A common question that builders get involves waterproofing for buildings at the top of hills. It usually sounds a lot like this: “I’m building on top of a hill, I don’t have to worry about water, right?”

It’s a good question. We all are taught at an early age the scientific fact that water runs downhill. Water pools at the lowest point it can reach. Rivers run down, and gather in ponds, lakes, and oceans.

So, theoretically, the house at the top of the hill should need very little waterproofing.

Here’s why that’s not absolutely correct.

Water management factor: soil

First, it matters a great deal what type of substrate the structure is built on. Your builder will do a site survey that will include this information.

The soil type and soil drainage class will have a large impact on your structure’s waterproofing needs. Builders are looking for, and might bring in solutions to create, a well balanced, well drained supportive soil. This will help keep your structure dry. But it does not mean that you will not need to do waterproofing.

Water management factor: slope

Another important factor is slope. Even at the top of a hill, the slightest slope can create water pressure on the walls of your construction. In a downpour, this pressure can be pretty obvious to see and understand. However, even light rainfall spread over a wide area then collects at the first obstacle it encounters.

Your builder will work to slope the surface away from your foundation, but under the ground are unseen slopes and forces that will deliver water to the base of your project. Waterproofing is still essential.

Water management factor: pressure

We have understood water pressure for years. We have mastered it by lifting water high above our communities in water towers to create pressure in our faucets and for our showers and lawns.

These same principles apply underground. In some places, water can build up underground and seek routes to vent that pressure, and those escapes can occur a long way away from the original source. In fact, these escapes can sometimes happen uphill from the location.

We call these spots springs when they occur in nature. But when they occur at a building site, they are an expensive problem.

For these reasons (and more) it is important to waterproof carefully even if you are building at the top of a hill.

Photo by Megan Ruth