Proper Orientation for Plywood Subfloor under 3/4" Hardwood

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My title pretty much asks the question, but for slightly more detail: I am prepping to put in some 3/4" hardwood flooring on the upstairs (2nd floor) of my 1970s house. My subfloor has had only carpet on it for the life of the house (I'm pretty sure) and is in pretty good shape. However, there are a few plywood butt joints that are not flush with each other - they are off by as much as 1/8" (see photo. yes, that's a Lego to show scale - it was sitting nearby).

here's a more zoomed out photo

I'm wondering how best to fix the problem - I'm no expert, but even I can tell that hardwood flooring will not sit nicely across that 1/8" height difference.

The plywood is nailed down, not screwed, and one idea is to put some screws in and see if that tightens it up. I thought I'd ask the collective wisdom on here before doing that.

Another interesting and possibly salient detail is that the plywood subfloor is nailed only every 4' which I am pretty sure is because...

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Re: 3/4 T&G Hardwood Floor Underlayment Options

I have just finished a project from hell..The house is 12 years old builder's model home with 3/4" solid white oak installed over 3/4" ply. One corner in the dining room and another beside the fireplace is moving up and down when stepped on. There is no indication of water damage or cupping.When question the homeowner, found out that there were some water leakage around both places years ago(DR-3 years ago, Fireplace 6 years ago).The customer believes that was minor and "should have been dried by this time".And adds that there were no cupping of the wood at that time.Measured with moisture meter;15% where the floor is moving, the rest is 8%.Despite that no cupping.
Ok, the only way to find...

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Thanks for the replies gentlemen. We finally decided on a floor, 3/4" Bella Cera Kempas and I'm going to do the carpet/underlayment removal and put down the new underlayment. We are doing a little over 500 sq. ft, including a small hallway, dining room, and living room/foyer. I've continued reading the forums for additional information and tips, but still have a couple of questions on preparing the sub floor (yes it is 3/8" plywood). I want to do this right, but I also don't want to over engineer either.

1) Plan is to use 1/2" ply as the new underlayment. Is 5/8" plywood needed, as I'm trying to keep the height under control as a few of the other rooms which the new hardwood is going to meet up with original hardwood to the house. The old hardwood is 3/4" x 2 1/4" hardwood nailed to the 3/8" plywood sub floor.

2) BCX, CDX or OSB? From reading many of the posts, and even the sticky, OSB seems to be the least preferred. So BC or CD?

3) The current 3/8" plywood...

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The subfloor is the bottom-most layer of your floor. There are several materials you can use as subfloors, depending on the type of finish flooring that you are using. The most commonly used types of subfloors are wood and concrete, but there are other subflooring alternatives that you might consider. The installation process including moisture and thickness of the subflooring material are critical for a successful installation so make sure moisture, pattern, and glue/nail use to attach the subflooring are adequate to support the installation.

Wood Subfloor

If you are installing a wood subfloor be aware that you can use a nail-down method, a floating installation or a glue-down method. Normally, and for economical reasons, wood subfloors are made of old wooden planks or, plywood.

No matter which installation method or product you use, it is important to remove all nails and screws and to make sure that the wooden planks are leveled. We recommend using the...

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# Posted: 3 Aug 2012 10:22
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I'm trying to decide between solid and engineered hardwood flooring in my unheated cabin. The sub-floor is 3/4" Advantech plywood. Unless you all tell me otherwise I'm not planning any insulation or vapor barrier under the flooring. The floor joists are 2x8 and the cabin will be on a post foundation a foot or more above the ground.
Monthly average temps range from 36 to 76 and monthly rainfall is about 4 inches.
My cabin is only 12x12, so I'm drawn to a wide-plank, long-length floor to make the space seem bigger and less visually busy, but I'm concerned about the stability of solid hardwood given that I won't be controlling temperature and moisture levels.
The engineered flooring will probably be more stable, but the lengths tend to be really short, like under 4 feet, and getting longer lengths of the engineered stuff puts me in the price range of the really gorgeous wide-plank solid floors.
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Installing Plywood Subfloor: Plan Ahead

Plywood is rated for use, based on whether it’s for interior or exterior applications, and what type of material you plan to use for the finished floor. Available in standard sheets of 4 X 8 ft (1.2 X 2.5 m), your work will be easier if you minimize the number of cuts you need to make to cover the floor. Lay out your floor carefully before you start attaching sheets to the joists. Also, plan the layout so that the cut edges are against the perimeter of the room. Make sure the sheets are spaced evenly and butt each sheet tightly. The first step is to snap a line across the floor joists to mark the position of the first sheet of plywood.

Installing Plywood Subfloor: Prepare Floor Joists

Apply a consistent bead of construction adhesive on the floor joists.

Installing Plywood Subfloor: Laying the First Piece

Keeping a space of about 1/8 inches (3 mm) away from the walls to...

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A floor underlayment is installed over a subfloor to provide a smooth surface onto which carpet, hardwood, vinyl or other finished flooring can be installed. The most commonly-used type of floor underlayment is plywood, as it is durable, strong and able to be sanded to a smooth finish. Plywood sheets are laid perpendicular to the subfloor, with no common joints and no adhesives between the layers. Sized in 4-by-8-foot sheets, you can install a floor underlayment to wood subfloors in a surprisingly short period of time.

Snap a chalk line along each line of fasteners connecting the subfloor to the floor joists.

Place a sheet of 3/4-inch plywood with the sanded side facing upwards into one corner of the room, perpendicular to the orientation of the subfloor. Slide the sheet two feet away from the corner, keeping the long side of the plywood in contact with the wall.

Measure a 2-by-4-foot section of plywood from the end of a full sheet of plywood with a tape...

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The subfloor is the decking installed on top of flooring joists; the finished floor is then installed on top of the subfloor. The thickness of plywood panels in a subfloor is regulated by the spacing of the joists. This number is known as a span rating. When installing a plywood subfloor on top of the slab at grade, the thickness is governed by how long the fasteners for the flooring will be.

Joists and Spacing

Floor joists are made from 2-by-8-inch or 2-by-10-inch lumber, or from engineered quiet beams, which are made from a thin layer of chipboard on edge between two lumber caps forming an "I" shape. Floor joist spacing has traditionally been 24 inches on center, or from center to center of the joists. Newer construction has floor joists spaced at 16 inches on center. You will need to determine the spacing before installing the joists, or measure the distance between joists for existing construction.

CDX Plywood

CDX plywood is traditional wood...

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Lay out several rows of loose boards along the length of the floor, following your plan. Mix boards from several bundles to avoid noticeable color changes from one bundle to another. Arrange lengths, wood-grain patterns and variations in board colors to create a balanced look. To avoid confusion when installing random-width strips or planks, lay out the boards in alternating courses based on width.

Choose long boards for the first row and align them along your chalk line with the board tongues facing out toward the center of the room (image 1). Place the first board in the row so that its groove end aligns with the chalk line at the end wall. Be sure there is a 1/2-inch gap between this row and both the side and end walls.

The first row needs to be face nailed. To prevent the wood from splitting, drill pilot holes along the gap edge every 10 to 12 inches then top-nail the starter course with 10d finishing nails (image 2). Try to make sure they go through the sub-floor...

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