diff --git a/geodata_pipeline/buildings.py b/geodata_pipeline/buildings.py
index 3cbebcd..4bb3531 100644
--- a/geodata_pipeline/buildings.py
+++ b/geodata_pipeline/buildings.py
@@ -493,13 +493,26 @@ def export_buildings(cfg: Config) -> int:
                 if dgm_ds:
                     gt = dgm_ds.GetGeoTransform()
                     band = dgm_ds.GetRasterBand(1)
-                    arr = band.ReadAsArray()
+                    # Calculate srcwin for the tile
+                    xmin, ymin, xmax, ymax = bounds
+                    xoff = int((xmin - gt[0]) / gt[1])
+                    yoff = int((ymax - gt[3]) / gt[5])
+                    xsize = int((xmax - xmin) / gt[1]) + 1
+                    ysize = int((ymax - ymin) / abs(gt[5])) + 1
+                    
+                    # Clamp to raster dimensions
+                    xoff_clamped = max(0, min(xoff, dgm_ds.RasterXSize - 1))
+                    yoff_clamped = max(0, min(yoff, dgm_ds.RasterYSize - 1))
+                    xsize_clamped = max(1, min(xsize, dgm_ds.RasterXSize - xoff_clamped))
+                    ysize_clamped = max(1, min(ysize, dgm_ds.RasterYSize - yoff_clamped))
+                    
+                    arr = band.ReadAsArray(xoff_clamped, yoff_clamped, xsize_clamped, ysize_clamped)
                     nodata = band.GetNoDataValue()
                     for idx, (vx, vy, vz) in enumerate(vertices):
-                        wx = vx + bounds[0]
-                        wy = vy + bounds[1]
-                        col = int((wx - gt[0]) / gt[1])
-                        row = int((wy - gt[3]) / gt[5])
+                        wx = vx + xmin
+                        wy = vy + ymin
+                        col = int((wx - gt[0]) / gt[1]) - xoff_clamped
+                        row = int((wy - gt[3]) / gt[5]) - yoff_clamped
                         if 0 <= row < arr.shape[0] and 0 <= col < arr.shape[1]:
                             g = float(arr[row, col])
                             if nodata is not None and g == nodata:
diff --git a/tests/test_buildings_memory.py b/tests/test_buildings_memory.py
new file mode 100644
index 0000000..0e367dc
--- /dev/null
+++ b/tests/test_buildings_memory.py
@@ -0,0 +1,67 @@
+import unittest
+from unittest.mock import patch, MagicMock
+import numpy as np
+from geodata_pipeline.buildings import export_buildings
+from geodata_pipeline.config import Config
+
+class TestBuildingsMemory(unittest.TestCase):
+    @patch("geodata_pipeline.buildings.gdal.Open")
+    @patch("geodata_pipeline.buildings._ensure_cityjson_for_tile")
+    @patch("geodata_pipeline.buildings._load_cityjson")
+    @patch("geodata_pipeline.buildings._collect_faces")
+    @patch("geodata_pipeline.buildings._load_ortho")
+    @patch("geodata_pipeline.buildings._compose_glb")
+    @patch("geodata_pipeline.buildings.ensure_dir")
+    @patch("os.path.exists")
+    @patch("builtins.open")
+    def test_ground_snapping_windowed_read(self, mock_open_file, mock_exists, mock_ensure_dir, mock_compose, mock_load_ortho, mock_collect, mock_load_cj, mock_ensure_cj, mock_gdal_open):
+        # Setup mocks
+        mock_exists.return_value = True
+        
+        # Mock file handle for manifest CSV
+        mock_handle = MagicMock()
+        mock_open_file.return_value.__enter__.return_value = mock_handle
+        mock_handle.__iter__.return_value = [
+            "tile_id,xmin,ymin,xmax,ymax,global_min,global_max,out_res,tile_key,tile_min,tile_max\n",
+            "tile1,1000,1000,2000,2000,0,100,1025,1_1,0,100\n"
+        ]
+        
+        mock_ensure_cj.return_value = "dummy.json"
+        mock_load_cj.return_value = {"CityObjects": {}}
+        mock_collect.return_value = (
+            [[10.0, 10.0, 5.0]], # vertices (local to tile)
+            [([0, 0, 0], "WallSurface")] # faces
+        )
+        mock_load_ortho.return_value = None
+        
+        # Mock GDAL for heightmap
+        mock_ds = MagicMock()
+        mock_gdal_open.return_value = mock_ds
+        mock_ds.GetGeoTransform.return_value = (0, 1, 0, 3000, 0, -1) # Origin (0, 3000), 1m resolution
+        mock_ds.RasterXSize = 5000
+        mock_ds.RasterYSize = 5000
+        
+        mock_band = MagicMock()
+        mock_ds.GetRasterBand.return_value = mock_band
+        mock_band.GetNoDataValue.return_value = -9999
+        mock_band.ReadAsArray.return_value = np.array([[50.0, 50.0], [50.0, 50.0]]) # Return 2x2 array
+        
+        cfg = Config.default()
+        cfg.work.heightmap_vrt = "dgm.vrt"
+        cfg.export.manifest_path = "manifest.csv"
+        
+        export_buildings(cfg)
+        
+        # Verify ReadAsArray was called with a window
+        args, kwargs = mock_band.ReadAsArray.call_args
+        # ReadAsArray(xoff, yoff, xsize, ysize)
+        self.assertEqual(len(args), 4)
+        xoff, yoff, xsize, ysize = args
+        # tile1 starts at 1000, 1000 (xmin, ymin) -> (xmin, ymax) = (1000, 2000) for GDAL
+        # xoff = (1000 - 0) / 1 = 1000
+        # yoff = (2000 - 3000) / -1 = 1000
+        self.assertEqual(xoff, 1000)
+        self.assertEqual(yoff, 1000)
+
+if __name__ == "__main__":
+    unittest.main()
\ No newline at end of file