Simply stated, it’s a path of least resistance to move water around the foundation of your home, instead of through it. Around, good! Through, not good.
Here in the Pacific Northwest, it rains for pretty much half the year. Somewhere around 4 feet of rain falls in this region per year. That’s a lot of water. Couple that with the fact that we have a lot of steep terrain and, great! now, all that water is moving, and it always seems to move downhill!
Without perimeter drainage, water can build up around your house foundation. This can cause hydrostatic pressure against the foundation wall. Any crack in the concrete or breach in the waterproofing or damp-proofing on the outside of the foundation will let the water into the house. The water can also “hydraulic” up where the basement or crawlspace concrete slab meets the foundation wall.
Houses make crappy boats, for this reason, we build them on land. To cope with these large volumes of water moving through the surrounding ground, a perforated pipe is installed around the perimeter of a house foundation. It can run to a sump (more on these later), which can be connected to a municipal storm system or, in some instances, to the municipal sewer system. Municipalities are slowly replacing sewer only systems with separate storm ones, but it is still quite common for the runoff from your property to reach the ocean via the sewer system. It can also drain directly to a creek, river, stream or ocean. In the last 20 years or so, municipalities and the Department of Fisheries and Oceans are requiring greater measures from new construction and larger renovations to minimize pollution and sediment from entering natural water bodies.
The perforated perimeter drain will usually lead to a sump. These have been installed around here since the ’50s. The sump is a vertical cylinder or box from 2’ diameter up to 4’ diameter. Older ones can be made of wood and are prone to rot, failure and collapse. Most, since the 70’s are concrete. The perimeter drainage comes into the sump along with rainwater leaders as inlet pipes (more on these to follow). It leaves the sump by way of an outlet pipe. The outlet pipe is lower than the inlet pipe and has a downturned 90-degree elbow on the end that is removable. This prevents debris from entering the outlet. The outlet takes the water magically away to the ocean. You are responsible for its management to the property line.
The sump serves two principal functions:
You know what this means right?… Yes! Your sump needs to be cleaned periodically! When is the last time you cleaned your sump? Tell the truth… (I have a sump?)
Once the debris piles higher than the outlet pipe it will surprisingly still function, just not so well. I recommend checking on it every 2-5 years. When the debris is getting near the outlet pipe, its time to roll up them sleeves, or call someone with sleeves. The easiest way to clean it is with a wet/ dry vacuum.
The perimeter drainage takes care of groundwater, and, up till the mid-80s when you were listening to Wham and choosing life (admit it), it also managed all the run-off from your roof. This, however, proved to be problematic in heavy rain events. The 3”- 4” diameter perimeter drains couldn’t handle the volume and water ingress into the living space would occur. To alleviate these occasional overloads, a second separate system came into use, the rainwater leader system. It is the same principle as the perimeter drainage, get water to the sump and away from your domicile, same material too, but it’s not perforated, it’s solid.
A rainwater leader system should run just below the frost line to function well. But it is imperative that your perimeter drainage runs below the elevation of your interior floor slab to ensure the water on the outside stays below your interior floor elevation.
Your homes reliance on an effective drainage system is directly proportionate to how deep your house is in the ground. Is it Slab on Grade, a concrete slab placed on the surface? Your reliance on good drainage, in this case, is minimal. Crawlspace? More reliant. Full basement? It is imperative that your drain tile system is fully functional.
There are 5 main types that I come across in the lower mainland of BC;
Concrete has been used as a material for drainage piping for over 2000 years, As far as a modern, commercially available product in North America, about 200 years.
Few buildings here are over 120 years old, chances are if have you are seeing concrete extending vertically out of the ground to meet your downspouts, you are looking at original. Of the two segmented types in our list, concrete and clay, both came in straight lengths of 18”- 24” and diameters of 3”- 4”. Concrete is the lesser performer for two main reasons. There were no elbows, tees, or tee-wyes to make meaningful connections, and the concrete corroded and wore quickly along the bottom, much quicker than clay, eventually collapsing and requiring replacement.
Clay/ Terracotta, much the same as concrete above, clay sections can misalign. They do include options for elbows and tees to improve flow. They are also much more durable than concrete. If installed well on stable ground, they can provide decades of good performance. But people, ya gotta keep em’ serviced! On some deeper basements, in my experience, the drainage systems were moving 100 gallons per hour in July. Its always working.
Trees loooove these pipes when they can find them. Their roots have sensors that can detect good water sources and will send additional resources to those roots that are closest to the source to accelerate growth. If you have a lot of trees on your property and concrete or clay drainage, it’s a good idea to stay on top of the annual maintenance. Try to find some kind of joy in the exciting realm of pragmatism. You can do it!! I know…drainage isn’t sexy. Until. Now. Time to talk about the Big-O.
Big O If I see Big O around a house with a shallow foundation and a dry crawlspace, I’ll make a note. If it’s a 1985 build-in Deep Cove with a full basement, it’s of much greater concern. It is more likely to crush under heavier backfill loads and does not maintain a straight, consistent slope to the sump or away from the house. It’s what I like to call “forcing the medium”, or trying to make a material do something it’s just not designed for.
PVC Polyvinylchloride Oh yeah, you know what time it is, if you’re rollin’ PVC. Common from the 80’s on, it is an integral sealed system that outperforms all others. What I like to see with these systems are several means to access them at key locations that they may be easily scoped, cleaned and maintained. The more clean-outs the better.
ABS Acrylonitrile Butadiene Styrene Used in residential under driveways where the pipe is close to grade. They are thicker-walled and can withstand the weight of heavy vehicles without deformation or collapse
All manner of patio and driveway drains will tie into this system and often act as auxiliary sumps themselves. Have a look at your exterior drains. If they are larger than 12 inches, square or round with a metal grate and have a downturned outlet pipe, you’ve got a little sump. Strip drain, often seen at driveways, need to be kept clean as they have no way of separating leaves and debris from running stormwater. These all should be kept as clean as possible. Wet/ Dry vacuum them 2- 4 times a year as needed.
If you haven’t had your perimeter drainage system inspected in the last 5 years, you should probably think about getting it done. As with all systems in a home, regular maintenance is key to keeping long-term overall costs at a minimum. Contact a qualified Perimeter Drainage Specialist to further evaluate your system and up your drain-tile game.
Thanks for reading!