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. Mar 5, 2016 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, 2016 at 4:49

3 Answers 3


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, 2016 at 4:51
  • Very good. I'll update my answer.
    – Iggy
    Mar 8, 2016 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, 2016 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, 2016 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, 2016 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, 2016 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???? Mar 9, 2016 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, 2016 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. Mar 9, 2016 at 4:26

So nobody gave any ideas on a waterproof membrane.Tisk.Tisk.

well first is to slope the floor. you have quite a few options. then obvious like everyone stated is use a wedge under the plywood.since your cutting it out and replacing it. that would be a good option.

All slopes that interact with water. (except roof pitch and pools) Building code minimum 2% slope(1/4 inch per foot) That goes for a balcony,concrete patio or soild floor deck.To Slope away from the house or to a drain(outdoor,indoor or shower).Also the pipes for sceptic,sewer and runoff drains should slope a minimum 1/4"per.ft.

Now the floor boards depends on what type of waterproofing you decided to go with. I will start at at the top. easyiest&fastest install durable least likely to fail longevity neefing replace,normal breakdown of materials

#1 Gavinsized corregated(flooring) sheet metal with flashing on sides. Then 1/2" concrete board on top. Waterproof ready to tile.It wont fail,warp,leak,mold,rot,get termites ,withstand 180mph hurricanes,moving flooding water(maybe a small tsunami) that can last, possibly centuries. Although I cant say the same for rest of house. Unless its also a galvinized steel framed home.(like Australian homes.Which honestly was sure how they would be made on the future(our future)Im baffled we dont have them here. Because I was learning how use the framecad program for the machine to make steel framed homes. Way Back in 1996 taking advanced drafting in high school.

, #2 Replace OSB and roll on the OSB coats of Aquaguard,Redguard or another water proof membrane. to trowel on and dry enough to roll 3-5 coats on top. Nexf day start tile.

#3 Remove OSB put down 3/4 plywood. then roll on a cheaper waterproof membrane(Henerys roofing elastomeric, or go oldschool and slop a ton of asphalt emulsion everywhere.Then cover it with roofing tar paper. stalpe down rows startng af bottom and overlap one above it 3-4"inches.Then screw down 1/2 concrete on top of that.but after screting thinset on bottom side of each concrete board first. (technically you could just use the elastomeric, or alsphat emulsion in sections without tar paper or thinset on the bottom if the boards before screwing down. but it would be extremely messy and wasteful.) Without a coatinf like redguard thinset wont (doesnt really) bond to a wood surface or tar paper. it is mainy used to fill any holes or voids under the concrete boards so that they are leess likely to flex and crack tiles or pop tiles off because houses move alot Especially a 2nd story.

#4 Keep exsting OSB and fix dips and create new slope on top. get.thinset and screet OSB filling voids, use 1/4" concrete board on top cut around any humps and screw board down then fill and flatten between boards around humps with thinset.

since balcony is 5ft before edge and aleady have one concrete board over entire surface .
". heres an easy way to make a 1/4" per ft slope. because each concrete board is 1/4" thick. just cut a row of concrete boards 4ft wide and put on top agajnst the house . then put a 3 ft wide row on top aganst wall. Then a 2ft and 1foot so it looks like 1ft 1/4"inch high steps. cut one more strip less than 1 inch wide at the top put againstw wall and screww them all down. Mix up some thinset a bit thicker to fill in steps. screet it with level or flat stick and you got your perfect 1/4" per ft slope next day after it dries roll on waterproof membrane on top and its good to go for tile next day after it dries.

you can also use the step technique on the boards under the OSB To make a new slope without having to try and make a bunch of giant long skinny wedgdes all the same hopefully correct slope. With out cutting your fingers off as well. Either buy some 1/4 wood strips or cut 1/4 strips from a 2x4 on a tablesaw.

then just cut each one on top 1 ft shorter with a 1 inch piece on top.staple nail,screw, the end of each step so if wont ever move.Then place your board on top. it works totally fine and the wood just flattens over time anyway.


  • This could use a good edit to clean up the typos. Capitalization, proper spacing and complete sentences go a loong way toward improving readability and the usefulness of an answer to the OP and the general public.
    – FreeMan
    Nov 28, 2022 at 13:59

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