# Screw size for top layer of double 3/4" plywood subfloor



## personalt (Feb 15, 2006)

*Double 3/4" Subfloor - Do you screw top layer in to joists or just 1st layer*

I am replacing some moldy plywood sublfoor (boiler cracked in basement under apartment and nobody noticed for 2 weeks).

The subfloor is two layers of 3/4" plywood. Of course I will change direction and stagger joints of the layers but I was wondering what others do as far as screw length for the top layer.

Do most of you screw the top layer in to the joists with long screws or do you just screw it in to the first layer with screws in the 1-1/2" range.

I feel like there is pluses and minuses to both ways.


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## avenge (Sep 25, 2008)

No do not screw the second layer to the joist.


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## mako1 (Sep 1, 2013)

Agree with avenge's post you do not want to screw that into the joist.Use 1 1/4" screws on the top layer.


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## mako1 (Sep 1, 2013)

Just reread the post and wondering why you are putting two layers of 3/4" down.What is the joist spacing and what is going over it?For the subfloor I would use something like Avdvantec (sp) that is T&G and glue with some PL and screw to the joist depending on what you are doing.Your question is a little vague for a good answer?


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## personalt (Feb 15, 2006)

Thanks.. that is what I figured.. I am sure there is some technically reason that has to due with expansion but it just seems to make sure sense to do not attach to joist. 

The sucky part is I doubt I can get smaller screws then 1 3/4" for my Senco collated Screw gun. Or at least in without getting them shipped in....

As to why I am using two layers, mostly because I am cutting out the subfloor in two rooms and the previous contractor used two layers. If I dont replace with 2 layers the subfloor height will not line up with the other rooms. I could shim it by putting strips on the top of the joists but I think this double layer is done for fire rating as it is an NYC apartment over a boiler room. . It certainly isnt because of span, the joist spacing is super close - 12" in most places. 

The apartment is over the boiler room. The old boiler for the building(size of a fiat or minicooper) cracked and made the boiler room a sauna for a week before someone noticed. The moisture caused my clients floor to buckle. The plywood is still at 15%-20% moisture 20 days later. 

My guess is the double plywood is to try and increase the fire rating slightly or cut down on boiler noise. I am leaning to the fire rating as the joist cavities also have 2 layers of Roxul. The boiler room ceiling is a thin layer of concrete over mesh so I think they tried to do what they could from the apartment side from a renovation done 2 years ago


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## mako1 (Sep 1, 2013)

I would use T&G such as Advantec glued and screwed to the joist with PL400 and then the second layer the opposite direction screwed to the first layer not into the joist.No need for Advantec on the subfloor.You will need to leave some room for expansion .


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## mrcharles (Sep 27, 2011)

3/4" + 3/4" equals 1-1/2"..... so why would you use 1-3/4" screws?

Get 1-1/2" or better yet 1-1/4" screw to allow for some countersinking for your senco.... They are readily available... 1-1/4" are very common for hanging drywall.


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## personalt (Feb 15, 2006)

I am saying that I would likely use 1-3/4" for the 1st layer as that is fairly common size screw for 3/4" subfloor in to the joist. 

http://www.homedepot.com/p/Senco-8-...n-Collated-Screw-1-000-Pack-08F175Y/100149246

Agree - Senco makes a 1-1/4" drywall screw. Actually drywall screws is what was holding the old floor. I guess if they don't snap that would be okay but I usually dont like to put flooring down with drywall screws. At least the cheap ones. 

http://www.homedepot.com/p/Senco-6-...ad-Drywall-Screws-1000-Pack-06A125P/100137909


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## mako1 (Sep 1, 2013)

Agree with you there.I never use drywall screws for flooring.It's just a bad idea for several reasons.


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## mrcharles (Sep 27, 2011)

I was just saying that 1-1/4" senco screws are actually the most popular size I see on the shelf... 

Here is a link for all the senco fasteners including 1-1/4" subfloor screws.... use the locate tool to find a vendor...I looked up for me and there were dozens of places to buy locally, including HD. 

http://www.senco.com/fasteners/underlaymentptnshgp/


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## Morning Wood (Jan 12, 2008)

I think they make 1 1/2 subfloor screws don't they? Maybe it's just 1 1/4 and 1 3/4. I'd use the 1 3/4 for screwing ply to ply


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## MarkJames (Nov 25, 2012)

1 1/2" at least, depending one what you have, preferably smooth-shank for the 3/4" below the heads (to pull the sheets together).


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## EricBrancard (Jun 8, 2012)

First, don't change direction on the top layer. The strength axis is still in the direction going across the joists. Just stagger both sets of seams. Second, what kind of flooring is going on top? The reason to not put the top layer screws into the joists is for isolation. Under carpet or hardwood, it doesn't really matter. Under tile, it's better to avoid it.


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## fjn (Aug 17, 2011)

Depending what is to go over the second layer,would dictate what type of glue I would use.If it is ceramic I would Not use PL. Michael Byrne the tile guru explains the "ribbons" of construction glue leaves room for flexing. He suggests using a yellow wood glue and a small notch trowel and be sure to have 100 % coverage .


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## EthanB (Sep 28, 2011)

I just bought a Senco last week. Grip Rite makes a 1.5" subfloor screw.


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## madmax718 (Dec 7, 2012)

Its not for fire rating. Its because that room gets mega hot, and super noisy, or they have moisture intrusion from the boiler room. 

if it were me... I'd put down sheet metal, and seal it with silicone. Fire block the edges with spray foam. 

If your client says that room gets incredibly hot/damp/noisy in the winter, consider ripping 3/4 ply and raising the height. Then sandwich 3/4" foam inbetween the rips. 

Top off with another 3/4 in ply. 

Im not a huge fan of Ply on Ply- moisture plays havoc on ply and buckles everything.


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