Gap Between Basement Floor and Wall?

Some construction methods, have a concrete floor poured into a space, after concrete walls were poured.... and occasionally, the floor will shrink, leaving a small crack or gap between the floor and the wall, though this is usually uneven.

If this gap is a uniform 1/2 inch all the way around though, then I'd guess that it was an attempt to help channel water seeping into the basement, around to a drain, or sump pump, as you mentioned.

Simple check - take a flashlight, and something small enough to poke into the gap, and see how far it will go into it. Check it at different locations, and see if there is anything clogging it.

Ideally, if this is a retrofitted 'drain' channel, it should slope from the highest point (furthest away from the sump/drain) down to the sump pump, or drain. A channel like this can become clogged with dirt, so you may want to try scraping it out, to ensure that the water will run where you want it to. A wire brush would work to clean out the channel.

Start with some simple steps first - clean the channel if it does actually run to a sump pump. Also check the sump pump to make sure its working properly. Add extensions to the gutter downspouts - get as much of the excess water away from the foundation as possible. Finally, check the grade (slope) of the ground around the perimeter of the house foundation. Getting the proper slope might mean you'll have to do some landscaping - a good 2 to 4 inches of slope across 2 ft from the foundation will really help direct a lot of the rain away from the foundation. Since you mention you're having problems when there is a heavy rain, I'd say it sounds like the water is seeping in faster than it can be drained off..... if you can redirect some of that water before it even gets in around the basement, it will help a lot.

I'd also consider how your room is set up...for example, don't let power cords run across the floor, instead, hang them on the wall. In a basement, I wouldn't leave wood furniture to make bare contact with a concrete floor to begin with, even without potential water pooling or seeping from the perimeter. Concrete tends to draw moisture up through it, so I like to put spacers, or furniture 'cups' under them, to keep them dry. Don't leave metal laying on concrete, for example, paint cans, or tools - they'll rust.

You've got some elbow work ahead of you, heheh.... but get it done, and you'll not have to rush around with towels.

Have Fun

The answer to the question is two-fold: it is either a floating slab or it is a French drain.

What is a Floating Slab?

A floating slab or floating floor is part of a typical stem foundation in areas where there is freezing and expansive soils. If the foundation is monolithic, the floor is not a slab but actually part of the foundation itself. If the home has a stem foundation, where the wall is constructed upon a footing, then it may have a floating slab which also sits upon the footing.

The floating slab is created by placing a rigid object, such as a 1x6” board against the wall before the slab floor is poured. After the concrete floor sets the board is removed a clean even gap is left between the basement wall and floor. This is a floating slab or floating floor.

Some areas have very expansive soil which means the soil expands and contracts. This expansive soil puts pressure on the walls when the soil expands and releases pressure when the soil contracts. These soil pressures cause the foundation to move. The wall actually rocks back and forth in very slow motion.

When the slab, or basement floor, is directly adjoining the stem foundation wall sitting upon the footing it is subject to the same movement that the wall is. If the slab is touching the stem walls on all sides, and the wall moves in a thirty-second of an inch, the slab has nowher to move to and cracks as a result of this pressure. Having the gap of space that the floating slab provides allows movement of the walls without the floor slab cracking.

What is a French Drain?

The French drain system is designed prevent water from flooding your basement. It is installed inside the basement, below the floor, around the perimeter. This French Drain controls the water, delivers it to the sump pump(s), and discharges it out of the house.

If your house has hollow block masonry walls, an interior French Drain system can capture the water entering the block system and drain the water into a sump where a sump pump will discharge it from the building. Weep holes are drilled into the lowest course of block, allowing the drainage to occur.

How can I tell the difference between a floating slab and a French Drain?

While both may have a gap between the basement floor and walls it is easy to tell the difference between the two. A floating slab is part of the original construction and a French drain is installed post construction.

Because the French drain requires a sub floor drainage channel, the slab must be cut around the perimeter to excavate this channel. When the channel is closed with concrete, it sets and cures with a different consistency than the original floor. The basement with the French drain will have a discernibly different strip of concrete around the perimeter.

It might be a French drain (named after the man who designed it) or air trench - same thing. This is used to allow water to drain away from the building and either be absorbed by the ground below. In addition, you will have a sump pump which is operated when the water reaches a certain level and pumps the water away to a drainage point. If this overflows, something is amiss or it hasn't been installed correctly. Contact the installer or have someone else come out to check its operation.

Any interior caulk that will remain flexible will do it. Cut the tip of the caulk tube to make a hole no more than twice the width of the gap. Run a bead of caulk along the gap, forcing caulk into the gap all the way around the slab. Also, check the weatherstripping and caulking around the vents and basement windows to see where the spiders are getting in. I can't visualize the "drain tiles" you are talking about, but spiders don't usually enter through drain systems.

Filling the gap in will make the problem ten times worse. I think I would move to the upper room during the rainy season.