diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/loads.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/loads.rs
index ce99962..5d51829 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/loads.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/loads.rs
@@ -252,6 +252,23 @@ impl Mask {
     /// `F = Σ_outer (σ·n − ρ u (u·n)) A − d/dt ∫ ρ u dV + ∫ f dV`.
     #[allow(clippy::too_many_arguments)]
     pub fn control_volume_force(
+        &self,
+        f: &Field,
+        dt: f64,
+        rho: f64,
+        mu: f64,
+        source: Option<&dyn Fn(f64, f64, f64) -> (f64, f64, f64)>,
+        bx: (usize, usize, usize, usize, usize, usize),
+    ) -> [f64; 3] {
+        self.control_volume_force_with_walls(f, dt, rho, mu, source, bx, false)
+    }
+
+    /// Route B on a box spanning the whole z range between NO-SLIP z walls
+    /// (`no_slip_z`): the walls' shear on the fluid inside the box (against
+    /// a wall at rest) is an outer-face stress and enters the balance;
+    /// without it (slip or periodic sides) the z faces carry nothing.
+    #[allow(clippy::too_many_arguments)]
+    pub fn control_volume_force_with_walls(
         &self,
         f: &Field,
         dt: f64,
@@ -259,6 +276,7 @@ impl Mask {
         mu: f64,
         source: Option<&dyn Fn(f64, f64, f64) -> (f64, f64, f64)>,
         (i0, i1, j0, j1, k0, k1): (usize, usize, usize, usize, usize, usize),
+        no_slip_z: bool,
     ) -> [f64; 3] {
         let g = self.grid();
         let (nx, ny, nz, dx, dy, dz) = (g.nx, g.ny, g.nz, g.dx, g.dy, g.dz);
@@ -341,6 +359,20 @@ impl Mask {
         // z faces on the domain's z sides: no momentum flux and no shear on
         // a slip wall, and the two faces cancel on a periodic pair — skipped.
         let full_span = k0 == 0 && k1 == nz;
+        if full_span && no_slip_z {
+            let a = dx * dy;
+            for j in j0..j1 {
+                for i in i0..i1 {
+                    for k in [0, nz - 1] {
+                        if !self.is_fluid_cell(g.cell(k, j, i)) {
+                            continue;
+                        }
+                        force[0] -= mu * uc(k, j, i) / (0.5 * dz) * a;
+                        force[1] -= mu * vc(k, j, i) / (0.5 * dz) * a;
+                    }
+                }
+            }
+        }
         for j in (j0..j1).filter(|_| !full_span) {
             for i in i0..i1 {
                 for (k, sign) in [(k1, 1.0), (k0, -1.0)] {
diff --git a/crates/specialized/rtx-cfd/tests/embedded3_dfg_2z.rs b/crates/specialized/rtx-cfd/tests/embedded3_dfg_2z.rs
index 622b894..a33f82a 100644
--- a/crates/specialized/rtx-cfd/tests/embedded3_dfg_2z.rs
+++ b/crates/specialized/rtx-cfd/tests/embedded3_dfg_2z.rs
@@ -129,7 +129,8 @@ fn dfg_3d_2z_on_the_device() {
                 0,
                 nz,
             );
-            let fcv = mask.control_volume_force(&field, dt, RHO, RHO * NU, None, bx);
+            let fcv =
+                mask.control_volume_force_with_walls(&field, dt, RHO, RHO * NU, None, bx, true);
             let zc = 0.5 * H;
             let p_front = mask
                 .pressure_at(&field.p, CX - 0.5 * D, CY, zc)