I would like to install a subfloor in my garage and turn it into an office. The floor is slanted all towards the middle where there is a drain. There has never been any water (that I've seen in the last 5 years anyway) come out of it. However, sometimes, when it rains VERY HARD there is a little bit of water infiltration beneath my garage door.

My question is - Can I build a sub floor over top of that? Do i need to leave access to the drain? should i be worried about the minimal water infiltration (we're talking about 2-3 cups of water)

  • 2
    If you are turning it into an office are you retaining the garage door that sometimes leaks, or replacing that with a wall?
    – Ecnerwal
    Dec 2 '15 at 2:25
  • If you are keeping the door both Lowe's and Home Depot sell rubber garage door gaskets. Dec 3 '15 at 2:16

Consider a subfloor system like DriCore.


These types of systems combine a subfloor base with a water barrier that has channels underneath to allow small amounts of moisture to drain or evaporate.

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  • Does it allow for leveling, or just follow the "slope to drain" underneath it?
    – Ecnerwal
    Dec 2 '15 at 2:28
  • 1
    It does allow a small amount of leveling. They sell shims to even it up. But if the slope were more than about 1/8 per foot, you would have to follow the slope or use sleepers.
    – bib
    Dec 2 '15 at 2:32

Yes, you can build a floor over it. Compensate for the slope and use pressure-treated lumber where it contacts the concrete. Do allow for drainage.


Sure you can. Really, this is no different than a slab-on-grade or crawlspace scenario. The key will be to minimize moisture contact with untreated lumber.

My approach would be to 1) seal the concrete with a suitable material, such as a DryLok product, and then 2) use treated lumber joist members ripped to level the floor. If flowing water is a concern, be sure to leave channels against the floor as appropriate for drainage.

  • I'm not a fan of any wood (treated or not) being in contact with concrete. You might consider sill gasket between the "joists" and concrete, even when using treaded lumber.
    – Tester101
    Dec 2 '15 at 3:22
  • You use gaskets for all your basement energy walls? I've been doing new construction and remodeling for decades and have never seen significant rot in those cases.
    – isherwood
    Dec 2 '15 at 14:22
  • Not sure what an "energy wall" is. But yes, wherever wood would touch concrete, I use sill gasket (or other separation technique).
    – Tester101
    Dec 2 '15 at 14:34
  • Here in Minnesota (warm, humid summers, cold, dry winters) we build 2x4 insulation walls around all basements against the foundation. They, along with any interior basement walls, are built directly on concrete slabs using treated bottom plates. This practice became standard in the 1970s, and is standard today for its success.
    – isherwood
    Dec 2 '15 at 14:40
  • We do use a foam seal under sill plates (on top of foundation walls), but that's mostly for weather sealing.
    – isherwood
    Dec 2 '15 at 14:43

have you considered not using a subfloor, but just levelling the concrete and flooring over it with vinyl or some such impervious material? you could integrate an electric radiant floor system for heat, and the water can be solved by just flashing and sealing the door. plus, if the floor is currently sloped to the door, levelling it will solve the water infiltration by effectively lifting the new floor above the current entrance height at the sill.


Although the dricore will work you are introducing a ton of cost, a ton of weight (well maybe a couple tons), and a ton of labor. Those squares are a royal pain in the ass and expensive.

If you want to elevate your wood flooring so you do get it wet then simply glue down 1" poly sheets and the plywood over it - if you can paint the under side of your boards with a waterproof paint you will be better off.

However you have two big issues to deal with here. First you will have a slope and hole in the middle of the room and the hole for your drain might be 2.5" higher than the drain. There is just no other way to describe this but odd. Then you have a giant lip at the front of the garage. You will need to create a faux stair and again this is kind of awkward.

My advice for wood floor is above. However it it was me I would put down some large (12"x18" or bigger) greyish tiles and some mosaics around the drain. Therefore it can make a really nice office - throw a rug over drain when clients are over - and can be converted to a really nice garage.

  • Couple of TONS? How big of a garage is this? :)
    – DA01
    Dec 6 '15 at 5:58

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