Do NOT use plywood over damp cement outdoors. In fact, don't use plywood over any concrete whether it is outdoors or not.

Contact your local carpeting store and ask them how they will go about installing your new indoor/outdoor carpet.

Thanks for taking the time to answer, but I thought this was a DIY forum? I must be on the wrong forum. 'Hire a professional' could be the answer to literally every question asked on any DIY forum.

Houzz is about home improvement, home building and is a place for professionals to offer up FREE advice to those looking at projects or who have dilemmas around a current build.

I may be tagged as a private poster but I'm a flooring expert. What I've learned over the years (and the 'Best Practices' tell all of us) is wood should NEVER touch concrete. Ever. Not without a TREMENDOUS amount of work.

Your enclosed porch may LOOK like an indoor space, but it is obviously acting like an outdoor space. I'm going to guess this space USE TO BE an outdoor space. That means the slab would have been poured like a garage slab = without a moisture membrane. And THAT'S what is causing it to be damp at times.

When concrete touches soil the concrete will take on all the dampness the soil below it has. Concrete is porous. It is the hardest sponge you will ever find. You can improve the drainage all you want, you will still have the same issue = slab sitting on raw earth. The water in the soil is going to evaporate through the slab until you remove the slab and do it again with a vapour barrier underneath.

When concrete touches soil the concrete will take on all the dampness the soil below it has. Concrete is porous. It is the hardest sponge you will ever find. You can improve the drainage all you want, you will still have the same issue = slab sitting on raw earth.

Plywood glued to concrete is, at the best of times with the use of all the bad-boys of the adhesive and sealer industries, is touch and go. Now you add in the 'exterior grade' situation and you have one big bucket O' mess.

Some things have simple solutions and others, like yours, have more complicated solutions. The solution you propose is not viable. One of the best solutions is to remove, clean/grind the concrete and install more of the outdoor carpet. If you don't like that solution then it is time to bring in a professional.

Thanks for that info. I don't think the porch was ever an outdoor area but i can't guarantee it has that moisture membrane. I read some info on adding carpet for a basement which is where I got the idea for the plywood but it makes sense why that is not the same as my situation. Thanks.

Oh, and if I go the rout of cleaning / grinding the concrete would some kind of vapor barrier help between the concrete and outdoor carpet?

The vapour barrier would be something that is sprayed or rolled on (2 coats). After that you apply a primer. Then you get to coat the concrete in more concrete (called a micro-topping). Once all of that is done and the new concrete has cured (1 day per 1mm) you can then put down the indoor/ourdoor carpet.

When professionals do the blast/spray/prime/microtop, the cost is around $5/sf. Then the flooring and labour to install is extra.

Thank you. I'll try and find a local flooring company and discuss the treatment you described. Sounds like that's what I need.

It's more of a sunroom than a porch. On the 'back' side is a sliding door leading into the kitchen, directly opposite that is a glass sliding door leading to the backyard, the left wall is another sliding glass door leading to the yard and the right wall is a solid wall. The moisture is rising through the slab, not coming from rain through screens or anything like that. But the previous owner and myself definitely don't treat it as an indoor room, like you said it's just astroturf laid on top of concrete, not even glued down or anything. I honestly use it more for storage but on some nice days I'll open the screen doors and have dinner there, though that's rare.

@Mike A (First, I am so sorry I referred to you as "her!" For some reason I read your name as "Mika." Sorry about that!) Assuming you're sure there are no leaks under all those sliding glass doors (I believe you, I'm just mentioning it in case--I recently read an article about a slab that extended past the walls/doors of the structure, which led to water seepage, which the homeowner initially thought was coming up through the slab), then yeah, I believe your main options are applying a waterproof coating (available at any home store), or laying waterproof sheeting and re-coating it with concrete as @SJ McCarthy described (or, of course, ripping up the slab and re-doing it).

I'm tempted to wonder if you could put some kind of stucco down, but I doubt that would work.

Personally, I'd try the elastomeric coating first, because that's the easiest and least expensive, and you can DIY that. But that's just me; you have to decide what you're comfortable with in your house.

Best of luck to you!

Thank you for all of the advice!