I have a patio attached to my house (like in the image below) like in this picture

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?
  • 1
    The slopes I find by Googling range from 1/4" in 12' to 1" in 5'. But, some of these values are for board-style decking where the water can fall between the boards, which isn't the case for you. And, snow seriously retards or even stops draining. – Daniel Griscom Mar 5 '16 at 18:12
  • Yes, I am thinking about making it a 1/4" slope over 1', that is a 1% slope. I just don't know how to add slope to an already existing flat surface. – wilsjd Mar 8 '16 at 4:49

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.

  • So I am tearing up the existing rotting OSB deck top. Beneath that is good framing to which I don't know how to add a slope. Are you saying that I can add slope with cement sealer? It is the "adding the slope" that I don't know or understand how to do. – wilsjd Mar 8 '16 at 4:51
  • Very good. I'll update my answer. – Iggy Mar 8 '16 at 5:06
  • Hmm. Interesting. So under the OSB is some "subfloor" decking that sits on top of the joists. I have a door on high side of where the slope should start and only have about 5 inches from that threshold of the top of the subfloor-deck. Can I build the slope on the existing deck subfloor or should I take that out too so I can build the slope directly onto the joists? – wilsjd Mar 8 '16 at 17:54
  • Sorry, I left the subfloor out...almost hourly distractions this week & I apologize for it affecting your potential resolution. I'll update again. But yes, options 1 & 3 can go on top of the subfloor. – Iggy Mar 8 '16 at 21:36
  • 1
    Oh come on, she'd love it. It's shiny :) . Yep, tile is plenty fine, but don't go smooth, get a slip resistant tile & you'll want to seal your grout lines (re-seal every 5-years too). Grout's pretty porous & you still want to avoid seepage due to cold temperature freezes, additional weight on the structure & possibly saturating everything beneath...including the sub-floor & framing, cheap & easy grout sealing is well worth it. – Iggy Mar 12 '16 at 14:59

just rib out the osb, rip some lengths of spruce 2x4 or 2x6 to give you the slope you want. just make these tapered cuts so that the minimum thickness is approximately 1". nail and construction adhesive glue them to the tops of the existing joists and then put new a new subfloor and a rolled asphalt roof on top.

you can use a laser transit or level to establish the datum for the existing structures slope. you can rent these everywhere or just buy a cheap one. once you know how it currently slopes or crowns/bows/saddles, you can tune your surf cuts to give you the exact slope you want so that the whole thing is perfect when done.

  • What if the existing subfloor is good? Just my OSB is rotted, but the subfloor (it's like a deck) still seems good to me, but I don't think it is sloped at all. – wilsjd Mar 8 '16 at 17:56
  • ???? your construction should be like this (vertically up): 1) joist, 2) subfloor, 3) membrane, 4) decking. so your osb should be your subfloor (subfloor is by definition below the flooring). i am a little lost as to what you have going on. please clarify. it almost sounds like your osb is on top of the subfloor - meaning the osb is the flooring???? – personal privacy advocate Mar 9 '16 at 2:06
  • Yes, that is right. I have the joists, then 2x4 decking "subfloor", then osb as the top layer. It's terrible; hence, the need to change it. – wilsjd Mar 9 '16 at 2:50
  • so, if i read this correctly, the 2x4s run perpendicular to the joists, on maybe 16" centers? that means they are acting as purlins and you could do your sloping by modifying them. if the 2x4s are laid perpendicular, on their flats, lets say abutting each other, then thats a different story. you could just lay your surf cut strips on top directly above the joists. – personal privacy advocate Mar 9 '16 at 4:26

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