Getting started on my install of 1/2" engineered, and the first step was to pull up ~60 sq ft of parquet glued directly to the 3/4" plywood subfloor. Wasn't too terrible (about 4 hours with a cat's paw prying up the 6" squares), but now I'm dealing with removing the adhesive. I've got a long-handled razor scraper and I've removed the vast majority if the adhesive residue, but the plywood is by no means "virgin" clean. My questions is, just how clean do I need to get it if I'm going to be putting 3/8" plywood overtop anyway (so I can run my flooring parallel to the joists)? Here's some before and after pictures:

Before scraping:


After scraping:


Good enough? The tiny bit that is remaining is probably the thickness of a sheet of paper at the very most, and that's only in a couple of places. Most spots there's no adhesive at all, the plywood is just stained dark from the adhesive that was there.

Secondly, I've read all kinds of different opinions on here about how to fasten the underlayment to the subfloor, ranging from staples, screws, nails, glue, and various combinations of the above. I found a publication from APA that specifies 3d ring shank nails at 3" on the perimeter and 6" in the field for 1/4" and 3/8" plywood underlayment over wood subfloors, so I'm assuming that's going to be sufficient? The existing subfloor is pretty solid, only a handful of squeaks that I'm going to eliminate with a couple of deck screws. I'll be stapling the hardwood with the HighPro AS-4090, probably using 1-1/4" staples.

If the high spots are only paper thick I think you are fine. I glue and nail. I use PL400 subfloor and deck around the perimeter and than zigzaged in the middle, with nail down I don't use that many nails. About every 6 inches around the edge and every foot in the middle I look at it as the nails or staples from the install are going through also.

Be concerned more on the look. instead of firing up nails, minimize it and put it on strategic spots. paper thick high spots are okay but it really depends on the durability. not all thick can hold and not all thin are bound to break.

24 or 36 grit.

How do you say, Gum Up

Visually, the subfloor looks ready to go. Check for flatness before adding the underlay ply.
Ring nails and glue (PL or the like), are fine for the install.

I was trying to stay away from glue so that if necessary in the future, the underlayment could be pulled out without the hassle of the glue. However, if I use glue, would that cut down on the number of nails I'd need? My calculations using the APA specs I quoted earlier put me at ~200 nails per 4'x8' sheet, which I'm not terribly thrilled about If gluing will cut that down considerably, I'm all for it.

EDIT: Forgot to ask: How far should I plan to get on a single tube of PL 400 (I'm assuming I'll be zig-zagging the bead)?

Well, you don't actually NEED the glue, just a nice extra touch to avoid any potential squeaking possible caused by the residual glue from the parquet causing small spaces between the underlay and the subfloor. BTW, removing that underlay in the future will be no picnic anyway if you use ringed underlay nails. If you are anticipating removing it at a future time, better to just use divergent staples.

I'm not really anticipating removing it, just thought that if something goes horribly wrong and I HADto, it'd be nice to not have to scrape 500 sqft of adhesive (the 60 sqft I'm doing now is more than enough, thankyouverymuch ).

If I could get my hands on a pneumatic stapler that shoots 16ga staples (recommended for plywood UL > 1/4" thick), I'd be all over that; much better than putting down 3000+ nails by hand. Unfortunately nobody around rents one, they've only got 18 or 20ga. I thought about picking up a Senco SNS40/41/45XP or similar on eBay, but that's another $100 at least, and I don't really have time to wait for it to get here.

If you have a Harbor Freight near you... 40 bucks for a 16 ga. staple gun . Just the labor savings from hand nailing 500' would be tremendous.

Believe it or not, a friend recommended I check out Harbor Freight for a cheap staple gun not two days ago, and I looked and found that there's one only about 35 minutes away, an easy drive down the interstate. However, I couldn't find a single 16ga staple gun anywhere on their website; the biggest I saw was 18ga. I did see some combination nailers/staplers that did 16ga nails but only 18ga staples. Got a link for me? I absolutely agree the savings would definitely be worth it and I'll make the drive and throw down $40 in a heartbeat if they have what I need.

Darn, your right. The one I was looking at is a "2 in1", 16 ga. finish, 18 ga. staple, weird gun They do have a few chepo staple guns on there that dont tell the gauge, I wonder if one of them is a 16?. May be worth a call.
16ga. staple guns are a dime a dozen in the pawn shops around here
Hey, with air fare and room packages it may save you in the long run to pick one up out here!

Much as I would love to make a day trip to Vegas, airfare alone would be something like $400 round trip, so I don't think that's gonna happen

Assuming I can't get hold of a 16ga stapler, would you folks recommend a nailer instead? What size/type of nails would I want to be shooting?

I've had that gun for 8 years and it works awesome. I just shoot staples with it for underlayment. Never put a finish nail through it.

Dude, no ones knockin the gun. He was looking for a 16 gauge STAPLE gun, not 18 gauge staple.