French Drains

French drains consist of perforated piping installed around the perimeter of a basement (either under basement slab or around the exterior of foundations footing) and back-filled with clean crushed stone. Flexible piping with small slits is prone to clog with minerals, silt or iron bacteria so this type of piping should be avoided. When using PVC piping, holes should always be facing upward. Silt, minerals and iron bacteria accumulate at the bottom of a French drain trench. Holes facing downward will clog or contaminate the pipes interior causing the French Drain system to fail prematurely. Water does not have to raise to the level of the holes to start draining. Water will flow through the gravel that is placed around the pipe. If there is an abundance of water, it will then filter through the gravel and enter the holes of the piping. This way the pipe will always remain free of silt and other contaminates.


How Do I Know If I Need A French Drain System?

There are four conditions that will determine whether you will need french drains:

  • Water seeping in between the basement floor and foundation wall seam.
  • Water seeping in through a crack in the basement floor.
  • Water seeping in around lally columns.
  • Leaking walls, especially those made of cement/cinder block, brick or fieldstone.

Only A Clean Drain System Will Last A Lifetime

When soil conditions are unstable and an abundance of mud or silt is present, proper drainage fabrics should be used prior to pipe installation to prevent piping from clogging. If the foundation is constructed of block, one, ½ inch hole should be drilled in every bottom block near the floor level. Piping is then back filled with clean ¾” blue stone or washed gravel. A plastic cove base drain is set against the foundation wall, then 2 ½ inches of concrete is then poured on top of the gravel to complete the system. The cove base will provide a small space between the floor and wall which will direct water downward from the holes that were previously drilled into the bottom block. If a foundation crack develops down the road, the water seepage from the crack will also be directed down behind the cove base thus keeping the floor dry. The French drain system should terminate at a sump pit with an adequate sump pump to handle a maximum amount of water flow.

Hydro-Static Pressure/High Water Table

This is when rain water saturates the ground and water levels within the ground begin to rise. As the water reaches the bottom of a basement slab, pressure begins to form under the slab. When a water table rises above the basement floor, water pressure begins to push up creating seepage into the basement between the foundation walls and floor or through cracks in the floor and sometimes around lolly column’s. Water tables are usually higher in spring months during winter thaw or during extreme rain events. The only cure for this situation is a French drain System also known as a pressure relief system. No attempt should be made to seal up and hold back Hydro-static water pressure.

Interior French Drain Systems

At Basement Waterproofing Solutions, we offer three different types of interior French Drain Systems to accommodate all situations.


Footing System

A patented rectangular shaped pipe sits on top of the footing set 4-6 inches below the floor. This system works great when there is not a consistent high water table. Ground water that seeps into the hollow sections of a block foundation or between the footing and the foundation wall is generally not a tremendous amount of water. This water will flow through the gravel and pipe and drain back into the ground under the floor. In some situations, the sump pump will run only during significant rain events (three or more inches) In some situations a pump may never turn on.

Exterior French Drain Systems

Generally exterior systems should only be installed in an area where a home sits up on a hill and piping can be gravity fed to a low-lying area. These systems are typically found in mountainous areas. Some may view these systems to be superior to interior systems due to the fact they do not have to depend on a sump pump for the final water discharge but the exterior discharge piping is prone to clog from erosion and snow-pack in the spring and winter months. Other applications for exterior French drain system can be found in neighborhoods where an existing storm drain pipe has been installed. In these situations, the storm drain piping must be 18 to 24 inches below the basement slab so all French drain piping can terminate into the storm drain. These storm drains should not be prone to backups and local zoning laws should be investigated prior to this type of installation.

Exterior French drains are much easier to install during new construction. The cost to install an exterior drain system after the home has been completed can be 5 to 10 times the amount of an interior French drain system. In most cases, it would be near impossible to complete a full perimeter exterior system years after the home has been constructed. Front porches, patios, slab built garages and landscaping are just several obstacles to consider prior to installing an exterior French drain.

We have witnessed foundation plans for new homes with piping from an exterior French Drain System terminating to an interior sump pit/and pump. If you are having a new home built and discover this design to your foundation/basement drain system, we highly recommend you get the design changed.

20 to 70% additional water can flow through an exterior drain system compared to an interior. In an interior system, rain water will flow into French drain piping only from a high-water table situation. Exterior drain systems will also see water flow from high water tables. Additional rain water that just seeps into the ground and flows downward up against foundation walls will also make its way into the French drain piping causing quite a bit of extra water volume to flow into your interior sump pit. Many sump pumps will not be able to handle this volume of water. How about an exterior sump pit? They are dangerous and should be avoided. Only when no other options are available should an exterior sump be considered.

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