The first step in laying plywood over the floor joists is to snap a chalk line. This gives you a straight line to follow. I always snap this line at 48 1/4" in off the edge of the rim joist. This ensures that in the course of installation the plywood (which is 48" wide) will not hang over the edge of the rim joist. It doesn't matter whether you start in the front or back of the house. For best results, start where you have the longest run without a jog in or out in the foundation.



After you have snapped your line, determine which joist the first sheet of plywood will break. If the joists where laid out 16" o.c. (on center) from the end of the building, the edge of the plywood would split the joist at 8 feet. Sometimes the roof layout determines the floor joist layout. This is usually the case when the roof is a hip roof. In this instance start with the joist that will allow the plywood to cover all the joists, even if it hangs over the edge of the first joist. This will be cut off later.

After you have determined where to start, apply construction adhesive to the top edge of the joists. Apply no more than 48" the width of the plywood. Lay the first sheet in the glue with the groove edge on the chalk line. Holding the sheet to the line, nail the leading edge of the plywood to the rim joist so it splits the joist. You'll be covering 3/4 of an inch of the joist with 3/4" exposed. Still holding the plywood to the chalk line, put a nail in the rim joist at the first joist. Now put nails in the rim joist where the floor joists are nailed into it. When nailing off the field these nails can be used as a guide to find the joists.

Now the groove edge can be nailed. On the leading edge, move the joist so the edge of the plywood splits the joist. Once you have the joist where you want it, nail the plywood to it. Now taking your tape measure, hook the leading edge of the joist you just nailed, and pull it along the edge of the plywood. Mark 16" centers on the plywood and pull the leading edge of the joists to this mark and nail them. This will help keep the joists in line and will help make sure future course of plywood break on 16" centers no matter which joist you start with.

Glue up the joists for the next sheet. Butt the next sheet to the one previously installed, making sure to hold it to the chalk line and nail the groove edge corner. Nail it to the rim joist just like the first sheet. Once again move the joist so the plywood splits it. Hook that joist with your tape, mark centers and move the joists to the lines. Keep laying the plywood in this fashion to the other end of the building.

Your now ready for the next course of plywood. If I've started with a full 96" sheet on my first course, I like to start my second row with a 48" piece. This works great if the building length is in increments of 4 feet (24',28',32',36' etc.). This isn't always the case. If the building is an odd length you can usually use your ending cutoff to start the next course. Stagger the joints a minimum of 32" apart.

Start the next course by gluing the joists. Do not apply the glue more than 4' beyond the first course of plywood. Stand the sheet on its tongue edge next to the groove edge of the sheet you are standing on. Make sure its butt edge is lined up on the joist it is breaking on and let it fall into the glue. As it hits the glued up joists, step on it and try to pull it in with your foot. Only under the right conditions will the tongue go completely into the groove. Sometimes the sheet can be jumped into the groove. This involves standing on the sheet and jump with force towards the the sheets in the previous row. In most cases it takes a sledgehammer and a 4' to 6' 2x4 beater block to persuade the sheets together. The block keeps the groove edge from getting damaged by the sledge. This is a two person operation. One stands on the tongue edge of the plywood to guide the sheet into the groove while the swings the sledgehammer. This will be the process for the rest of the installation.

Once the piece is in place, nail off the tongue edge, making sure the leading edge is breaking on a joist. Move the joist so the groove edge breaks on the joist. Pull 16" centers from that joist, mark the plywood, move the joists if necessary, and nail them off.

To keep the joists at the ends of the building straight, do not glue or nail them. Ideally we'd like the plywood to be hanging over the ends. After all the plywood is in place, snap a line from one corner to the other and cut this over hanging plywood off. Move this joist to the cut edge to make it straight and nail it off.

Tips:

- Field can be nailed as you go (recommended to set sheet in glue) or after all plywood is in place.

- do not glue more than 4' out from sheet. Keeps glue off your tape when you pull centers.

- When nailing groove edge, nail at least 2" from edge to keep from collapsing groove.

- Before installing plywood, check for damaged grooves and tongues.

Mike Merisko (c) 2006