Building a Deck on Different Types of Soil

It doesn’t take a geologist or engineer to realize that the soil you build on has a huge affect on a foundation – whether it’s for a deck or a home. Knowing what it is under the ground will help you better prepare, and avoid costly fixes, later on. Before you start figuring out the construction analyze your soil first.


Basically the soil is the start of your deck’s foundation. The support posts go into the footings, which are sunk into the ground. The footings help spread the load of the deck throughout the soil surface. What the soil consists of will affect how it handles the load.

There are three primary types of soil: sand, clay and gravel.

Here in Central Texas we have a good deal of limestone under a shallow layer of soil that has a lot of clay in its makeup. The type of clay varies from one region to the next, but to the east of Austin the soil becomes a little more sandy.

Building a Deck on Clay

Clay Characteristics: Can’t see particles with the naked eye, can be clumped up into a ball, sticky texture, wide variety of colors

Clay Classes: Clay, Sandy Clay, Silty Clay, Clayey Slit, Silt and Sandy Silt

Load: 1,500-2,000 lbs/sqft

Water drainage: Poor

Clay holds the least amount of load, so be very conservative when building on this soil. If you don’t know the exact makeup of your soil it’s suggested to treat it as if it is clay and your structure should be sound no matter what.

Building a Deck on Sand

Sand Characteristics: White/tan hue, gritty texture, small particles that can be seen

Sand Classes: Sand, Silty Sand, Clayey Sand and Silty Gravel

Load: 2,000-3,000 lbs/sqft

Water drainage: Good

Sand is a good middle ground for building a deck on. It’s not uncommon to see sand characteristics creep into soil that has more of a clay or gravel base.

Building a Deck on Gravel

Gravel Characteristics: Visible small pebbles, doesn’t clump up, rocky texture

Gravel Classes: Silty Gravel, Clayey Gravel, Sandy Gravel and Gravel

Compressive Load Strength: 3,000-5,000 lbs/sqft

Water drainage: Very Good

It may sound unstable but gravel soil can actually handle the heaviest load. Water will also drain more easily in gravely soils compared to other types making it good for decking.

The soil you build on is inherently a part of the footing foundation. Most soils, even in Texas, have a mixed composition. It’s always a good idea to err on the side of caution and plan for building the deck to support 1,500 lbs/sq ft.

No matter how much work you put into the footings and supports if it isn’t built for your type of soil there’s a good chance you’ll run into issues at some point. You may find that you need more footings to support the load, different types of footings or that they need to be located in different points for better support. Soil and weather also affect the depth and size of the footings.

So, know your soil. It’s what provides stability and solidity for your deck – it’s where everything begins.

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