Level the sleepers after laying taped XPS on the floor to inhibit moisture rise and insulate.

See Building Science Corp. here.

We've lived in homes where basements were done exactly as you propose. Rot and mildew everywhere.

I didn’t mention the membrane - but it was always my plan to build the shims over a membrane . (I live in a cold climate)

I was more wondering if my the gaps I intend to allow between shims are close enough together so there’s no bounce in my plywood.

Dr. Lstiburek suggests dimensional lumber. It seems you could plane those boards as necessary.

Or simpler, perhaps, use a concrete floor leveler. If you haven't used a leveler before, consider getting a pro for that. Or do a sample area first. It doesn't flow like water!

Leveller is out. Too much leveller needed . Way too $$$.

You don't mention the type of slab. A full 4" slab should not require more than SLC. A mud slab (<2" thick) in an old house should not have a floor built over it.

I'm a bit unclear on the 6"x6" - you are referring to a square piece of plywood you will stack as necessary to shim a subfloor? If it's a uniform slope, shims @ 16"o.c. seems doable, but if it's a mudslab and there are pockets and bulges everywhere and anywhere, then you will probably not get your 16"o.c., you might have a 30" or a 40" here or there or the shims are sitting on top a ridge instead of getting the full 6"x6" supported. And subfloor should be 3/4" nomimal if it's spanning and you are having habitable space.

It’s a concrete slab. At least 4”. Slab is in very good condition. Just has a gradual slope from one corner going kitty corner to low point at sump pit. 2.75” difference from high to low. House built in 1964.

And yes I intend to stack the 6x6 pieces as needed to level up to the high point. And then tapcon the stack thru the membrane and concrete.

Question: what is “SLC” ?

Thanks

Duh - self levelling cement. Oops.

3/4” osb luckily is only a few bucks more per board. I have no problem using it versus 5/8”. I just want my floor to not have that hollow feeling .

Why do you need it level? Most flooring just requires flat to within 1/8" over 10'.

My basement is 28’ wide. With a 2-3/4” slope. And most of the slop occurs over 15 feet. So that’s a pretty steep drop over 10’. Much more than 1/8”.

Fair enough. I would do the PT wood sleepers then like mentioned above.

Sounds like the floor is sloped to a floor drain. I'd be surprised if the slope was only meant for a sump pump, and if it was, that might be telling about the possibility of water intrusion.

Sleepers would be the best method, but I can't imagine you have the headroom to lose. If it's just 15' of the 28', I'd first have my room layouts consider the slope, then I'd personally keep it cementious or gypsum leveler (depending on thickness) and no subfloor.

I have a 7.5 ft ceiling. I should have some buffer to work with. I have no floor drain. If there ever is water intrusion , it would go under the dimpled membrane, and slope towards the sump and/or evaporate under there. I am designing it to keep the wood dry - barring a flood, which there’s no real safeguarding against .

And I contemplated adding a small step in the middle of the room where one section is higher than the other . I envision people tripping over this constantly , so won’t be doing that.

I think I’m going to go with my original plan. And replace the 5/8 OSB with 3/4”. I will lose anywhere from 4” to 1” of headspace. I think I can live with that.

I would have loved to use SLC. But I have an average of approx 1.5” over 550 sq feet.

At $40 a bag of SLC, the math is grim.

Also, I’d much rather keep the slop and have the water move to the low point if ever I had a leak.