Q.: We build concrete shelters for farmers who want protection from tornados. The shelters have a 10x10-foot footprint and walls are 7 feet high. We cast the floor slab first, leaving a keyway at the edges, then cast the walls and roof in one pour. The shelter sits in the ground with about 1 foot of the wall exposed and there's a metal door in the roof.

Here's the problem. We're getting moisture leakage into the shelters, making them damp and uncomfortable. Sometimes it leaks in around tie holes and sometimes at the joint between the floor and walls. In one case we put a 6-mil vapor barrier on the ground and brought it up around the edges of the slab but still got water coming up through the floor.

I have two questions. How can we economically waterproof the shelters we've already built so the owners will be satisfied? We don't want to dig if it can be avoided. And how can we build shelters that don't leak? If we put in granular material and drain tiles at the bases of walls, what do we drain the water to?

A.: If the shelters periodically dry out, wait for a dry period and use a so-called negative side waterproofing product that can be applied from the inside. Polymer or cementitious products can be applied by a do-it-yourselfer. Follow manufacturers' instructions. A list of waterproofing material manufacturers can be found in the Concrete Construction buyers' guide company directory. If the shelter is perpetually wet, you might solve the problem by injecting a hydrophilic foam through holes drilled through the concrete from the inside near the source of the leakage. You'll probably have to hire a specialty contractor to tackle this job and cost will be higher than for using surface-applied products.

To prevent leakage during new construction you could try several approaches. Instead of using drain tiles and granular material, you could place the concrete slab on bentonite panels, use bentonite rope in the keyway between floor slab and wall, and apply bentonite panels to the walls before backfilling. You could also use adhered or loose-laid sheet membranes or liquid-applied solvent systems. With these methods you're trying to keep water out even if it rises and puts a liquid head on the floor and walls. Obviously, the floor and walls have to be stout enough to resist the pressure caused by a liquid head.

If you use underdrains and granular material or drainage boards to direct water away from the shelter, you'll need to dig a separate pit below the level of the shelter floor, fill the pit with granular material, and run the drainpipe to the pit.