Step 1: Cut the Angled Aluminum

Cut the angled aluminum to size using a miter saw:

nine 36" pieces for the cross supports
two 60" pieces for the front legs
two 72" lengths for the back legs (adjust the height of the legs according to the height of your window)
two 24" pieces for the side shelf supports
two 20" pieces for the side shelf supports
two 16" pieces for the side shelf supports
one 48" piece to attach to the house

Step 2: Build the Frame and Walls

Use the two 60" pieces (verticals) and four of the 36" cross pieces (horizontals) of angled aluminum to build the front of the frame (Image 1). Have one shelf sit level with the bottom of your window. Pre-drill holes in the aluminum and attach the pieces together using nuts and bolts. In order to install the mesh (hardware cloth), it has to be put in place while building the frame. Lock it into place while attaching the 36" cross pieces to the 60" supports. Once the cross pieces are attached the mesh will be held in place (Image 2).

Assemble the two side walls. Use the 72" pieces for the back vertical legs (these pieces will be against the house). Use the 16", 20" and 24' for the side horizontal cross pieces. Angle the 16" and 20" pieces so two of the inner shelves slant downward. You may want to adjust the length of these pieces depending on how much of a slant you want. Attach the mesh while assembling the frame (Image 3).

Use two of the 36" pieces on top to attach the front and back verticals. This will leave an overhang on the top, front of the structure.

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    Image 1

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    Image 2

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    Image 3

Step 3: Cut and Install the Shelves

Cut the shelves from the plywood:

one at 24" x 35-1/2' for the top shelf
one at 19" x 35-1/2" for the middle shelf
one at 16" x 35-1/2" for the bottom shelf

Cut out a corner from both the top and middle shelf for the cats to have access to all levels. The cut out should be a minimum of a 6 inches square. Cover plywood with indoor/outdoor carpeting. Glue and staple the carpet into place. Set floors into place in the structure on the shelf supports.

Step 4: Add the Scratch Wall and Finish Framing

Cut wood for the scratch walls to size. Drill a 3/4" hole in one corner and insert one end of the twine and tie a knot to secure. Spread glue evenly on the plywood then tightly wrap the twine around the plywood. Finish the wall by drilling another hole in the opposite corner and tying off the twine.

Attach the 48" piece across the top of the back of the structure (attaching the two 70-inch pieces). Attach the rest of the 36" pieces across the back to the structure enclosing the plywood shelves.

Drill the holes in the Plexiglas and in top frame pieces. Do not attach yet, wait until the structure is attached to the house.

Step 5: Attach the Structure to the House

Install the cat door into the window according to manufacturer's instructions (Image 1). Depending on the amount of space between your window and the structure, it may be necessary to create a tunnel. Use plywood and bendable PVC plastic to construct the tunnel.

Attach the frame to the house using the appropriate hardware for the home's siding. Attach Plexiglas roof using nuts and bolts and silicone adhesive.

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    Image 1