# Plywood underlay to subfloor - size screw?



## ygeorgiay (Jul 25, 2008)

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.

Thanks.


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## angus242 (May 1, 2008)

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


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## JazMan (Feb 17, 2007)

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. 

Jaz


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## ygeorgiay (Jul 25, 2008)

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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## HotRodx10 (Aug 24, 2017)

JazMan said:


> 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?
> 
> ...


I agree - 3/4" screws won't bite in enough to countersink flush, so they'll be sticking up all over the place.

In case anyone reads Jazman's post and thinks that the difference between 1/2" ply and 5/8" is not much, 5/8" ply is about 50% stronger and almost twice as stiff as 1/2" ply. Btw, one layer of 3/4" ply is a little stronger and nearly 70% stiffer than 2 layers of 1/2".


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## XSleeper (Sep 23, 2007)

1 1/4 deck screws would be fine.


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## chandler48 (Jun 5, 2017)

12 year old thread.


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## XSleeper (Sep 23, 2007)

What the heck!!


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## HotRodx10 (Aug 24, 2017)

chandler48 said:


> 12 year old thread.


My apologies. I don't know how it ended up on the first page of new posts. A glitch with the new and improved site?


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## Oso954 (Jun 23, 2012)

Beware of the “recommended reading”. 
If you look below, the most recent I see is 2014.


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## HotRodx10 (Aug 24, 2017)

Yeah, now that I'm aware of it, I'll make sure to look at the dates before posting comments.


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