Subfloor Installation 1: Get the First Sheet Right
The first sheet sets the layout
This three-part video series was shot on a jobsite of David Joyce's, in Concord, MA. Covering squeak-free subfloor installation, it does not include subfloor adhesive in the air barrier system. Because David will wrap the house with housewrap and two layers of rigid insulation, it allows David's crew not to glue the rim so that it can be adjusted after the subfloor is installed.
This ensures a flat wall spanning the second floor that won't telegraph humps through the siding. On a first-floor subfloor installation, the rim can be glued as part of an air barrier strategy.
Engineered I joists are great because they can span long distances to make a flat and solid subfloor. By tying the joists together with blocking, the floor is made stronger and stiffer.
So one of the last things that need to be done before sheathing the deck is to fill in the blocking.
To make the blocks, David cuts sections of i-joist 1-12 inches smaller than the space between the joists. He then nails a scrap of 3x4 inch subfloor to each end of the block, making large nailing fins. The subfloor strips are ripped to be about a ½ inch shorter than the height of the i-joist so that the flanges will not stick above the top of the joists and cause humps in the floor.
He passes the block up to Calvin, who lines it up with the tops of the floor joists and nails it in place. Top and bottom, on both sides.
Meanwhile, the other guys are stacking sheets on a platform for the inevitable lifting ritual. A few sheets are tacked into place as a staging platform, for adhesive, nails, coffee cups, and a radio.
With a safe place for the Dunkin’ Donuts cups, Dave measures to where the first course of sheathing will line up to. Not surprisingly it is at 48 inches from the corner. He pulls the string tight and snaps a line.
David: "Pull that string nice and tight on the arrow, snap that line and now all of our sheets on that line will be nice and straight, the seams will look good..."
Now its time to grab the subfloor adhesive and squirt it on the joists.
"We don’t always do our rim joists, because we’ll have a wall over there, so not worried about squeaks, and we don’t nail that part yet."
For second and third stories like this, David waits until the floor is fully sheathed before nailing the perimeter so that the rim can be adjusted in or out. This will allow him to maintain a flat plane for siding.
"We want a nice generous bead; we have 2-12 inches of joist to cover, we want to put plenty in there so it can squeeze out.
On the joist that is eight feet from the rim, run the adhesive along the edge, rather than the middle, because two sheets will break here, you’ll need two beads of glue.
The first sheet is walked over and carefully placed on the line. It is critical to get the first sheet in the right spot, or it can throw off the whole layout. When placing the sheet, don’t slide it around on top of the joists or you’ll scrape off all of the adhesive.
Instead, walk the corners until the edge is where it needs to be and lower the other edge into place onto the line. Make sure the tongue runs along the edge of the building, and the groove edge is on the line.
—David Joyce is a builder/remodeler in Lancaster MA. This is the first in a three-part series on installing a subfloor.