Our cat, Kiki, likes to sit and look out our front window. We used to have a nice catwalk for her made of laminated fiberboard with carpet on top that she would climb up to for food and to spy on our neighbors. Fiberboard shelves are pretty flexible, so we had originally supported it with brackets screwed to the window frame. Last year we had our damaged wood windows replaced with vinyl ones, and vinyl windows almost always come with a warranty that can be voided by drilling into the window frame. We needed to replace the catwalk with a new one that would be rigid enough to hold her but only be supported at the ends.
Kiki is about 12 years old and is not able to jump straight from the couch up to the level of our upper window, so she also needs a step in the middle that is wide enough for her to jump onto and turn around. The last one we had was made of fiberboard, but it was damaged a bit when we took it down, so I decided to make a new one.
The old shelf just sat against the wall and window, but the new window is recessed into the wall. I picked up a very straight pine 1×10 board from the hardware store for the long shelf and used the remaining piece of wood from our stemware shelves to make the step shelf.
I cut notches out of the corners of the shelf so that it tucked into the recessed window opening. This wood can split pretty easily, especially when it experiences temperature cycling in a window, so I made sure the inner corners were smoothly rounded to reduce the stress concentrated there. I also cleaned up the corners of the wood by picking off splinters, gluing down big chips, and sanding. This is extremely important, because the last thing I want is for Kiki to get a splinter from the shelves.
For both of the shelves, I applied two coats of Danish oil for water-resistance and a soft finish. This was probably a good idea just for the water resistance, but the smooth wood did not provide enough grip for Kiki. When we tested the shelves out the first time, she fell trying to jump onto the middle shelf. I took them down and carpeted the tops so the she would have more grip when jumping.
Now that the shelves are installed, she has started climbing up there again to look out the window and eat her food.