Perforated piping installed around the perimeter of a basement (Either under basement slab or around the exterior of foundations footing) is considered a French drain system. The system should be installed with a minimum of a 3 inch pipe for interior systems and 4 inch for exterior then backfilled with clean crushed stone. 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.
Hydrostatic Pressure/High Water Table
This is when rain water saturates the ground and water levels within the ground begin to rise. As the water table rises above the basement floor level, water begins to enter the basement between the foundation walls and floor or through cracks in the floor.
Water tables are usually higher in spring months during winter thaw or during a severe weather event. 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 Hydrostatic water pressure.
Interior Drain Systems
Interior French drain systems should be installed with a minimum of a 3 inch pipe. All piping should be laid next to footing and installed 8 to10 inches deep then backfilled with clean ¾ inch crushed stone. 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, 2 to 3, ½ inch holes should be drilled in every bottom block at or below the floor level. 2 ½ inches of concrete is then poured to complete the system. The system should terminate at a sump pit with an adequate sump pump to handle a maximum amount of water flow for a particular basement.
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 systems can be found in particular 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 installed during new construction. The cost to install an exterior drain system after the home has been completed can be 3 to 5 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 new construction has been completed. 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-make your architect change it.
20 to 70% additional water can flow through an exterior drain system compared to an interior. Rain water will flow into an interior drain system only from a high water table situation, also known as hydrostatic pressure. Exterior drain systems will also see water flow from high water tables and additional rain water that just seeps into the ground and flows downward will also flow into the piping causing quite a bit of extra water volume. We do not want this additional water flowing into an interior sump pit.
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.
French Drains For Monolithic Foundations
For most parts a traditional French drain installation (Pipe & gravel installed below basement floor) is not an option for basements with monolithic slabs. A monolithic slab-built foundation is different than a standard build basement. With a standard basement, a footing is poured 1st. The footing is generally 1Ft thick and 2Ft wide. The basement walls are then built on top of the footing and then the basement floor is poured on top of the footing up against the foundation walls.
With a monolithic build foundation, the basement floor is poured 1st and the foundation walls are built on top of the slab. In certain situations, breaking through the basement floor around the entire perimeter of a monolithic foundation can cause a foundation structural problem. In these situations, Basement Waterproofing Solutions will install an above floor baseboard French drain system. This system consists of a plastic cove base (similar in looks to baseboard heating) epoxied down to the floor. Water will track behind the cove base and work its way to a sump pit/pump. Personally, we prefer to install a traditional below floor French drain, but our options are limited when dealing with monolithic foundations.