How do you waterproof an indoor/outdoor space? You can use the same idea as flashing a window and create a sill pan.In this video, Matt Risinger shows a new build that has a beautiful covered porch. The porch itself should be sloped enough to ensure water doesn't seep back into the house; however, he still has to address the possibility of a big storm blowing water all the way to the house.On the sliding glass door," you can see how he sets the door into a stainless steel pan that has a lip on the indoor side. Any water that would come up to the door would only touch the stainless steel pan and not affect the flooring.You can use a similar approach on the walls. Check out the full video to see how.