# How to install 5/8" Plywood over OSB Sub-Floor ?



## sirjeff (Jan 20, 2010)

Whats the preferred method of installing a 5/8" of T&G Plywood over a 5/8" T&G OSB subfloor ? My OSB Subfloor is in good condition. The house is 28 years old.

1) Basically do I have to install the plywood sheets perpendicular to the OSB subfloor and overlap the seams or should I just overlap the seams with the Plywood in the same direction as the OSB Subfloor ?

2) Should I screw into the joists or just the OSB or both ?

3) The seams of the Plywood, should they start and end at a joist ?

4) Along the seams, should I fill in with some filler or just leave a 1/8" gap and a 1/4" gap along the walls ?

I will be installing 3/4 inch hardwood floor nailed. 

Thanks !!


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## HABSFAN2006 (Feb 25, 2009)

sirjeff said:


> Whats the preferred method of installing a 5/8" of T&G Plywood over a 5/8" T&G OSB subfloor ? My OSB Subfloor is in good condition. The house is 28 years old.
> 
> 1) Basically do I have to install the plywood sheets perpendicular to the OSB subfloor and overlap the seams or should I just overlap the seams with the Plywood in the same direction as the OSB Subfloor ?
> 
> ...


 
They should be perpendicular to the other sheets, with staggered joints.
Use 1 1/4" screws every 18" square, and 3" screws could be used on the joists every 18", but this might be overkill for hard wood floors. Sheets do not have to start and end on joists. Always leave your gaps, it's critical.
The best option, would be to give 4-5 days for your newly installed t&g, and your hard wood flooring to acclimate in the area it will be installed before being installed.


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## jlhaslip (Dec 31, 2009)

The OSB should be running perpendicular to the joist system. go the same direction.
Rip a sheet in thirds length wise to offset the long joints.
Place the new 5/8 ply in the same direction , joints over joists, and screwed with some thing that would bite into the joists. Pattern would be 6" spacing on the perimeter and 12" across the fields.
White glue under the ply would be a good option.


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## JazMan (Feb 17, 2007)

SirJeff,

I don't completely agree with portions of the answers. But, to give you better info you should tell us what you plan to install on this floor.

Jaz


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## sirjeff (Jan 20, 2010)

I have 2 different opinions concerning laying the plywood over the OSB. Should the plywood be installed perpendicular to the OSB or Parallel ? The OSB was correctly installed when the constructed the house 28 years ago.

On the bottom of my post I indicated that I will be installing Hardwood floors. I'm redoing the whole house and it has 2 floors (excluding the basement of course). The Hardwood floor will be Maple (Canadian) 3/4" by 3 and 1/4" width...


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## rusty baker (Feb 13, 2009)

I would suggest you listen to the installers over on the pro forum where you asked the same question. I have installed for 37 years and occasionally, I see advice offered here that I totally diagree with.


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## JazMan (Feb 17, 2007)

While we're here I'd like to answer so that other DIY'rs won't install underlayment wrong. 

Your new plywood MUST be installed perpendicular to the joists, so yes it goes the same direction as the original subfloor. You of course have to offset the seams in both directions.

Jaz


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## sirjeff (Jan 20, 2010)

*This is what I dug up with the help of a member :
*

*
*

*GENERAL RECOMMENDATIONS FOR UNDERLAYMENT*

To assist the contractor, we summarize our ideas for underlayment placement and orientation, panel end, edge, and perimeter gaps, and nailing. The recommendations given for nailing are more conservative than specified in ANSI A108-1999, Section AN-3.4.1.3, which states, “locate nails at 6-inch centers along panel edges and 8-inch centers each way throughout the panel…" The closer nail spacing in Table 1 will better guard against voids between the subfloor and underlayment sheathing layers, improve the composite action of the two layers of sheathing thus reducing sheathing curvature under service loads, and it will increase the buckling resistance of the underlayment thereby minimizing the potential for buckling of the underlayment due to seasonal moisture content changes.

Place underlayment panels (Exposure 1, plugged-face plywood of minimum 3/8˝ thickness) such that the following conditions are met:

1. Butt all underlayment end joints at 1/4-points between joists as depicted in Figure 3. Example: Butt underlayment panels on either side of the joist centerline at: 4˝ for 16˝ o.c. joists, 5˝ for 19.2˝ o.c. joists, or 6˝ for 24˝ o.c. joists. Underlayment end joints should be placed as far away from subfloor end joints as possible.

2. Underlayment to overlap edge joints of subfloor by 1/2 of the width of the subfloor panel (24˝). At restraining surfaces, overlap may be less than the 24˝ when the subfloor panel is less than 48˝ wide.

3. Gap underlayment panels 1/8˝ on all ends and edges, and 1/4˝ at perimeter walls, cabinetry, or other restraining surfaces. 









Description of the type of plywood underlayment and the recommended fastening schedule are given in Table 1.

The following guidelines are recommended when fastening underlayment panels.

1. Use ring-shank nails or screws (no drywall screws).
2. Fasteners should pass through entire thickness of underlayment and subfloor panels with minimal or no penetration into joists. 

Here's the link to the complete article : http://www.schluter.com/5138.aspx


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## jlhaslip (Dec 31, 2009)

Interesting to read that info, sirjeff.

I was familiar with quarter pointing joints on Beams, but never realized the impact could be applied to a sub-floor.

Thanks for the link.


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## JazMan (Feb 17, 2007)

That is all correct. That info was taken from an article co-written by Frank Woeste and Peter Nielsen of Schluter. A link to the article: http://www.johnbridge.com/vbulletin/showthread.php?t=72423

Jaz


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