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Install Plywood Underlayment for Vinyl Flooring

Floors, Vinyl October 2, 2009 Sonia

We load our sheets of underlayment material on sawhorses for easy layout and cutting.

Vapor Barriers

Moisture content in the subfloor should be low before installing underlayment. In some cases a vapor barrier should be used between the subfloor and the underlayment, particularly for first-floor applications when using particle board underlayment over a subfloor other than exterior-grade plywood.

If a vapor barrier is not required by code, you need to decide if the subfloor provides protection against moisture. If it’s car decking (tongue-and-groove dimensional lumber) with gaps and knot holes, a vapor barrier is a good idea. Vapor barriers are usually polyethylene sheets (4 mil or thicker) or asphalt-layered kraft paper.

Determining the floor break in kitchen areas often requires a careful examination of the cabinet plan.

Floor Breaks

OK, your subfloor is ready, the material’s been selected and protected with the proper vapor barrier and now you’re ready to start running that new circular saw, right? Hold on there, Sawyer—you gotta figure out your floor breaks next.

We use a T-square to mark underlayment sheets for cutting. Look in the drywall tool area to find a good T-square for your underlayment install.

Floor breaks are where the vinyl meets the carpet (or whatever other flooring is adjacent to the flooring you are preparing to install on your underlayment). The edge of your underlayment is the place where the vinyl and the carpet meet. If, for example, you are installing vinyl in a bathroom, you will want to make the floor break under the door going into the bathroom.

Set the depth of your circular saw to about 1/4″ beyond the material thickness.

If the door swings into the bathroom, then the floor break will extend into the door opening about 1 inch from the face of the drywall inside the bathroom. If the door swings out of the bathroom (such as into an adjacent walk-in closet), then you’ll extend the underlayment into the door opening until it is about an inch toward the bathroom from the inside face of the wall of the adjacent room. In other words, figure out where the door is and place the edge of the underlayment right under the center of the door panel.

We use Porter Cable’s new 18-volt Lithium 6-1/2″ circular saw for our underlayment project.

There are a few exceptions I make to this floor-break-under-the-door rule. One is in the case of bifold or bypass doors where there is vinyl on one side and carpet on the other. Because bifold doors tend to be a bit higher off the floor and bypass doors do not line up with each other, it’s often preferable to have the floor break on the inside of the closet area where it is unlikely to be seen when the doors are closed. Just imagine the flooring behind the doors peeking out into the living area and you’ll make the right call.


After notching the sheet so the floor break extends into the door opening, you can then layout other cuts as measured inside the walls.

When the floor break is in an open area such as a transition between a nook area and the living room, you’ll have to decide how to make that happen so it looks best. If there is an archway or half wall on either side, it’s generally preferable to split the flooring types in the middle of the dividing wall or arch or cased opening where there is no door.


The preferred method for carrying a sheet of underlayment is one hand under and one over. This gives you good control to negotiate hallways and door openings.

In the case of a kitchen or bar area, you need to know the cabinet layout so you can make the floor break at the edge of the outside cabinet box. If there are cabinets on both sides of the opening, plan the floor break to go in a straight line between the outside corners of each of the outside toe kicks.

Be careful not to damage walls and tubs as you maneuver sheets into place.

Sheet Orientation

Once you’ve decided on the floor breaks, you can measure, cut and install your underlayment. In most cases I begin with the biggest piece I can fit in a room (usually a 4-by-8-foot sheet) right in the door opening so you avoid putting seams in the areas that will be walked on most.

Keep the piece on plane with the down edge parallel to the wall as you ease the piece into position.

Whenever practical, run the long direction of the underlayment sheets in the opposite direction of the subfloor material so they cross each other. This adds to the floor’s structural strength. Avoid placing sheets of underlayment in such a way that the seams of the underlayment correspond or line up with the joints in the subfloor.

Check the piece for parallel to the walls before tacking it into place.

If the floor break takes place in an area other than under a door, I prefer to lay the first sheets along the floor break edge and work back into the vinyl area of the room. This allows me to cut and fit the smaller pieces against the walls and under the cabinet areas where the tolerances are less demanding. Also, if you start with a nice straight factory edge along a carefully established floor break it will greatly improve the transition between flooring types.