Home Building and Laying Tongue and Groove Plywood Sub Floor

How to Install a Subfloor with 3/4 inch Tongue and Groove Plywood

By Mark J. Donovan

After you’ve installed the center beams, sill plates and floor joists it’s now time to install the subfloor. Installing a subfloor involves laying tongue and groove plywood over the floor joists to create a rigid deck for constructing the rest of the home.

When installing a plywood subfloor you always want to use 3/4 inch tongue and groove plywood or Oriented Strand Board (OSB). Tongue and groove plywood or OSB sheathing will ensure a solid and strong deck. I prefer plywood over OSB as it is less susceptible to water damage.

If you installed the floor joists on 16 inch centers the process of laying out tongue and groove plywood and creating the subfloor should be a breeze.

Since this type of sheathing comes in 4’x8′ panels the sheathing when installed properly should end up centered over the 2xN floor joists.

The first thing you want to do to install your subfloor is snap a chalk line 48-1/4 inches in from the edge of the rim joist. This will help to prevent plywood sheets from hanging over the edge of the rim joist when installing the subfloor deck.

Start the layout and installation of the subfloor along the longest run on the floor joists without a jog in the foundation wall. This will make your work go much faster and smoother.

Next, after snapping the line figure out which floor joist the first sheet of plywood will break over. Assuming you placed the floor joists on 16 inch centers the edge of the plywood should break over the joist at 8 feet.

Floor joists and laying tongue and groove plywood on top of it.

Once you’ve figured out where to start the placement of plywood panels apply a thick bead of construction adhesive to the top edge of the floor joists where you plan to place your first sheet of plywood. Then place the plywood sheet onto the floor joists and into the adhesive.

The groove edge of the panel should be on the chalk line. While holding the panel to the chalk line nail the leading edge of the panel to the rim joists so that the far end of the panel splits the joist in half. If you do this right the floor joist will have 3/4 inch of exposure left over to allow for the abutting piece of plywood panel to rest on.

While still holding the plywood sheet to the chalk line, place a nail in the rim joist at the first floor joist. Then place nails into the rim joist where the floor joists are nailed into it. These nails act as guides to the floor joists when nailing off the field of nails in the plywood sheets.

Next you can nail the groove edge down, however before doing so adjust the floor joist edge so that the plywood panel evenly splits the joist. Then place one nail in the corner edge of the plywood and into the floor joist.

Now take your tape measure and hook the end of it on the leading edge of the floor joist you just nailed and pull it along the edge of the plywood.

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As you do so make pencil marks on 16 inch centers along the length of the plywood panel. This will enable you to keep the floor joists in line as you nail the plywood down and ensure the next course of plywood panels will break on 16 inch centers as you install them.

Now apply construction adhesive to the floor joists for the next plywood panel. Butt the next panel up against the one you just installed while in the process making sure you hold it to your chalk line. Then nail the groove edged corner of it. Start by nailing the panel to the rim joist like you did with the first sheet. And again, adjust the floor joist so that the plywood panel splits it on center. Again, place the end of the tape measure on the joist and mark 16 inch centers along the edge of the plywood panel. Continue to layout the tongue and groove panels along the entire length of the home.

Once the first course of panels have been installed you can move on to your next course. If you started with an 8 foot panel on your first course start the second coarse with a 4 foot panel.

This means you’ll need to cut a 4’x8′ panel in half with your skill saw. By interleaving 8 feet with 4 feet panels between rows you’ll achieve a stronger subfloor by staggering the courses.

Begin the installation of the second course of plywood panels by applying construction adhesive to the floor joists for the first panel in the next course.

Then stand the plywood panel on its tongue edge next to the groove edge of the panel you are standing on. Make sure that the butt edge is lined up perfectly on the floor joist it is breaking on and then simply let it fall into the construction adhesive.

Afterwards, walk on the panel and adjust it with your feet to pull it in tight with the abutting panel. Most likely the tongue won’t easily slide into the groove between the adjacent panels. Most of the time you’ll need to help it along by using a sledge hammer and a 4 foot length of 2″x4″ (a.k.a. beater block) to help work the tongue into the groove of the abutting panel. The beater block is required to protect the far end groove edge of the panel from becoming damaged with the sledge hammer.

After positioning the panel into place nail the tongue edge in first, but making sure in the process that the leading edge of the panel is breaking over the center of a floor joist. If necessary adjust the location of the floor joist so that the groove edge breaks on the floor joist. Then pull 16 inch centers from that floor joist and mark the plywood panel. In the process, move the floor joists if required and nail them.

To ensure the floor joists at the ends of the building structure remain straight don’t glue or nail them. Preferably let the plywood panels hang slightly over the ends of the floor joists. After laying out all the tongue and groove plywood panels and nailing them down snap a line from one corner to the other and cut the the overhanging plywood sections off with a skill saw. Then move this floor joist to the cut edge and nail it off. This way the subfloor edge should be perfectly straight.

Finally, before installing each panel check to make sure the tongue and groove edges are not damaged. Also, I highly recommend nailing in each course of panel field as you install it. In addition, only apply construction adhesive when your about to lay a panel. This prevents a big mess and potentially a danger. Stepping on a floor joist with adhesive could cause you to slip and fall.

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