Interior Drainage System
Beneath the Floor Slab

By far, the most effective and unsurpassed interior drainage system is a perforated drain pipe system properly installed inside the perimeter of the footing. This requires removing and replacing concrete at the wall/ slab edge.

Perimeter Drainage works most effectively when placed beside the foundation footer, instead of on top of the footer. This practice allows you to lower the effective water table an additional 6-8" below your basement floor.

Note: Above or on top of the footer installations allow the water table to rise high enough to contact the floor slab. This can cause cracking, seepage through floor cracks, excessive humidity in a basement, and in certain cases complete failure of the system during extended periods of heavy rain.

Preferably, we pitch the pipe slightly to drain the area most effectively. The drainage pipe connects to a sump liner. The sump liner has an airtight, childproof cover. 

If a permanent, multi-layer waterproof seal will not be applied on the foundation wall(s) a critical component of this system is the dimpled membrane placed against the base of the wall and beneath the slab edge along the top of the gravel. When exterior waterproofing is basically impossible and wall seepage is expected, the dimpled sheeting permits free drainage down the wall and across the footing into the drainage system. This membrane also prohibits water vapor from passing through the  floor slab at the drainage area, that may otherwise cause damp spots on the concrete floor to appear and excess humidity in the basement during heavy rain.

For basements that may have water penetrating cracks in the foundation wall(s) that will be finished , the dimpled membrane is extended over the entire wall. The membrane serves as a vapor barrier and it creates a drainage space that leads to the drain pipe below. This is particularly an effective means of dewatering if the basement is to be finished, and exterior waterproofing is basically impossible. A second interior vapor retarder should be avoided, if batt insulation is to be used.

This method of interior dewatering is NOT, and should NOT be confused with waterproofing. 

National Waterproofing Systems highly recommends that, when attainable, this method of interior drainage be combined with an exterior seal for a waterproof and structurally sound basement, certainly in situations where wall cracks are present. Sealing a wall on the inside alone traps water within the wall and actually speeds up the deterioration of the wall. Prolonged water penetration through foundation cracks, freeze and thaw conditions, expanding and contracting soils, other ground conditions, and high soil pressure can compromise the integrity of the structure and lead to costly and serious damage (cracked floors, leans, bulges, bowing and complete buckling of foundation walls). It is also recommended that with this type of interior drainage system a moisture control/mechanical ventilation unit be installed to keep relative humidity levels below 55% to prevent mold growth, and to assist in venting radon and other soil gases.

See page Interior drainage for more information on our superior installation process.

NOT RECOMMENDED - INTERIOR DRAINAGE SYSTEMS

Never contract for a water control system if dampness, seepage and or cracking on the walls are your concern, as this may result in further damage to your foundation.

1. Interior Drainage Channel on Top of the Floor Slab

In cases where water is entering a basement through the cove area (where the wall and floor meet), some water control companies will install an on top of the floor, or above the slab interior water control system. The simplest and least costly approach is this drainage channel, which is adhered at the base of the wall on top of the floor slab.

By no means whatsoever should this system be confused with, or called waterproofing. 

Weep holes are drilled along the floor at the base of the wall to allow water to enter the drain channel. Heavy rain entering the ground around a foundation will rise to seek its own level, as it penetrates the floor-wall joint and rises up through the holes drilled in the floor, only then does it enter the drain channel.  Water is directed into a sump using another channel placed on top of the slab, then through a trap to the sump basin.

The idea is that the “removal” of this water under the slab lowers the water table around your foundation, thus relieving hydrostatic pressure (this soil pressure can crack floors/walls or cause cracked foundation walls to bow or buckle). Although pressure is being relieved through the holes drilled in the floor, the water under the slab is not being “removed” at all. In fact, as intended, water is being permitted to drain through the floor slab and into the drain channel on top of floor. When the ground is no longer saturated and the water level begins to drop, water will remain on top of, within, and under the saturated floor slab. Water can easily soak into concrete, and water vapor readily passes through concrete. Granted, it does not transfer at the same rate as perhaps paper or cloth, but it does move to an environment of lower vapor pressure (the basement air). With this type of system the water is not removed from the space. 

The result of these indoor gutter systems are excessive humidity, which can cause mold and mildew.  These systems also permit structural deterioration, which will ultimately lead to costly structural damage in the long run.

2. Interior Drainage Channel within the Floor Slab Edge

Another approach, used exclusively by some water control system companies, is to place a drainage channel at the base of the wall on top of the footing. This requires removing and then replacing the concrete along the wall/slab edge.

The drainage channel is connected to a drain pipe leading to the sump. This approach is, at most, effective on masonry walls with water problems because it drains the block cores. Holes must be drilled at the base of every block core to permit drainage.

Note: This type of system cannot drain groundwater from under the floor slab.

During heavy rain, as water seeks its own level, the water will rise under the floor to the height of the drainage channel, allowing water to remain in contact with the floor, causing vapor transfer (excessive humidity) into the home, damp spots on the floor, and possibly floor water (certainly if floor cracks are present). For this reason, water control system companies routinely try and sell sub-floor drainage tiles to channel the floor water that their systems permit, and/or a device to deal with the extra humidity their systems create. 

In some cases where poured concrete foundations that have wall cracks, flexible patches (wall band-aids) are often used or combined with these systems to stop wall leaks. Note: Sealing a poured concrete foundation wall from the inside alone will trap water within the wall and can actually speed up the deterioration of the wall.

Never contract for a water control system if dampness, seepage and or cracking on the walls are your concern, as this may result in further damage to your foundation (bowing, bulging foundation walls).