What is your concern with the existing foundation and veneering new concrete? You could use pressure treated runners ripped and scribed with 3/4" exterior tongue and groove plywood as a sub floor, but you are going to want to get a reputable carpenter in there that knows what they are doing in order to get a decent/flat surface. Going over the existing is going to mean scribing each runner independently to a series of string lines in order to pull that off, and not everyone is capable. If you go that route, you might want to consider using foam board panels inbetween the runners with the runners and all over a 6 mil visquine barrier. Personally I would drill 3/4 - 1" holes every four feet everywhere in the existing slab about a couple inches deep, and pour out fibered concrete. It doesn't finish well because of the fibers, but it eliminates the need for wire meshing and is great for veneers that aren't full depth slabs such as yours.

Plywood and sleepers would allow room for some rigid foam insulation.

Terminology wars, east vs. west :-)

"Sleepers" are commonly referred to as "runners" out here. Not to be confused with carpet "runners", table cloth "runners", or marathon "runners". Everything about west coast terminology is confusing sometimes!

Thanks to both of you for responding.

@sierraeast - My concern with concrete is the fact that the room is already closed off and accessible from inside the house only. Not sure if I will be able to get the equipment in.

One stupid question - What dimension sleepers should I consider? I am guessing that for height ~2 inches but any recos on the width?

Concrete pumping collaborates with the mixer truck to pump concrete in situations such as yours typically through a window. It's more expense and more help needed with one person at the window controlling the hose to keep from damaging the opening.

You can get two rips out of a 2x6 pressure treated length, ( 2x6 = 1-1/2" x 5-1/2"), for the 2" sleepers. To do it right though, each sleeper will have to be scribed to any imperfections in the existing slab or the sleepers will have to be shimmed in areas, or both in order to get you a decent flat joisting system for the plywood to lay nice and level. The 1-1/2" width is fine similar to standard joisting, but I would run them at 16" on center minimum and double them up where the plywood ends butt. It would be layed out similar or same as floor joisting or decking. You can run blocking perpindicular to the sleepers where the sides of the plywood,( the long side), run as well.

Hey Macv & Sierraeast;

Assuming the garage floor was flat & level, is there a type of rigid foam insulation that you could just lay flat and then lay plywood over? No sleepers/runners? I guess not or you'd have mentioned it - I suppose it'd crush down over time, even though the weight of people/furniture would be distributed by the layer of ply on top.

(I've done the scribed sleepers for my in-law's outdoor deck over a slab and that was a bit of work.)

Thanks,
Bill

Hi Bill, I've seen where ridgid foam boards were layed down,(glued), and then 2x4 sleepers layed flat were tapcon screwed through the foam boards and into the existing slab, lated out for the plywood. I guess that would be another option if the existing slab is fairly decent as the foam would no doubt contour to any minor imperfections in the existing slab.

I searched and found this thread as I have a very similar situation. Please follow up and tell us (me) how you accomplished your project, and how it turned out. My area to be leveled (filled in) is about 10' x 16' and the depth ranges from about 1/2"-3/4" up to 2". I'm leaning toward the 'drill holes and use fibered concrete' idea as the finish is not my main concern, level is!

sorry for jumping into this thread, but I have a similar project and I have to raise the slab floor 2 1/4 inches. I have decided to use 2x4 sleepers with 3/4 plywood on top. I am trying to find the best way of securing the sleepers to the slab, and from what I have seen a powder nailer, such as a Ramset is a common solution. I tried drilling holes for tapcon screws using the tapcon bits in a hammer drill, but they don't make any impression on the concrete. If I go up to a 1/4" bit I can make a hole, but I can't find tapcon screws that large. I am a bit nervous about using a ramset because I have heard that they are dangerous to use. Just how dangerous are they if used correctly? Do I need a bullet-proof face visor and kevlar gloves, or is that overkill?

Any other ideas for what fasteners I could use?

Use a 3/16" bit for 1/4" concrete screws. That's a standard size.

Treat powder-actuated tools like guns and you (and those around you) will be fine.

Here is a link that might be useful: Ramset Safety

worthy, the problem I am having with tapcon screws is that I can't find any bigger than 3/16" and I can't drill a hole smaller than 1/4".

Also, I don't see how I can treat the ramset like a gun - I can imagine no scenario where I would press the barrel of a loaded gun to a concrete slab and pull the trigger