converting part of the garage into an office

[Larry Browning]

We are having a storage nook in the garage converted into an office. As a part of this we want the floor to be raised to be even with the rest of the house. This means raising the floor about 6 inches. My solution is to build it up with a 2x6 framework and then lay T&G plywood on that to add the engineered wood flooring to.
My question is, should I lay down a vapor barrier between the cement garage floor and the 2x6 frame? If not, is there anything I should do other than just put the frame down directly on the floor? The little bit of online research is mixed as far as how to do this.

[David Helm]

Did a similar thing in my shop (detached building) so just have a step up into my office from the shop. I did not use a vapor barrier but made the sleepers out of ground contact pressure treated wood. Used foam insulation between sleepers, then T&G plywood subfloor. It is currently five years old and everything is fine.

[Larry Browning]

I don't think the builder is planing on using PT lumber. Should I insist on that?

[John Lankers]

I would use PT lumber on the concrete floor or at least strips of 6 mil Poly if it was mine. I just finished a few walls in my new shop and put poly under the PT bottom plate to be safe.

[Thomas Bank]

Even with the poly, there will likely be condensation with any humidity against the cold concrete. I’d use PT framing with another vapor barrier below the subfloor.

[Jim Becker]

Even with the poly, there will likely be condensation with any humidity against the cold concrete. I’d use PT framing with another vapor barrier below the subfloor.
I agree with this. Vapor barrier, PT lumber for the framing and subfloor, rigid foam insulation between the sleepers.

[Robby Tacheny]

I agree with all above too. We just redid our kitchen and took it down to the studs. It is in the oldest part of our house which is cinder block with untreated 3/4" strips attached directly to the walls and no vapor barrier. We were lucky and the strips were in perfect condition and wall bone dry, especially considering the house is almost 70 years old. The outside wall has probably been painted enough times that it is no longer wicking moisture.

Since the concrete slab is in direct contact with the ground I would definitely put vapor barrier and use PT lumber or at minimum use PT lumber. Better safe than sorry.

Is this your own private woodworking office or one that you'll have to share? :)

Robby

[Bud Millis]

Larry,

You should use PT where it comes in contact on the cement. As far a a vapor barrier just put it on top of the plywood and under the engineered flooring and you'll be fine. Use the stuff found in your local BORG in the flooring section and skip the plastic, which will only invite trouble.


Bud

[Wayne Lomman]

Larry, does the existing concrete floor have a vapour barrier under it? If it does, you would not need a further vapour barrier which can actually create a localised moisture trap. Cheers

[Von Bickley]

Larry, does the existing concrete floor have a vapour barrier under it? If it does, you would not need a further vapour barrier which can actually create a localised moisture trap. Cheers

I agree with Wayne on the vapor barrier. If there is no vapor barrier, I would put one down, use PT lumber, and Advantech for the sub-floor. I would also use construction adhesive on the sub-floor.

[Larry Browning]

Everyone,
I probably should know, since we built the house about 18 years ago, but I do not remember if the contractor put down a vapor barrier in the garage area. Probably not though.
We have never had an issue with the floor being wet for any reason either. As of now, the contractor is using PT where it contacts the cement. He is using 1X6 at 45% over the floor joist frame and then 3/4 T&G plywood over that, then engineered hardwood flooring on that. I don't think he is planning on a VB at this time. If he does put down a VB, where should that go? On the cement? On top of the joist frame? between the 1X6 and plywood? I am thinking all this is going down today, so I have to decide within the next few hours.

[John Lankers]

Everyone,
I probably should know, since we built the house about 18 years ago, but I do not remember if the contractor put down a vapor barrier in the garage area. Probably not though.
We have never had an issue with the floor being wet for any reason either. As of now, the contractor is using PT where it contacts the cement. He is using 1X6 at 45% over the floor joist frame and then 3/4 T&G plywood over that, then engineered hardwood flooring on that. I don't think he is planning on a VB at this time. If he does put down a VB, where should that go? On the cement? On top of the joist frame? between the 1X6 and plywood? I am thinking all this is going down today, so I have to decide within the next few hours.
Here is what I did, the slab is insulated and has vapor barrier underneath.
338702

[Larry Browning]

Here is where we are now.
338707

[Von Bickley]

That will probably never give you any problems. Not the way I would have done it, but different strokes for different folks. Good luck with your building project.

[David Helm]

Unlikely that 18 years ago a vapor barrier would have been put under a garage floor. Typically they just pour the slab over whatever happens to be there without doing any smoothing even. In your progress picture, is the non PT wood off the floor and just nailed or screwed to the pieces of PT? Here in the Northwest, that construction would not fly.

[Von Bickley]

Unlikely that 18 years ago a vapor barrier would have been put under a garage floor. Typically they just pour the slab over whatever happens to be there without doing any smoothing even. In your progress picture, is the non PT wood off the floor and just nailed or screwed to the pieces of PT? Here in the Northwest, that construction would not fly.

I would have used all PT for the floor system. I don't think it would have cost much more.

[Stew Hagerty]

As a former General Contractor, this is how it should be done. If it even has one, the vapor barrier under the slab ALWAYS has holes so using a second one is not only perfectly fine, but I would not build without using one. You absolutely need to use PT lumber against the concrete. While you don't need to, I would place pieces of rigid foam between the sleepers. Roll out a continuous vapor barrier between the sleepers and the plywood. Use T&G Ply for the subfloor, add a piece of Luan on top if you are going to use tile of vinyl.

[Larry Browning]

Unlikely that 18 years ago a vapor barrier would have been put under a garage floor. Typically they just pour the slab over whatever happens to be there without doing any smoothing even. In your progress picture, is the non PT wood off the floor and just nailed or screwed to the pieces of PT? Here in the Northwest, that construction would not fly.


Yes the non PT is off the floor. Here in Arkansas we have mild winters and VERY hot summers.

[Larry Browning]

Yes the non PT is off the floor. Here in Arkansas we have mild winters and VERY hot summers.

Well, it's too late now. The floor is down.
No vapor barrier, but at least he used PT against the concrete. I am really not too worried about it. The concrete has never sweated or been wet in the past 18 years. Our winters are pretty mild here, I think I can count on one hand the number of times it has gotten below zero since we moved here 30 years ago. We didn't get any snow this year. Most years we might get 2 or 3 snows that are more than a dusting.