), funnelled: fly(SYN_FUNNEL) };
   } catch (err) { venErr = String((err && err.stack) || err); }
 
+  // SPRINT16 [B]: flyGarden grew a `porosity` param (storm_06's membrane bet
+  // needs the audit engine to fly both fabrics). Two things must hold or every
+  // pinned number in THREADS quietly changes meaning: the DEFAULT is still the
+  // 0.30 shade cloth every pin was measured on, and membrane genuinely flies
+  // as a different fabric (double the pressure term → heavier corners).
+  let fabRun = null, fabErr = null;
+  try {
+    const flyFab = (porosity) => flyGarden({
+      anchors: SYN_ANCHORS, bed: SYN_BED, stormDef: SYN_STORM,
+      siteDef: { wind: { venturi: SYN_FUNNEL } },
+      ids: ['a1', 'a2', 'a3', 'a4'], hw: hardwareByName('rated shackle'),
+      ...(porosity === undefined ? {} : { porosity }),
+    });
+    fabRun = { byDefault: flyFab(undefined), cloth: flyFab(0.30), membrane: flyFab(0) };
+  } catch (err) { fabErr = String((err && err.stack) || err); }
+
   return [
     ['gardenfly: the pinned separation line, flown in BOTH chains — the 12s-skew disagreement, pinned', () => {
       // ⚠ THE STATE OF PLAY (2026-07-18, [B] THREADS, receipts in the entry):
@@ -168,6 +184,31 @@ export async function buildGardenflyTests() {
           `the siteDef's venturi is not reaching gardenfly's wind (windForSite must receive the SITE def)`);
       }
     }],
+    ['gardenfly: default fabric is still the pinned 0.30 cloth, and membrane flies heavier', () => {
+      if (fabErr) throw new Error(`fabric flight died: ${fabErr}`);
+      const { byDefault, cloth, membrane } = fabRun;
+      // omitting porosity === saying 0.30, to the digit — every pinned number
+      // in THREADS was measured on the default and must not move under it
+      assert(byDefault.hp === cloth.hp, `default flew hp ${byDefault.hp} vs explicit cloth ${cloth.hp} — the default fabric drifted`);
+      for (const d of byDefault.peaks) {
+        const c = cloth.peaks.find((p) => p.id === d.id);
+        assert(c && c.peakN === d.peakN, `corner ${d.id}: default peak ${d.peakN} != explicit-cloth ${c?.peakN} — the default fabric drifted`);
+      }
+      // porosity 0 must be a genuinely different fabric: the solid membrane
+      // catches the full pressure term, so every corner flies heavier
+      for (const m of membrane.peaks) {
+        const c = cloth.peaks.find((p) => p.id === m.id);
+        assert(m.peakN > c.peakN,
+          `corner ${m.id}: membrane peak ${m.peakN} N not above cloth ${c.peakN} N — the porosity param is not reaching the rig`);
+      }
+      // Measured 1.23x on this synthetic yard (browser, 2026-07-20) — less
+      // than the raw 1/0.7 pressure ratio because relative wind bleeds load as
+      // the heavier-loaded membrane accelerates. Floor at 1.1: the per-corner
+      // strictly-greater asserts above prove the wiring; this only guards
+      // against the difference collapsing to epsilon.
+      const up = membrane.peaks.reduce((s, m) => s + m.peakN / cloth.peaks.find((p) => p.id === m.id).peakN, 0) / 4;
+      assert(up > 1.1, `membrane only ${up.toFixed(2)}x cloth load on average — porosity is wired but not biting`);
+    }],
     ['gardenfly: unknown hardware names throw — the shop has no mystery tier', () => {
       let threw = false;
       try { hardwareByName('titanium dream'); } catch { threw = true; }