Gonna try to be diplomatic.
It's extremely hard on elderly pets to be changing their living arrangements like you are proposing doing. Any change is very upsetting to them, especially when they can still basically see, smell, and sense where they used to stay which will be the case with your elderly girl.
Talk to your vet about pills to help with incontinence. Layer news paper, or put down puppy pads to catch the bowel movements. Look into getting her dog diapers. Do what it takes to let her live out her remaining years where she is most comfortable. Your deck can be sanded and refinished when she passes on.
I do understand where you are coming from, I do know the frustration of having to deal with accidents, I went through it with my two former dogs. My girl became incontinent, but loved to sleep on my couch, I put down a baby, rubber backed, mattress pad on the couch at night so that she could sleep up there and not ruin my couch. I eventually got her the incontinence pills which did help but I still wonder what it did to her overall health. She was younger then your dog though.
My old, elderly, boy had pee accidents when I left the house, even if it was only 10 minutes, due to anxiety caused by going suddenly blind. I put him in a small area and papered the floor with taped down puppy pads. We worked though that and the accidents lessened, then came the accidents from him going senile and forgetting to let me know he needed to go out. I got really good at timing taking him out on a set schedule and that helped with those accidents. The last week of his life I scrubbed my floor because of his nearly uncontrollable diarrhea, and did my best to keep him on the puppy pads. All that took place over the course of nearly 2 years. I never once considered making him stay outside where he would have been extremely stressed, he gave me his all in his younger years, he deserved my all in his elderly ones. I scrubbed the heck out of my carpet after he passed away.