Most people know that keeping cats indoors is good for them and wildlife, but many worry their cats do not get enough fresh air and exercise living inside. This is remedied easily with an outdoor enclosure connected to the house by walkways, where they can patrol their domain or sunbathe.
Analyze Your Cat
No, don't send her to a shrink. Merely make mental notes about her energy levels and interests, whether she is a couch potato who lies around on a window sill all day or an adventurer who climbs on and pokes her nose into everything. Assess her responses to games and other stimuli to decide whether her new walkways should offer entertaining ups-and-downs, and exciting twists-and-turns, or merely go from point A to point B without fuss. You want your cat to use the walkways, and the best way to get her to do that is to make it according to her preferences.
Some Basic Design Considerations
The walkways should conform to your cat physically, as well as temperamentally, so measure your cat from head to foot, and nose to rump before getting out your building tools. As for width, healthy-weight cats will fit easily through any opening that measures the width of their whiskers. In general, a floor 10 inches wide with a ceiling height around 18 inches should work fine. That is wide enough for her to turn around in if she wants to come back inside without going all the way out, and allows two cats to pass each other, if you have more than one.
Laying the Groundwork
Now that you have an idea what you want to build, look over the available terrain. Sketch simple plans of the yard, noting special features and areas to avoid. Decide which door or window you will use to build the entry/exit from the house and measure it carefully, as well as its distance from the ground. Begin sketching the run and walkways, keeping your cat's personality in mind. If she is adventurous and energetic, include a few extraneous bends or dips in the walkway, or add a detour through the branches of a large tree to make things more exciting. Build some wider areas within the walkway, where she can survey her domain, watch birds from a shady perch, or take a nap in the sun.
Safety and Comfort
For safety and comfort, use welded or woven wire mesh big enough for good ventilation and easy visibility, but not so large she can squeeze her head through it; ½- to 1-inch square, 16-gauge mesh is reasonable, but anything larger may tempt fate. Make solid plank walkways to protect kitty feet, then arch wire up and over to enclose the top and sides completely. Alternatively, tack the mesh to a wooden, box-like framework. Either way, make provision for drainage, such as using two narrow planks side-by-side with a 1/4-inch gap between, so rain won't stand on the wood, leading to mold or mildew that can harm your cat's respiratory system. Avoid using treated lumber as these contain poisons, such as arsenic, which are extremely dangerous for pets and children.
No matter how well-designed, runs and walkways will need occasional cleaning. Make the chore easier with human-sized entrances into runs, and small clean-out doors at intervals along the walkways. Those also may serve as emergency kitty exits if you need to get your cat out in a hurry. Hose off walkways periodically to head off unpleasant accumulations of fur and hairballs, and provide litter boxes in runs to avoid contaminating the ground. An unsanitary run soon will keep you and your cats away, so keep it clean and enjoyable for all.
Avoid placing runs in front of doors, gates or windows, or where they must forever be stepped over or ducked under as well. Unless you can afford to give up your whole yard to your cats, keeping the walkways around the perimeter of the yard works best for the rest of the family.
Notes on Indoor Walkways
For those who prefer to keep kitty completely indoors, except for the fact that there is no necessity of enclosing the walkway in wire or waterproofing against rain, there is little difference in designing indoor or outdoor walks. Kitty still needs a solid platform and an interesting layout -- with access to a sunny windowsill or two -- and you need to ensure that the shelves and walkways do not interfere with normal operation of windows, doors or cabinets. Never put a walkway above a hot surface, like a stove or furnace, and be aware of potential hazards posed by electrical wires, lights or outlets within your cat's reach. Hang walkways using the studs in the house walls as support, exactly as you would put up wall shelves. Paint them the same color as the walls to blend, or paint in contrasting colors to make a feature of them. Carpeting a few areas for comfortable cat naps is another possibility.