To collect or convey? Poking holes in the use of Perforated Pipe


A perforated, corrugated drain pipe.

by Keith Tripp, 2020

Sump systems used to be associated with rural properties or older house. Sumps are now, for about the last 15 years or so, making a comeback in new construction. They are installed in new subdivisions where the City does not want to assume the risk of damage to houses caused by storm system backups, and on new construction in areas with older infrastructure, where the height of the house foundation drainage system may not match the existing City storm water system.

Staring down the barrel of this sump infeed pipe, a few things become evident:
a) this is a perforated pipe with slots cut around the circumference.
b) there is no top or bottom to the pipe, it is installed with random orientation
c) the pipe looks quite clean, there has not been much water flow through the pipe

Add in a dash of common sense: a pipe full of holes will lose water unless it is submerged in water.
Conclusion: this is the wrong pipe to use as a sump infeed.

This pipe is deigned to collect water from the surface of saturated and compressed soil . This is the type of pipe commonly used here in the Toronto, Ontario, Canada region for foundation drainage around the perimeter of the footings. As per OBC (Ontario Building Code) they are OK as COLLECTION pipes to be installed alongside the footings, with the top of the pipe being below the level of the underside of the basement floor slab, and must be installed on undisturbed or compressed soil (not on gravel) and installed level, not sloped. In this application, the pipe will collect water that rises above the level of the saturated soil. Rising water will enter the pipe through the slots, then seek the path of least resistance, which will cause it to flow through the pipe until an escape route is reached. That escape route may be out through the slots in areas where the soil is not saturated or where there is no water above the soil surface, or it may be the ends of the pipe where it terminates at the sump basin, or flows in to a City storm system, or to “daylight” if the house is built on a hillside. So in theory, ( the theory being the assumption that the pipes are installed level and on compressed soil), the pipe is moving water from wet areas to dry areas, until all areas are wet and then flow out of the pipe will occur. This may explain why some some sump basins are always dry, the pipe may just be moving water from the wet side of the house, to the dry side of the house where it dissipates in the soil.

If the pipe is installed on gravel. It will only collect water that rises above the gravel, and it will quickly lose water back on to the gravel. Pipes installed on gravel or  are basically useless.

To pass from the exterior to the interior where the sump basin is installed,  pipes will pass through a sleeve in the footing.  Water will be lost at this sleeve if a perforated pipe is used. At the areas around the sump basin, there is often gravel, and the soil has been disturbed by the digging and installation of the basin. A perforated pipe will lose water as it approaches the basin. This can be a considerable volume of water, as it has been collected from the full perimeter of the house. This water may cause erosion damage, or weaken soil by ongoing cycles of saturation and drying.  Sump basins are typically installed just inside the basement wall, where they are close to the structural footings. Long term soil weakening around or under the footings is undesirable, and may result in movement of the structure.

The pipes passing through or under the footing and under the basement slab to the sump basin should be solid pipes (not perforated) that CONVEY water without losing it along the way.  These are the pipes that connect the foundation drainage pipe (a.k.a. weeping ) tile, to the sump basin which acts as a collector. From the sump basin, the water will be pumped to the exterior.