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how to lay plywood over an existing plywood subfloor

michellep's picture

I have a 12'x16' Building/cabin. It is a blank slate. 4 walls, roof, door, windows with Exterior sheeting on it. We bought it like this. It has 2x10 floor joists 16 OC and frame. 2x4  exterior walls. But the guy who built it used 1/2 plywood for subfloor. It is to weak and giving in places. I need to strengthen it so it can support a shower room, Kitchenette, and mini set of stairs to a loft. I was planning on laying a second layer of 3/4 plywood but unsure of the best way to do so. Any advice will be greatly Appreciated.

You could lay the 3/4 right (post #214054, reply #1 of 4)

You could lay the 3/4 right over the pattern of the existing stuff but I'd rather break all the joints in both directions by a half sheet. I haven't seen that done in more years than I would care to mention but it used to be not uncommon.

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Reinforce the floor for (post #214054, reply #2 of 4)

Reinforce the floor for ceramic tiles, and it will be strong and stiff enough for any other flooring.  For an occasional use cabin, this may be seem to be a lot more work than warranted, but you will end up with a robust floor. 

The Tile Council OF North America ( TCNA)  Handbook identifies various types of construction for different uses.  In your case, a double plywood floor, identified as  F150-03, is similar to what you are thinking of.

An older version of the TCNA handbook is available on the Laticrete website:   F150-03 is on page 22

As to how to install the second layer of ply, have a look at this:  tileletter-underlayment-prevent-cracked-tile-grout.pdf.  If you use screws to attach the second layer, get a good screw gun.  If nailing, get good knee pads.  There a lot of fasteners to drive.  If using a nail gun, reduce the fastener spacing  to 2/3  shown on the tables, to compensate for the thinner wire in nail guns.

The quantity of fasteneres are required to get both layers working together.  If you leave out the field fasteners, and fasten the second layer to the joists only, the two layers work individually and are not as stiff as the fully fastened floor.  Also, the floor will squeak as the two layers slide on one another.

The process I use when adding layers is:

  • Check existing floor for flat and level, and repair / replace as required.  Shim any gaps between joists and ply, careful not to cause high spots. Fill in low spots with layers of thin plywood ( door skins) to bring to flat.  Glue down shims using carpenters or PVA glue and tack or staple in place. Do not use construction adhesive, as it is too thick and will leave gaps.  Use a belt sander to knock off any ridges or local high spots, and the smooth edges of any fill pieces. 
  • Screw down the existing floor to the joists, making sure there are no squeaks.
  • Lay down additional plywood layer, using the offset and fastener pattern shown in the TileLetter.  Check both sides of plywood  for smooth before laying.  Ensure the two layers are in full contract when driving fasteners, This may require weighing down if your own body weight does not close any gaps.  Work from one corner of the ply diagonally to the opposite corner
  • Check each sheet for flat before moving on the the next.  Check and fix squeaks as you go.  A squeak means there is a gap between the two layers.  Additional nails, or removing and redriving screws one at a time in the squeak area usually fixes the problem.

This is a fair amount of labor.  Power tools help a lot.


Thanks! This will help alot. (post #214054, reply #4 of 4)

Thanks! This will help alot. Not scared of work lol. Have plenty of power tools but got to get my generator working. No power at the cabin. Have already replaced 1 rim joist and sheeting do to rot. Added 4 concrete Footers To support a 8x6beam down center for added floor support. Fixed window framing and framed for new door. We are weekending it currently. But it will be home when we start building our off grid cabin next year. This little cabin will then be a home away from home for our teen boys who will be in college by then so a robust floor fits the bill.

Thank you! It helps to have (post #214054, reply #3 of 4)

Thank you! It helps to have Experienced people to talk it out with. Im a self taught go getter but I second guess myself alot.