diff --git a/openmc/mesh.py b/openmc/mesh.py
index c9c76552b..3f53582d5 100644
--- a/openmc/mesh.py
+++ b/openmc/mesh.py
@@ -182,6 +182,40 @@ class Mesh(IDManagerMixin):
 
         return mesh
 
+    @classmethod
+    def from_rect_lattice(cls, lattice, division=1, mesh_id=None, name=''):
+        """Create mesh from an existing rectangular lattice
+
+        Parameters
+        ----------
+        lattice : openmc.RectLattice
+            Rectangular lattice used as a template for this mesh
+        division : int
+            Number of mesh cells per lattice cell.
+            If not specified, there will be 1 mesh cell per lattice cell.
+        mesh_id : int
+            Unique identifier for the mesh
+        name : str
+            Name of the mesh
+
+        Returns
+        -------
+        openmc.Mesh
+            Mesh instance
+
+        """
+        cv.check_type('rectangular lattice', lattice, openmc.RectLattice)
+
+        shape = np.array(lattice.shape)
+        width = lattice.pitch*shape
+        
+        mesh = cls(mesh_id, name)
+        mesh.lower_left = lattice.lower_left
+        mesh.upper_right = lattice.lower_left + width
+        mesh.dimension = shape*division
+        
+        return mesh
+
     def to_xml_element(self):
         """Return XML representation of the mesh
 
diff --git a/openmc/tallies.py b/openmc/tallies.py
index a5341cbed..74f342903 100644
--- a/openmc/tallies.py
+++ b/openmc/tallies.py
@@ -248,7 +248,7 @@ class Tally(IDManagerMixin):
 
     @property
     def sum_sq(self):
-        if not self._sp_filename:
+        if not self._sp_filename or self.derived:
             return None
 
         if not self._results_read:
diff --git a/tests/unit_tests/test_mesh_from_lattice.py b/tests/unit_tests/test_mesh_from_lattice.py
new file mode 100644
index 000000000..4d79fc579
--- /dev/null
+++ b/tests/unit_tests/test_mesh_from_lattice.py
@@ -0,0 +1,116 @@
+import numpy as np
+import openmc
+import pytest
+
+
+@pytest.fixture(scope='module')
+def pincell1(uo2, water):
+    cyl = openmc.ZCylinder(R=0.35)
+    fuel = openmc.Cell(fill=uo2, region=-cyl)
+    moderator = openmc.Cell(fill=water, region=+cyl)
+
+    univ = openmc.Universe(cells=[fuel, moderator])
+    univ.fuel = fuel
+    univ.moderator = moderator
+    return univ
+
+
+@pytest.fixture(scope='module')
+def pincell2(uo2, water):
+    cyl = openmc.ZCylinder(R=0.4)
+    fuel = openmc.Cell(fill=uo2, region=-cyl)
+    moderator = openmc.Cell(fill=water, region=+cyl)
+
+    univ = openmc.Universe(cells=[fuel, moderator])
+    univ.fuel = fuel
+    univ.moderator = moderator
+    return univ
+
+
+@pytest.fixture(scope='module')
+def zr():
+    zr = openmc.Material()
+    zr.add_element('Zr', 1.0)
+    zr.set_density('g/cm3', 1.0)
+    return zr
+
+
+@pytest.fixture(scope='module')
+def rlat2(pincell1, pincell2, uo2, water, zr):
+    """2D Rectangular lattice for testing."""
+    all_zr = openmc.Cell(fill=zr)
+    pitch = 1.2
+    n = 3
+    u1, u2 = pincell1, pincell2
+    lattice = openmc.RectLattice()
+    lattice.lower_left = (-pitch*n/2, -pitch*n/2)
+    lattice.pitch = (pitch, pitch)
+    lattice.outer = openmc.Universe(cells=[all_zr])
+    lattice.universes = [
+        [u1, u2, u1],
+        [u2, u1, u2],
+        [u2, u1, u1]
+    ]
+
+    return lattice
+
+
+@pytest.fixture(scope='module')
+def rlat3(pincell1, pincell2, uo2, water, zr):
+    """3D Rectangular lattice for testing."""
+
+    # Create another universe for top layer
+    hydrogen = openmc.Material()
+    hydrogen.add_element('H', 1.0)
+    hydrogen.set_density('g/cm3', 0.09)
+    h_cell = openmc.Cell(fill=hydrogen)
+    u3 = openmc.Universe(cells=[h_cell])
+
+    all_zr = openmc.Cell(fill=zr)
+    pitch = 1.2
+    n = 3
+    u1, u2 = pincell1, pincell2
+    lattice = openmc.RectLattice()
+    lattice.lower_left = (-pitch*n/2, -pitch*n/2, -10.0)
+    lattice.pitch = (pitch, pitch, 10.0)
+    lattice.outer = openmc.Universe(cells=[all_zr])
+    lattice.universes = [
+        [[u1, u2, u1],
+         [u2, u1, u2],
+         [u2, u1, u1]],
+        [[u3, u1, u2],
+         [u1, u3, u2],
+         [u2, u1, u1]]
+    ]
+
+    return lattice
+
+
+def test_mesh2d(rlat2):
+    shape = np.array(rlat2.shape)
+    width = shape*rlat2.pitch
+    
+    mesh1 = openmc.Mesh.from_rect_lattice(rlat2)
+    assert np.array_equal(mesh1.dimension, (3, 3))
+    assert np.array_equal(mesh1.lower_left, rlat2.lower_left)
+    assert np.array_equal(mesh1.upper_right, rlat2.lower_left + width)
+    
+    mesh2 = openmc.Mesh.from_rect_lattice(rlat2, division=3)
+    assert np.array_equal(mesh2.dimension, (9, 9))
+    assert np.array_equal(mesh2.lower_left, rlat2.lower_left)
+    assert np.array_equal(mesh2.upper_right, rlat2.lower_left + width)
+
+
+def test_mesh3d(rlat3):
+    shape = np.array(rlat3.shape)
+    width = shape*rlat3.pitch
+
+    mesh1 = openmc.Mesh.from_rect_lattice(rlat3)
+    assert np.array_equal(mesh1.dimension, (3, 3, 2))
+    assert np.array_equal(mesh1.lower_left, rlat3.lower_left)
+    assert np.array_equal(mesh1.upper_right, rlat3.lower_left + width)
+
+    mesh2 = openmc.Mesh.from_rect_lattice(rlat3, division=3)
+    assert np.array_equal(mesh2.dimension, (9, 9, 6))
+    assert np.array_equal(mesh2.lower_left, rlat3.lower_left)
+    assert np.array_equal(mesh2.upper_right, rlat3.lower_left + width)