Raising sunken den

[bayoufrank]

Looking to raise a "sunken" den, drop is 5 1/2 inches. Plan to use treated 2 X 4 on edge, 12" centers, alternating layers of 23/32 tongue and groove plywood subfloor, and either 1/4 or 1/2 inch hardibacker.

Questions -
1. What is the best way to secure 2 X 4's to the concrete?
2. Should I glue plywood to the 2X4's?
3. Screws or ring shank nails to fasten plywood?
4. Should I ventilate the space under the floor or seal it?

We live in S W Louisiana, not in flood zone,


Thanks

Bayoufrank

[Brian in San Diego]

Frank,

We probably need to get CX or Bob our Ingineer in here to answer these questions. There is a floor system you can install over SOG and I have seen it mentioned on this site before but I couldn't dig it up and looking for it through Google was fruitless. I did come up with the City of San Diego's interpretation of the building code and it may have some interesting points for you. I do know the plywood definitely needs to be glued to the "joists" with something like Liquid Nails or PL Premium. I would use screws to fasten the plywood to the sleepers. According to the literature I read you either have to seal (hot mop 'round these parts) the concrete or ventilate it. I wonder if a roll on membrane like Laticrete Hydroban would work for that purpose. I don't see why not. Provided there is no hydrostatic pressure from beneath the slab.

Brian

[Brian in San Diego]

I forgot to post the link to the city of San Diego building newsletter 23-4. Don't know if it's of any help to you of not.

[bayoufrank]

Thans Brian, looks like I have some more research to do -
I remember my parents building a house in the '50's, and the slab under the oak floor was hot mopped with 2 X 4 sleepers stuck in the tar. I think I'll explore a few options before I use tar - maybe an epoxy/waterproof coating.

After a little more research, I'm leaning toward Ditra instead of Hardibacker, looks like the installation and overall grief would be easier on the installer (me).

Bayoufrank

[jgleason]

Originally Posted by bayoufrank

alternating layers of 23/32 tongue and groove plywood subfloor

Not much to add for you but the mention of alternating layers of plywood stuck out.

Plywood should be laid perpendicular to the sleepers. Both layers. The second layer should be offset from the 1st layer, no overlapping seams. Second layer should be fastened to the 1st layer only avoiding the sleepers.

[bbcamp]

I think CX would recommend filling the area with concrete.

I think I would lay down a vapor barrier (4 -6 mill poly) then fasten treated 2x4 sleepers on the flat using powder driven nails. Set these 16 inches apart. Over the sleepers, glue and screw 2x4s (un-treated) perpendicular to the first set, also 16 inches apart. This second layer allows you to shim and correct for any slope or dips in the concrete. Glue and screw the first layer of plywood to the 2x4s, then install the second layer, without glue. Install your choice of underlayment and tile.

If the tiled area extends beyond the sunken area, you'll need a movement (soft) joint where the sunken area ends.

[cx]

Welcome, Frank. If that's the name you use, how 'bout you change that for us in your signature line so's we'll know.

Originally Posted by Injineer Bob

I think CX would recommend filling the area with concrete.

Yep, he would. He'd prolly argue against unnecessarily introducing a wood-framed floor system with all its inherent problems when a fella has the opportunity to pour a few inches of concrete and top it with deck mud.

But lotta peoples don't pay no nevermind to CX, as we all well know.

[Trask]

I'd put concrete in there toowood over concrete is always a bad idea from what I've seen.

[bayoufrank]

You guys are great! This plan started about a week ago, and has evolved based on conversations and research. Glad I found this forum.

Concrete is a distant maybe, distance to pump and curing times are factors I have to consider.

Now it looks like waterproof layer, sleepers nailed in 2 directions on 16" centers, two layers of 23/32" T&G plywood in the same direction at right angles to the sleepers, no overlapping seams. Ditra or hardibacker over the plywood. The sunken area is about 350 sq ft, the whole area being tiled is about 650 sq ft. I plan to t=run the backerboard/Ditra over the joint where the new and old tie in.

Looking for options on waterproofing.

Thanks

Frank

[dhagin]

Concrete is a distant maybe, distance to pump and curing times are factors I have to consider.

I agree with others, don't put wood over concrete. These are typically nothing but trouble. For the pumping distance, consider "line pumps". These are cheaper than conventional pump trucks and are worth some phone calls to find a guy near you. They work with the redi-mix trucks and drag a hose instead of using a pump & overhead boom.

Edit: For cure time, use Ditra over green concrete and worry no more Check out the Ditra install handbook over there--->

[bayoufrank]

Ok, I'm about convinced to go with concrete - Do I need a moisture barrier between the slabs?

[Davy]

I don't see why you would need a moisture barrier between the two.

Put me down for concrete also. Especially if you plan on bridging tile across the two areas. Once it's poured, I would probably use a membrane over the whole slab just to be on the safe side but that's just me. It's cheap insurance.

[Trask]

I agree, let the new concrete bond to the old and go over everything with a membrane like Noble or Ditra.

[Edthedawg]

Would it be a good idea to drill and pin the old slab to the new pour w/ rebar stakes set about every 24" or so?

[dhagin]

Skipped right over reinforcement for that big thing.... At 5-1/2" & 350 SQFT I'd consider laying 6x6 wire in the center of the new slab, at least, and not pin it to existing slab. Let it break free from underlaying slab if it wants to, I see no problem with that. Not sure what the sunken area abuts, but Frank will have to honor construction joints and provide for expansion at perimeter. I may want to pin it to an adjacent slab or foundation wall as well.

Schluter requires any construction joints to be carried thru the tile work, Noble provides a detail for tiling over their membrane and construction joints. Link to NobleSeal membranes over there too ------>

Any of this makin sense Frank?