How Do I Calculate Slope in My Yard?

by Cassi Henes

The importance of creating and maintaining a positive downhill slope condition away from your foundation is paramount, and happily, is one of the most relatively inexpensive and easily achieved preventative measures available. Our focus in this article will be on the practical how's and why's of good positive drainage.

This is usually defined in most soils reports as a condition where at least 6"-10" of drop occurs in the first 10' of run out from the foundation. Another way of expressing the same thing is to express the angle of the slope by means of a percentage; 6%-10% positive slope for mulched or rock bed areas, and 2%-3% positive slope for hard paved areas, such as sidewalks, driveways, or parking lots. (A lesser slope is usually acceptable for paved areas because hard surfaces like concrete or asphalt naturally conduct water away from the foundation much more efficiently than a porous surface like a mulch bed).

With a few basic tools and a bit of patience, these slope percentages may be determined by the average property owner wishing to prevent needless structural and/or water damage to the foundation of his/her property. The process of doing so is as follows:

• First, choose a point along the foundation wall and place one end of a 10' long straightedge (such as a 2 x 4) at the same point while extending it out perpendicularly from the foundation.
• Next, place a carpenter's level on top of the 2 x 4 while you gradually raise the far end of the 2 x 4 until the bubble indicates a level condition has been achieved.
• Finally, simply measure down from the elevated end of the 2 x 4 to the ground directly beneath is and make note of that dimension. For this example, let's suppose that our vertical measurement is 10".

Now, you have all the raw data you need to calculate your slope percentage.

The math is simple: divide the vertical dimension, known as the "rise" by the horizontal dimension, which is known as the "run". Don't forget to convert your units of measure as needed so that they are consistent. In our example, we would have 10" (our rise) divided by 120" (our run), equaling .083, which we could round to 8%. As a rule of thumb, an 8% positive slope downward would be considered to be a safe and effective amount of slope to quickly move water away from the foundation before it can penetrate to deeper layers of soil and cause problems. Sometimes, calculating slope or determining what a safe and effective grade may be difficult, so it is possible to have a geotechnical engineering firm or other foundation related company come out and do the calculations.

Now that you have a slope calculation, it’s easy to find problem spots on your property and change the grade to protect your home. You should check the slope of your property, especially when your home is a new construction, after spring melt. Settling and changes in the grade are often caused by changes in moisture and run off. Preventative maintenance like this can save from a flooded basement, a muddy crawlspace, or even cracked and settling foundations.