Hardwood flooring question

[jlm]

So, I've been researching options for a hardwood floor for LOML's new (old) house (a 1959 ranch with slab foundation), and I keep coming across 3/8" thick solid hardwoods that are supposedly safe to glue down directly to concrete using a vapor barrier adhesive like Bostik Best. These aren't engineered floors, just thinner tongue-and-groove solid hardwood.

My question is this: almost no manufacturer will recommend or warranty a 3/4" hardwood floor glued to concrete (for obvious reasons; it seems like a guaranteed recipe for a cupped, warped disaster down the road), so why is a thinner 3/8" floor considered more stable or less likely to have moisture problems. Seems to me thinner wood should be more likely to warp, not less likely. What am I missing here?

FWIW, my current preference is to put down a vapor barrier of 6 mil poly, cover that with plywood, and nail a solid hardwood to the plywood. It'll raise the floor a bit, but a) those urethane glues are a mess to work with, and I'm not sure I trust solid wood glued to concrete, no matter how great the glue is, and b) most engineered floors in our price range have an excruciatingly thin wear layer. Seems like it'd be pretty easy to scratch right through such a thin veneer.

[newood2]

This is a pic of my basement woodshop. It's the 12 X 12 3/8 parque with 1/8 foam backing.It's installed on bare concrete with adhesive. I leave 1/4" gap around the walls(hidden by baseboard) for expansion and contraction. My basement is very dry - no dampness, still in the summer months of high humidity some tiles will rise up, but it goes back down in the fall. It's a minor problem that I'm sure a dehumidifier would resolve. It's been 10 years and it still look great.
At the time of installation I also thought of the plywood underlay with 3/4 flooring on top, but I would have
lost an 1 1/2" off my 93" ceiling height.
If you go with the 3/8 make sure the slab is flat throughout.
If your installation is above ground with A/C in the summer you may not have much to worry about. I,m no "pro" just my 2 cents opinion. Good luck.
Howie

[crokett]

I glued down an engineered hardwood floor in my daughter's room almost 4 years ago. It has worn fine and looks good, even allowing for it being a kid's room. It was one of the lower priced ones from Lowes. My subfloor isn't but it is rated for being glued over concrete. I'd take engineered over solid wood on concrete. Another option is some of the floors are now rated for floating/gluing or nailing. You might look at floating the floor.

[JimD]

The living area in my walk-out basement is laminated oak flooring glued directly to the concrete. It stays flat and is wearing fine. The scraps make good zero clearance throat plates.

Jim

[chopnhack]

I have installed nearly 1k sq ft of floating hardwood floor in Florida - still very nice after 2 years. Just make sure you do your prep work well and you will be happy. Avoid glue down in my opinion, to hard on the feet!

[dkerfoot]

I suspect they don't recommend gluing down the thicker floor because when it does cup it will be more likely to overcome the adhesion of the glue and pop. Thinner floor, less force?

I installed a floating engineered bamboo floor in my basement after doing a ton of work to waterproof the basement. I was very relieved when we got the 5" of rain in under two hours and had flash flooding around town, yet not a drop in the basement.

You might consider painting the floor with Zinsser watertite. It contains hydraulic cement and creates a water and vapor proof seal. Two thick coats are required. It also can provide some moderate "leveling" of low spots. I wouldn't glue flooring to it though - I think the adhesive might pull it up. I still used a top notch vapor barrier/foam between the watertite and the flooring.

I regret not also installing plywood sheathing first. After installing my flooring, I discovered two slightly "squishy" spots where the floor wasn't quite as level as I thought. It wouldn't have been a problem with shorter/thicker flooring, but my planks are 7' long and 5/8" thick. That is a long span to cover and my 3' level wasn't long enough to pick up the gradual slope. Most other people never notice it, but I feel it every time I walk across the floor.