As you can see in the photo, one of the supporting beams of my balcony has "dry rot."

1) I plan to cut away the rotten wound and replace it with new wood affixed to the remainder of the existing beam. Any thoughts spring to mind? (type of wood, what not to do, etc...)

2) How do I prevent this from happening again in the future?
- Obviously painting it - any particular type of paint?
- Boron wood injections?
- Some kind of sealer?
- Some kind of plastic covering or layer under the floorboards of the balcony?
- Anything else?

 

How are your beams attached to the house? Are they cantilevered out from the house? I'm guessing that you won't simply be able to "attach" to the remainder of the beam. Where they meet the house is often where rot is the worst.

 

Yes, they are cantilevered. Where they meet the house looks okay, but I will take a closer look once I take off the 2x4's that make up the floor of the balcony. (That part is protected by the eves, so that might be why it was spared)

 

Was hoping for some suggestions... What I think I will do it put down plywood underneath the 2 x 4's to keep the water off the supporting beams.

 

I would nix the idea of cutting anything off the sides of those beams. Remove the ballusters, get some wood epoxy and seal the sides up so they are paintable again. Some brands sell a compatable wood hardener for prep. There are a lot of brands so it's hard to single out just one. Depends where you shop or if you buy online.

If it is too far gone, consider adding an additional layer of rough cedar on the outside of the beam just for looks. Keep wood away from the roof 1-2" so it can dry.

If you are taking the decking off, you should cap all the beams with either metal caps or a membrane to prevent them from getting/staying wet where the decking sits on top of them. (Not plywood)