I'm planning to put in the purpleheart flooring in two upstairs
bedrooms. It is solid hardwood, about 5/16" thick with tongue and
groove milled onto the edges. It comes in random lengths of about 12"
to 7', about 3" wide.
The subfloor is 2" thick slabs of some kind of compressed paper pulp
fastened over the joists, installed when the house was built in 1979. It
seems to take nails fairly well. The carpet strips were nailed down into
it.
I don't think it needs any additional underlayment, except maybe to
allow the whole floor to slide easier with expansion. The reason they
used the pulp stuff was for it to be acoustically and thermally
insulated and softer than hard subfloor. (That worked okay upstairs and
in the living room, but downstairs in the kitchen, bathroom and water
heater closet, it was a disaster after some water leaks soaked it.
nobody has ever heard of this stuff or knows exactly what it is. I've
already hauled 1500 lbs of it to the dump!)
I have been trying to decide whether to glue it down or nail it. I will
probably end up getting a compressor soon, but don't have a nailer. It
would be handy to have one, but the pneumatic floor nailer is
probably a rental item.
My wife thought she saw in a magazine somewhere that T & G hardwood doesn't
require either glue or nails. I got to wondering if I can get by without
either. It seems like fastening down the wood will not let it flex.
I could just facenail it at the edges under the baseboards to keep the
edge rows from shifting, but I can't imagine the planks going anywhere
if they're not fastened down. Maybe just an occasional blind nail ever
few feet? I suspect that I can just let them free float with no
fastening if I force pieces of stiff foam between the edges and the
floor plate, up under the baseboards.
Most the web DIY flooring instructions I've seen talk about nailing.
Some mention that T&G can be free floating but need to be glued together.

Who is the manufacturer? And what do they recommend? Most solid hardwoods are not suppose to be floated.

I'll have to check this weekend when I go back. I don't recall any major name. It was a Lumber Liquidators clearance.
I did see some mentions of gluing T&G planks together. What kind of glue would work for that? I think silicone dabed in spots along the bottom of the groove would be strong, yet flexible, and make it possible to rip apart later.
(I think anyone who installs ANYTHING should keep in mind that someday, be it two years or fifty, has to take it back apart.)

Now thats scary.

This occurred to me when refinishing our old 1927 pine floors(different house).
- strip off old carpet, padding and carpet strip. no big deal.
- remove old 1950s linoleum tiles with big heavy scraper. tedious and strenuous but not too bad. most pieces chipped right off. tested negative for asbestous.
- remove black tarpaper(underlayment?). used above mentioned scraper for the loosest parts. the stuck parts required carbide scraper.
- remove black mastic adhesive. tried solvents, voodoo spel. ended up heating small patches with heat gun to loosen up glue and remove with carbide scarper.
the kitchen, utility room and bathroom are next. in addition to the glue, linoleum, etc. they had 1/4" plywood nailed on top, and peel and stick linoleum tiles(circa 1990?) . tile adhesive is turning into gummy nastiness, plywood is warping and it's like walking on a wimpy trampoline. looking forward to moving on to this part.
all in all, redoing about 1200 sf of this has occurred over two years so far. many times I cursed the original installers and their surviving descendants.

Hello Sharpstick,

Solid wood floor cannot be floated unless you purchase a specific underlayment that warranties a floating installation with solid wood like the Elastilon underlayment. I would first figure out exactly what type of subfloor construction you have. Normally, solid tongue and groove wood that is 5/16" thick is a glue down or nail down installation. You can glue down to concrete or plywood. I would not recommend gluing to gypcrete, particle board, or masonite materials. To me it sound like the subfloor material is like a masonite or particle boad this type of subfloor look like compressed sawdust that is glue together. These types of subfloors will fall apart and crumble really easy when wet and they don't hold nail well at all. That type of subfloor is not recommended to glue or nail too. I would suggest pulling up the existing subfloor and installing a 3/4" CDX plywood in its place. Nail or glue your 5/16" solid wood to the plywood. When nailing use 20 gage 1" long cleat nails or glue with a wood urethane adhesive.

Tongue and Groove engineered flooring can be floated over an approved pad by applying a PVA carpenters wood glue to the tongue and groove of each plank. This is only recommended if the manufacture states it can be done.