non-T&G subfloor

[Prestressed Guy]

Ok, this one is personal.
I recently purchased a new (to me) home built in 1995. We are replacing the carpet and discovered that the subfloor is 3/4" CDX with no tongue and groove. There are gaps up to about 1/4" wide and you can look down and see the joists and crawlspace insulation. The joists are engineered I-joists @16" on-center. I have never seen a non-t&g single layer floor.
Any suggestions on how to proceed? I am hoping not to need to put down underlayment. It is about $20 per sheet and would force us to remove all of the baseboard and door trim to move it up for the increase in thickness.

 

[XR250]

Caulk it and cover it up. 1/2" plywood meets code if I'm not mistaken and I don't believe there is such thing as T&G 1/2" plywood.
That being said 1/2" subfloor feels awful as you can feel it deflect under your feet.

 

[Prestressed Guy]

The subfloor is 3/4". It just is not T&G like I expected it to be. At a minimum I plan to either caulk it or use expanding foam to fill the gaps to keep bugs and smells out.

 

[epoxybot]

Family had a 1970's townhouse with 1-1/4 non-tg plywood. The problems we had were on the second floor. Nails back out as the plywood & joists dried out. Subsequent movement of the plywood, when walked on, caused squeaking. I went through a couple boxes of screws, chasing the squeaks. In some cases I had to run a saw, where two sheets abutted, to stop them from binding & squeaking. A decent non-sag polyurethane caulk seems like it would do the trick for gap filling. I'm not a fan of latex caulks for long-term applications.

 

[Ron247]

Other than really old houses, I have never encountered a floor with only 1 layer of 3/4" plywood or 3/4" planks. They all had a 2nd layer of something on top of the subfloor. There was always a vapor barrier such as tar paper between these 2 layers. The upper layer may be more plywood, a heavy cardboard-like material or particle Board but there was something there to add stiffness to the floor and something to prevent vapor from entering the house from the crawl space. I think 5/8" is the minimum decking thickness for 16" on center.

Unless I was having some problems related to this, I may repair the areas I am already doing other work on but do not think I would change other areas intentionally. The house has been this way for 24 years.

 

[TehMightyEngineer]

Per APA documents this is acceptable for your span if you use 3/4" thick wood strip or wood block flooring perpendicular to the unsupported edges, or use a lightweight concrete flooring. Agree with epoxy that this will squeak over time due to the nails loosening up. Caulk the joints to allow them to flex but be sealed and screw the panels down 6" oc edge and 12" oc field to add additional strength and keep the panels from squeaking.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[BridgeSmith]

If you are concerned about the stiffness at the joints and have access to the underside, you can put either a 1X4 or a strip of 3/4" plywood along the joints between joists and screw them to the subfloor on both sides.

 

[TehMightyEngineer]

I suppose if you really wanted you could also cut little access notches at the joints enough to get some plywood clips onto the edges.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[Prestressed Guy]

Hotrod10 I had thought about reinforcing the joints from below but the entire floor is insulated so that would require pulling all insulation and replacing it.
TehMightyEngineer I do plan on screwing all edges and field and will redrive any loose nails. I haven't pulled any nails yet but they have a waffle head pattern and based on the size of the head are common nails 12d or 16d.
Ron247. I agree that I have not seen this construction for as long as I have been in engineering 1992 and construction 1975. The floor is stiff enough and has surprisingly few squeaks. I am also concerned about vapor, smells and insects coming in. The crawlspace has a 6-mil plastic film over the full area of the crawlspace and perimeter drain tile. It is dry for the most part. We are thinking about installing either laminate flooring or solid T&G board flooring. At a minimum we will caulk or foam the joints, apply a Tyvek tape over the seams and put down a full vapor barrier.

 

[XR250]

The subfloor is 3/4". It just is not T&G like I expected it to be. At a minimum I plan to either caulk it or use expanding foam to fill the gaps to keep bugs and smells out.

I understand. My point was that 1/2 plywood is accepted so your situation is not a minimum.

 

[BridgeSmith]

3/4" CDX should be fairly stiff with the joists that close, even with free edges like you have. I wouldn't try to put something like a ceramic tile on it without stiffening up the joints or adding a substantial backer board, but other than that, you should ok.

 

[Prestressed Guy]

I ended up just putting down 7/16" OSB over the entire floor. It is cheep, stiff and uniform. With the smooth side up it makes a good surface for the floating laminate flooring. It also eliminated a lot of draftiness that was coming through the floor and the rooms were quieter, warmer, stiffer and no more squeaks.
Thanks all.