What, is your favourite color?
Bridges are one of the biggest undertakes you can have with a treehouse. If you have two or more platforms, you have no choice when it comes to practicality, adventure and plain sweetness. It's just gotta be a bridge. If you have a love/hate relationship with the ground like I do, and just need some time from each other, bridges provide all the distance you need to declare your independence from the dirt below. It's over between us.
The Plan, Stan
Even the shortest of bridges are going to require some level of engineering. Trees are naturally accustomed to heavy downward pulls, without which we could never build a treehouse to begin with. But bridges produce a sideways pull, completely unnatural to the trees they're going to be in. If your treehouse is supported by posts, great! You solved that problem, though you may still need extensive bracing or possibly even a guy wire or two. I'm all in favour of people doing things themselves, but if you only have limited experience dealing with rope, cable and carpentry related tasks, I ask that you at least consider hiring a bridge builder and check out this treehouse website on bridge construction.
Solid bridges are the easiest and safest to construct if you plan on building a bridge on your own. They don't bounce and sway like rope bridges usually do and are much more comfortable to walk across, especially for little tikes. Keep in mind that with a 2 X 10 joist box, the longest span you should use really use is about 16'. Make sure to always use pressure treated or weather resistant lumber for all your structural building material needs. Remember that all trees sway and are going to move the platforms with them. Constructing a solid bridge without restricting tree movement is crucial to the integrity of the bridge. Check out these special treehouse brackets for floating connections on trees to help give you an idea.
Rope bridges are a lot of fun but dangerous if not installed correctly. They sway and should not be played on during windy days. The best way I know how to build a rope bridge is to have two climbing ropes or cables strung along the bottom connected by I-bolts reinforced into the two platforms with large washers and redundancy attachments. Planks with U brackets should then be spaced about every 2 to 3 feet apart with decking boards screwed on top. Rope railings should be just as strong as the bridge to prevent any accidents down the road. Typical deck code states that spindles should be no father apart than 4" apart. This is almost impossible to achieve unless netting is used to cover the sides and exposed openings.
This is my all time favourite type of bridge. The adventure and facetiousness is found in it's very simple design, working well for very short to 100' or more! It almost doesn’t feel right how far and how much fun you can have with just three ropes making that famous "V" shape. Burmas put much less strain on your treehouse and require only a fraction of time and material. Make sure to put a few connecting ropes to keep the the ropes from flying around. The only gotcha is when you want to transport larger objects that'll require both hands. With the excitement you'll get out of balancing on one rope, seeing the world beneath you, I think it's well worth it.
As I have stated multiple times already, making any mistake with a bridge can cause serious injury. Please make sure you're confident and skilled enough to take on this task, if not, please contact a professional builder. When building any kind of bridge, always use synthetic manilla rope to prevent any compromise in strength and ascetics.
As I said in the rope bridge section, code states that spindles must be spaced 4" or less apart. If you are using spindles for a solid bridge, please keep that in mind. For netting, make sure you can't stretch the mesh openings larger than this about. Children should always be supervised when moving from one platform to another. For further treehouse safety tips, visit the treehouse safety page.