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ygeorgiay 07-31-2008 12:41 PM

Plywood underlay to subfloor - size screw?
Hey folks, I'm a DIY newbie - just moved into my first house after years of apartment dwelling. I've been reading these forums for a few days and really appreciate people's willingness to share their knowledge.

I'm putting down 1/2" plywood underlay on a 1/2" plywood subfloor in prep for laminate (over 2x6 16" spaced joists)

My understanding is I shouldn't be fastening the underlayment through to the joists, but rather just to the subfloor.

What size screws should I use? Since my total depth is only 1", should I be using 1" or even 3/4" screws? Or is it ok or even preferable to use 1 1/4" which will protrude through the bottom of the subfloor.


angus242 07-31-2008 04:04 PM

I'd say 3/4". This way, you can recess the screw head some into the ply so you don't have any protrusions that may affect the laminate.

Good luck

JazMan 07-31-2008 07:42 PM

I disagree, 3/4" screws will not work, not enough bite.

Please double check the thickness of your subfloor, and try to read the markings from underneath too. No way should any subfloor be just 1/2" Is it possible another layer was removed because of re decorating in the past?

In any case, if it is only 1/2 thick, adding another 1/2" may still not be enough? Years ago many homes where built with a 2 layer sub system consisting of 1/2 + 5/8". That is the minimum I'd recommend.

Then you say you have 2x6 joists, YIKES! What is the unsupported span of those joists? Measure from face to face of the supports, accurately.


ygeorgiay 08-01-2008 11:36 AM

Sorry Jaz, I mis-spoke about the joists. They're 2x10s. I'd just got done telling a friend how I put some 2x6 blocking in to support a patch of subfloor that I'd cut out.

There had been another layer of 1/2" or maybe it was 5/8" fiberboard or particleboard - I'm not sure what the right word for it is but whatever it was had been terribly damaged by pet urine so I tore it up and decided to put a new layer of BC ply down. The one area of SF that I cut was also because the pet urine had seeped that far down.

I can't find anyplace on the SF where the stamp is still readable, but from the piece I cut out -- it really measures 1/2".

It's a bedroom - and I'm putting laminate down -- no bath/kitchen/tile involved. So do you think the 1/2" SF and 1/2" underlayment will be enough in that case? And 1" or 1 1/4" screws?

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