I have a patio attached to my house (like in the image below) 
As you can see, it is covered and over my garage. Although it is covered, it still gets rain and snow on it and, unfortunately, the top decking material is just painted OSB and it is seriously rotting. Under the OSB is good flat framing. I am going to rip out the OSB and I want to add a waterproof deck top, but understand that if I do that, I need to add slop.
My main question is this: how do I add slope to the existing frame?
(I am assuming that there is none).
Some bonus questions:
- How would I check if there is slope already? It doesn't appear that
there is slope to the eye. Would it show up on a long level? - Any suggestions on waterproofing material?
Best Answer
Slope is quite easy. 3 ways. First, is to cut full length wedges out of 2x4's or bigger & screw them to the top of the sub-floor at the joist locations. Second, is to remove the sub-floor too & sister-on pitched or sloped 2x4's or bigger. Third, is to use concrete or cement over the sub-floor & new decking, the floor needs to be real stiff to avoid future cracking. Typically, just 1/8" per foot is enough to keep everything heading out ...anything more is better.
Yes, a level should easily indicate if the floor is already sloped outward or not or even sagged in either direction. As far as good waterproof material. I'd probably opt for a 3/4" plywood (not OSB) with Cement Board on top of Options 1 & 2 for all taped & cemented seams & screw heads to then be covered in Concrete Resurfacer.
This would just need an Exterior Masonry Sealer to prevent dusting &, if desired, could be painted with a porch or garage floor paint. Glued-down Aluminum Diamond Plate Panels or Textured Thick Plastic Sheeting would be my next option, seams would just be caulked & painted (so they stay nice looking). I wouldn't try to protect any kind of wood though, as you've discovered it's a failure waiting to happen...this is true whether sloped or not.