I purchased 3 5/8" wide prefinished oak flooring and I am getting quotes for its installation (appx 2200 square feet). One installer said that due to the width of the flooring he'd have to both glue and nail it down to prevent cupping.

I was under the impression that this was only necessary on 5" and wider planks. Is this a better idea than just nailing or is this an unnecessary thing to do? The price for doing both is significantly higher.

Also, the manufacturers instructions for installation don't even mention the word glue - it only has instructions for nail down installation. I'm a little worried that doing this could void the warranty. I'm calling the manufacturer about this but any experience you've had with this issue would be helpful.

Thanks in advance.

No need to glue.

Couple of things :
-Glue / flooring adhesive wont prevent cupping .A glued floor will still cup most of the time and the cupping is caused by moisture from below the flooring , not from lack of holding strength of the fasteners.

-The only time an adhesive will block moisture is if you use the same system that is used for gluing flooring to concrete slabs in which case a moisture blocking material is also used and is full spread across the substrate .

-IS your plywood subfloor 3/4" thick ? If it is 1/2" then perhaps the installer feels fasteners alone wont be strong enough to keep the floor tight.

Cupping is moisture related. If you have moisture problems they need to be addressed first and foremost. Biggest swings in r/h come from uncontrollable crawl spaces and that's where I see most cupped floors are over poorly designed vented crawlspaces. If your over a heated/cooled basement most problems can be controlled by use of a dehumidfier during spring and summer. A 35/8 floor is a Chinese import and those can be all over the board as far as moisture content goes. If your installer doesnt have a meter then send him packing and find someone who does.

Thanks everyone. I wondered why "cupping" was referenced. I did call him back and asked about it. He's now saying its not "cupping" that's the issue but glue helps with squeaking and keeping the floor in place. Maybe I misunderstood him the first time. The subfloor is 3/4" thick. He said its just his preference to do it this way as he gets better results. We're still considering several quotes and others did not include glue so I was really curious if this was necessary or not. It doesn't sound like it is; it sounds like it is the installer's personal preference. However, it still makes me wonder which is the better way to have it done.

Its not a bad practice its just seems like overkill in your situation. But no one here has been to your jobsite, its impossible for anyone here to give you an absolute answer without actually being there.

There's nothing wrong with using a professional adhesive for the install but as mentioned, it is overkill .To avoid squeaking and movement, it's important to secure the subfloor to all joists with screws if it isn't already.

Then during install, use 2 inch staples .These hold significantly better than L shaped flooring cleats/nails.If you use adhesive , you can either full spread or you can partially spread by troweling the adhesive in a straight line perpendicular to the floor direction. You could space the adhesive lines every 8 to 10 inches .This method combined with stapling would yield a super stable floor but use less adhesive and cost less money versus a full spread of adhesive.

Not sure if anyone mentioned it but if you glue right to the subfloor and don't add an additional underlayment, your going to have large issues getting it up if there's a failure or something.