To flatten plywood subfloor (2nd story) prepping for engineered wood install (float or glue-down), how do you determine whether to lower the high spot or fill in low spots?? If filling in low spots, what material do you use?
Thanks!

Use a straight-edge like a six foot level to find the high spots and mark them with a pencil. Be sure that the sub-floor is fastened well first. The high spots should be cut down with an electric planer, setting nails first with a quarter inch punch and hammer so the blades don't get chipped or an edger with coarse sand paper.
The low spots can be marked for filling using the straight-edge after this step.
Underlayment mix can be used for the low areas. The straight-edge will help to spread the mixture, but after it hardens it can be fine tuned with coarse sand paper on the edger, after checking with the straight-edge.
If you use asphalt saturated felt and cut it to fill the depression like a contour map, you can fill the spots in layers. Fasten the layers with a stapler so they don't move around. You should be able to get within one-eighth of an inch by building up layers, but plan how thick you need in different areas before you start stapling the layers down because there will be some air between the layers until you set the flooring on it and the weight flattens the wrinkles.

Thanks!
OK...Here's where I get confused so please bear with me.
1) Where do I start in with straight edge? (I've got an 8' level)
2) If I'm doing BOTH lowering the high spots AND filling in the low spots, won't I give myself the same problem just opposite? I don't know, if this makes sense but shouldn't I just do one or the other?? How do I know what my "baseline" is...The highest spot, the lowest spot, somewhere in between, where the floor meets the wall, somewhere else???


How can this method be used for glue-down install?

You can use 1/8th inch plywood, or veneer to build up a low area.
Asphalt saturated felt or tarpaper doesn't stick to most glues for sale today.
Use the same glue for the shim strips as for the flooring and you will be fine.
You can use latex self-leveling underlayment compound for the low area, too.
It can be sanded after it sets, usually the next morning for me, to get a more perfect patch. Glues stick well to this after it has hardened.
Some patching compounds require a primer. Read the directions at the store where you get it.

Thanks!
I contacted one of the flooring manufacturer reps. He told me, if I'm gluing the flooring down to just add more glue to the low spots... And this would not void my warranty...Hmmm???

Sure sounds like a guy that hasn't been taught the right things to say.

Use your instincts. You know he's not right.

As far as your questions.

1) Just start anywhere
2) Baseline? You'll discover what's going once one you get going.

In did some videos on this subject earlier this year, but didn't get heavy into the filling of low spots. The guy I filmed became camera shy and didn't want to play anymore. He was a bit on the odd side anyway, but knew what he was doing.

http://www.hardwoodinstaller.com/prepar ... veling.htm

Thanks!
And thanks for the videos.

My instincts won't help me keep the warranty, necessarily.

As for where to start and baseline...
I read someone's post on another board, who may have said it better than I "...sometimes what appear to be low spots are simply the floor surface next to a high spot. Best sequence is to locate high spots first and remove them THEN deal with any remaining low spots.".