Level a concrete floor for subfloor installation

January 17, 2021 11:10 AM

I’ve got a 10 yr old detached garage that was built on a driveway pad. I’d like to convert it into shop with an insulated floor. I’m trying to figure out the best way to level the existing concrete - as the structure was built on a pad, there’s about 5” slope across the existing floor. I’ve assumed self-leveling concrete is the answer, but 5” is a lot to concrete to add, and it will “climb” the wall in that lowest corner (not sure how else to describe it). What do ya’ll recommend?

After leveling, I’m planning on using something like Dricore for a subfloor and then cork flooring on top of that. I’ve got some experience with subfloor installation, but I’ve never done any self-leveling concrete. Thanks in advance!!

posted by chuntered inelegantly from a sedentary position to Home & Garden (13 answers total)

Would you consider building it without completely leveling it? Five inches is a lot for self leveling concrete. It's a shop not a dance floor so you might be able to get away with having the lowest level an inch or two less than the highest. This might reduce your problem to a more manageable issue.

Have you considered putting down the self leveling concrete in layers, building it up? It dries quickly so you could put down a layer each day for a week.

They don't recommend more than an inch or so of thickness without using aggregate. Aggregate is not too hard to use, but you would potentially need to rent a mixer because of the weight - and at that point perhaps you should just go with making a form and using regular concrete and just using the self leveling concrete on top.
posted by Jane the Brown at 11:33 AM on January 17, 2021 [2 favorites]

Best answer: You could just rip treated 2x6 down to make sleepers, and then deck over that. That would also make it possible to pull out the floor and have a regular garage again, which is not a bad thing either.
posted by rockindata at 11:57 AM on January 17, 2021 [11 favorites]

That is going to be a LOT of (pretty expensive) self-leveling concrete. What if you want to sell the house and a garage is a must for most buyers? That is going to be a nightmare to pull out. You're going to have the climbing-the-wall problem no matter what you do, but I think sleepers are the way to go, plus it will allow you to insulate.
posted by HotToddy at 12:12 PM on January 17, 2021 [2 favorites]

Response by poster: Making sleepers was my initial inclination too. Then I could insulate it as well for a warmer floor. Would I need to put a vapor barrier down if I did that? Any concerns for treated lumber in contact with concrete?

The concrete floor itself is dry, but obviously porous. No visible water seeping.
posted by chuntered inelegantly from a sedentary position at 12:16 PM on January 17, 2021

Are the walls and ceiling insulated? Will it have a garage door with all the gaps that entails or something else? What kind of workshop?
posted by amanda at 12:31 PM on January 17, 2021 [1 favorite]

Response by poster: Walls and ceiling are currently insulated. However covered in plywood which I plan to replace with sheetrock. No garage door - it appears to have had one when first build, but has been replaced with a wall. Garage door removal seems to have been done decently well, it’s fairly seamless on the outside. Planning on making this my music room / light woodworking shop.
posted by chuntered inelegantly from a sedentary position at 12:39 PM on January 17, 2021

Have you considered slab jacking?
posted by Zedcaster at 1:14 PM on January 17, 2021 [1 favorite]

We converted a garage with a sloped floor into a workroom/office and we hired a guy to pour and level aggregate concrete across the floor. Took a couple hours, cured in a couple days. And because it was still a garage, we could just back the truck up and dump the concrete right in.

It worked okay because the wedge was 2" at the shallowest and 6" at the tallest, and there were concrete footers in place around the edge, so no wall climbing. And it was in Florida, and we didn't care about super insulating it, we just put down padding and carpet.

In your situation with reduced access and a smaller "wedge" and walls to protect, I concur with other respondents: just raise the floor with sleepers to insulate as you feel necessary and allow for drainage, and don't worry about the floor not being perfectly level. If there are shelves, large tools or tables you want level, use shims or adjustable feet.
posted by seanmpuckett at 1:17 PM on January 17, 2021 [1 favorite]

Self-leveling cement is not the way to go for this type of job. It's relatively expensive except for very thin layers.

Wood sleepers is the way to go. You shouldn't have to do anything special regarding vapor barrier as long as the surface does not get visibly wet. Homes and garages are framed all the time with wood directly on slabs.

There are two options to arrange your sleepers and the choice might depend on the longest dimensions of your garage, as to which is easier.

One way is to place your sleepers down slope, which means ripping your sleepers on a taper. The other way is to place your sleepers across the slope, so that each sleeper is ripped to a constant width but each sleeper is a different height as you move down slope.

I would plan on ripping with a bit of margin so that you can level the sleepers with shims.
posted by JackFlash at 1:35 PM on January 17, 2021 [1 favorite]

Also, buy a decent laser level. I have this one, and it is an absolute game changer for making uneven things even again.

If there is any chance of flooding, you should run the sleepers downslope so that draining happens.

Finally, I am going to question the idea of replacing plywood walls with drywall in a shop. With plywood you can mount all but the very heaviest things without having to find studs or deal with toggle bolts.
posted by rockindata at 2:17 PM on January 17, 2021 [5 favorites]

I have exactly the same problem and am considering using deck jacks with some kind of diy flooring pallet - some jacks are adjustable re floor angles - here's Buzon from Belgium - and come as short as ~25mm. I priced them for a job and the 25mm ones are NZ$6.50 each with a joist support.

My floor could run partly wet at times so this allows lots of air flow as well as cable duct space.
posted by unearthed at 8:58 PM on January 17, 2021 [1 favorite]

Here's one method I've considered for a few garage conversion projects recently. However, none of these conversions have come together yet and contractors keep pushing folks to just polish the floors and put down rugs so I don't have final advice there. Given that you don't have a leaky garage door and you have insulated walls and ceiling, a polished and sealed garage floor with either some click rubber mats (one example) or area rugs or a combination could work pretty well. Just need to get a nice heater in there! A mini split would be lovely for heating and cooling, especially if you plan to store any musical instruments which need temperature regulation.
posted by amanda at 8:21 AM on January 18, 2021 [1 favorite]

Best answer: I put down sleepers on two layers of 6 mil vapor barrier, and insulated between the sleepers. Down-slope, ripped to the angle, which I did by putting the sleeper in-place, shimming it level, and running a pencil with a spacer down it, to account for the fact that the concrete isn't necessarily flat. Cut along with with a circular saw by eye (rather than with a rail).

No problems so far, a few years in. Also gives a little "give" to the floor, which is nice vs slab.

(That's my garage, in my workshop I laid down 3/4" treated sleepers and put a plywood floor over it, same reason, and I highly recommend it.)

I'm a fan of replacing plywood with sheetrock, for fire resistance. You can always rip a french cleat trim and run that 6" below the ceiling for easy hanging of cabinets and such.
posted by straw at 3:04 PM on January 18, 2021 [2 favorites]