Converted garden shed with concrete floor is wet during rainy season

Question

We converted a small garden shed, about 8' x 12' into a small free-standing office space. It works great for that purpose, except the floor (concrete pad) seems to wick water if it rains too much.

We had to take out the first installed floor (laminate over a vapor barrier on top of the concrete floor) because there was clearly seepage and the floor was wet after heavy rain. We have tried a concrete sealer on the pad but it doesnt seem to help. Floor is still getting damp.

The shed is located on a slope, and its gets pretty muddy on the uphill side. We thought about installing a French drain on the uphill side to assist with drainage, but not sure that will help or make any real difference.

Will several more coats of sealant work better? Double layer of vapor barrier beneath the new laminate flooring? Do we need to wait until the end of the rainy season (we are in Seattle) and the floor is completely dry before applying any more sealant? Maybe put some kind of pressure treated lumber frame, and plywood to elevate the floor off of the ground, and install the laminate flooring on top of the plywood?

Any suggestions much appreciated!

Answer 1

A "sleeper" subfloor would resolve the problem. I'd put down heavy polyethylene sheeting (at least 6 mil), caulking it to the wall plates with acoustical sealant (commonly used for vapor barrier sealing). Then pressure-treated 2x2 sleeper joists. You could rip 2x4s in half if you can't find treated 2x2s.

Then overlay standard 3/4" tongue-and-groove subfloor, screwing it to the framing below. Install flooring as you like.

I asked in a comment if the wall plates are treated. If they're not, expect them to rot out within a few years, or replace them (not as difficult is it sounds).

Answer 2

The garden shed most likely did not have vapor barrier put down before the slab was poured. Therefore the concrete will always be wicking up moisture from the ground.

You can try to seal the floor with a water proofing used for shower walls such as Redguard or Aqua defense that is available in the big orange and blue stores.

Answer 3

As you state, the best thing to do would be to create an elevated floor with a small "crawlspace" underneath. That would also require raising the shed, which would be useful anyway, because it will quickly rot out in direct contact with the wet slab.

The French drain would also be helpful, but likely not sufficient by itself.

Answer 4

Sauna style floor?

The cement down there will still get wet. The wet will not get on top of the wood. The spaces between the wood will let it breathe. If you use cedar or redwood it will last many decades not in the sun. It will smell good in the shed.