Building a House? Don’t Forget a Perc Test!

Sherry NoyesPosted by Sherry Noyes on March 5, 2017

When you buy land to build a house from the ground up, you discover a world of stuff to think about. And one of the first and most important things? Sewage.

Most people don’t give much thought to what happens when a toilet flushes or water drains from a dishwasher. But if you build a home in rural Vermont or New Hampshire, sewage will be one of the first things on your mind.

In fact, before you even purchase a piece of land to build on, it’s essential to know if the parcel includes on-site sewage treatment, otherwise known as a septic system. And if it doesn’t, you need to find out if it’s feasible for a septic system to be installed.

Let’s look at the components of a traditional gravity-fed septic system. It will consist of:

  • A waste pipe that carries effluence away from the house to a large holding tank.
  • A septic tank made of concrete, plastic, or fiberglass.
  • A leach field, which is a collection of perforated pipes set in a gravel-filled absorption trench about two feet below the ground.

There’s a lot of natural science involved in the way a septic system organically transforms household effluence into fertile ground. A septic system also filters filthy water and makes it suitable for drinking again as it travels back into the aquifer from which it originated.

But what you need to know before home construction begins is if the soil will actually absorb all the waste a household produces.

For that, a percolation (or perc) test needs to be conducted. This, too, gets pretty technical, which is why a soil technician is hired to perform the test. Basically, the soil technician digs holes into the site of the proposed leach field, soaks those holes with water, and determines how far the water level drops in 30 minutes.

What’s being documented is the percolation rate, or the time it takes the water to fall 1 inch. Standards vary from state to state, sometimes even from town to town. But the cut-off point is generally somewhere around 60 minutes per inch. That is, the water level drops one inch in 60 minutes.

What happens if your site fails the perc test?

The bad news is that the land might not be suitable for building on. You’ll need to find a different parcel.

The good news is that an engineer might be able to “rehabilitate” the land and make it suitable for a septic system. Also, as land becomes increasingly scarce—even in rural areas—alternative septic systems have been developed that can be used on almost any type of land.

The only truly bad news, then, is if a perc test is not performed and home construction begins before you know what you’re getting into. Always get an expert to help determine if the land you love is good for building!

As your house plans begin to come together, you’ll need to know more about septic systems. Read this blog on four things you need to know about septic systems!