The Perc Test

While shopping for my first house in 1995 I learned about something called a “Perc Test.” I was considering building a new house on a piece of raw property without access to sewers, and before you can even consider such a thing you need a Perc Test. “Perc” is short for percolation and it has to do with how quickly water will drain through your soil.

If you intend to build a house with a septic system, the perc test will quickly tell you if that is even possible. If your soil does not sufficiently perc, your septic system will never work right and you will live a life of smelly, backed-up plumbing. Banks and real estate agents learned a long time ago that it is difficult to sell a house with a bad septic system, so before they even let you make an offer on a piece of property you must perform a perc test.

The procedure for determining the “perc rate” for a property involves digging holes, pouring water into the holes and measuring how quickly the water disappears. The actual steps are more involved, but that is essentially it.

I ended up buying an existing house with a functioning septic system, but what I learned about percolation would come into play years later.


In the summer of 2008, we tore out an old deck and installed a flagstone patio in our backyard. Where we live in western New York, we go through a great number of freeze-thaw cycles every year, so the installation of such a patio is quite an involved process:

  • We dug out ten cubic yards of dirt to make room for all the gravel – it does not sound like a lot of dirt, but imagine a block of dirt, three feet wide by three feet high and thirty feet long
  • After the hole was properly shaped and leveled, we brought in the coarse gravel and tamped it down
  • Then came the medium gravel, which again was tamped down
  • Finally came the fine gravel, which after tamping had brought the level of the hole almost even with the rest of the yard

After all this, Emily and I did the math… we had wheel-barrowed thirty-two tons of gravel. The reason all of this is necessary where we live is the aforementioned freeze-thaw issue. As we all know when water freezes it expands and of course when it melts it contracts. If your flagstone patio is built upon a shallow bed of gravel and sand, it will only take a few winters for it all to fall apart.

So, here in the great frozen north, we must dig deep holes and fill them with huge volumes of gravel.


The patio project was a combined effort involving me and Emily on most of the grunt work, and our friend Josh who is an artist with stone. Once we finished with the gravel, Josh got busy installing the patio and the stone walls on the northern side. The final piece of the project was the outdoor kitchen that I was building.

Our flagstone patio

Our flagstone patio


Josh was finishing up his part of the project just as I was getting deeper into mine, which involved a cinder block foundation dug thirty inches into the dirt, right next to the hole-o-gravel. As I worked on the foundation, every time it rained, it filled up with a lot of water. To keep working I pumped the water out, but to get it dry I ended up pumping many multiples of the foundation’s volume.

Where the heck was all this water coming from?


The soil in our yard is very high in clay. Clay is wonderful for lots of things, but it does not perc. As it turns out, we had basically constructed a ten cubic yard swimming pool full of gravel, and every time it rained it filled up with water. Since all the gravel was surrounded by clay, the water just sat there. If the first freeze had come with all this water sitting in the gravel our new patio would have been destroyed almost immediately.

To remedy the problem I had to install a drain. Not just any drain, but an underground drain, running about 100 feet clear across the yard into a lower wooded area.

I dug the trench, laid a perforated drainpipe, filled the trench with coarse gravel and covered it back up with the grass I had removed.

When I pulled off the last inch of clay-rich soil surrounding the gravel bed of the patio a gush of water started that was nothing short of horrifying. It ran down the three-inch drainpipe and out the other end for nearly half an hour. If I had to guess I would say it was at least 1,000 gallons. Freeze-thaw disaster averted.

If you look closely between the two white lines you can see the depression from the drain - from space!

Click on the image and look closely between the two white lines, you can see the depression from the drain – from space!


A few years later when we were remodeling the basement, we had a wetness problem in one corner. This part of the basement had been wet since we bought the house and it was not a big deal until we were about to install new walls and carpet.

After a thorough investigation we figured out that the problem was essentially the same one we’d had with the patio. When it rained, water was running into the well around the basement window, but since that well was surrounded by non-percolating clay, the water would fill up the well and leak through the basement window, running down the wall. We had never seen it because it was behind the paneling.

The solution was a similar one, install a drain. However, this time it was a shorter, but more complex task. I was able to tie the window well drain into our existing downspout drains that run into the storm sewers. The basement has been dry ever since.

The "keep the basement dry" drain

The “keep the basement dry” drain complete with check-valve


How often do we do everything right within our area of responsibility, only to see problems occur downstream? The patio was built to exacting standards, but we failed to take into account the makeup of the surrounding soil.  In that case we got lucky.

Take the time to learn and understand the things upstream and downstream from you, in every part of your life. Not only will you have a richer sense of your place in a larger world, but you might find a way to make changes in your efforts that could benefit others. At the very least you should be able to keep your patio flat and your basement dry.


Copyright © 2014 - Stephen S. Nazarian - All rights reserved.

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