The first part of a deck to be built is usually the footings. They must carry and support the full weight of the finished deck, all the furnishings, and all those who occupy it at any one time. The deck footing depth is key to a solid, safe, level, and durable deck that can be enjoyed for years.
Decks attached to heated structures should have footings below the frost line. Freestanding decks 200 ft² and smaller that are close to the ground don’t require a footing. However, those larger or higher off the ground should have footings 12” or deeper below the surface of undisturbed soil.
In this guide, we’ll explain why footing depth is important and the factors that affect their depth. We’ll discuss the frost line as it applies to construction, footing and post depth, code requirements, and using a frost depth map to identify deck footing depths. Plus, we’ll look at footing depths for free-standing decks and pool decks, and what happens if footings aren’t deep enough. Our goal is to provide you with the information necessary to get your deck onto the right footing depth.
- Why Deck Footing Depth Is Important?
- What Factors Influence the Depth of Deck Footings?
- What Is a Frost Line in Construction?
- How Deep Should Footings Be for a Deck?
- How Deep Should Deck Posts Be in the Ground?
- Minimum Depth for Deck Footings Code Requirements
- Deck Footing Frost Depth Map
- Footing Depth for Freestanding Deck
- Pool Deck Footing Depth
- What Happens When You Build Deck Footings Not Deep Enough?
Why Deck Footing Depth Is Important?
Deck footings provide a stable base or foundation for a deck and prevent it from settling, tipping, or sinking under load. The footings support the weight of the deck structure (dead load) and any furnishings or people (live load) and spread it to the surrounding soil or ground.
The depth of the footing ensures the safety and integrity of the deck. That is why it is identified in the International Residential Building Code (IRC), and most national, state, and local building codes.
To make sure the footing is stable, it needs to sit 12” below the surface of stable undisturbed soil, on bedrock, and/or extend 6” or more below the frost line. Since the stable soil strata, bedrock, and frost level vary from location to location, the depth of the footing also varies.
If the footing rests on disturbed soil or above the frost line, it is susceptible to movement concerns. That is why the depth of the deck footing is an important issue.
Pro Note: The IRC is used by the U.S. and Canada, as well as other nations, to support or form all or part of their National, State, Provincial, or local building codes. The most current code is the 2021 IRC, and Section R507.3 addresses footings, with R507.3.2 addressing the minimum depth of footing.
What Factors Influence the Depth of Deck Footings?
The main factors that influence the depth of deck footings are the frost line depth, load on the footings, type of soil, and the bearing capacity of the soil or ground. There are other factors, such as the foundation depths of adjacent buildings, utility locations, and water table levels that may also need to be considered too.
The frost line is how deep the ground freezes on average each year in a geographic area. In severe winters the frost depth will go deeper than in warmer winters, so the frost line is typically an average. If a deck footing isn’t below the frost line, the freeze-thaw action can cause it to lift, tip, drop, or move laterally.
The movement often results in damage to the deck and to any structure to which it is attached. The frost level can be nonexistent in places like southern Florida to more than 96” deep in some places in northern Minnesota and into the central prairies of Canada.
The frost line depth often depends on the depth of the soil and climate, so can vary greatly even within a couple of miles or even feet. In many situations where a footing is required, the local building department can provide the necessary depth when applying for a deck permit. Additionally, a footing inspection is often required to check footing hole depths prior to pouring or placing the footing.
The load a deck places on a footing often combines the dead and live loads, but may also include the snow and wind loads. The dead load refers to the weight of the whole deck, including railings, stairs, metal brackets, fasteners, bolts, etc. It typically is 10 to 15 psf (pounds per square foot).
The live load is the weight of furnishings, people, BBQ, planters, etc., and is usually 40 psf. The design load is the combined live and dead load, so is minimally 50 psf. Adding a hot tub or spa can increase the load values to 100 psf or more.
Snow load varies from region to region and ranges from 40 to 70 psf, with some isolated locations as low as 20 psf and others up to 90 psf. Wind load usually applies to areas that experience high winds that can apply lateral and lift forces. It is usually reflected in the types and locations of fasteners, bracing, and structural members.
The load affects the minimum diameter or side measurements and thickness of a footing. It also determines the spacing between footings based on the tributary area and load-bearing capacity of the soil strata. So, the load determines the footing size based on the live or snow load values, whichever is deemed greater.
Bearing Capacity and Type of Soil
The soil upon which the footing sits is an important factor in determining footing size and depth. Footings need to be on undisturbed soil, so don’t toss some back in if you dig down too deep. It should be well-drained compacted clean fill. Although there are many kinds of soil compositions, there are three common types of soil used to identify load-bearing capacity, plus bedrock.
Gravelly soil contains small rocks or pebbles and some dirt, it isn’t easy to form into a ball and drains easily. It is the best soil for supporting a load and is rated to carry loads of 3,000 psf.
Sandy soil has a rough, granular, or gritty texture as it is made up of tiny particles and is often light brown to white in coloring. Forming a ball with dry sandy soil is difficult, although adding water does help it hold its shape, however, the moisture passes out of it quickly. Sandy soil is rated to carry loads up to 2,000 psf.
Clay soil is composed of tiny particles and often has a sticky feel and can easily be formed into a ball. Depending on the minerals in the soil, the clay can be red, brown, gray, or other colors. Clay tends to retain moisture and is considered the weakest; it will support loads up to 1,500 psf.
Bedrock is a solid rock stratum and isn’t often affected by frost, so placing a footing on it at any depth can be done without worry.
What Is a Frost Line in Construction?
Frost line construction is ensuring that footings are below the frost level, or are protected with rigid insulation to keep the surrounding soil from freezing. The R-value of a foot of soil is about R3, while an inch of rigid insulation, depending on the type, ranges from R3 to R5. So, using 4 inches of rigid insulation is equivalent to or better than using 4 feet of soil.
Make sure to use insulation that won’t degrade underground. Additionally, there should be good soil drainage to below potential frost depths to reduce the amount of moisture that can freeze and cause damage.
How Deep Should Footings Be for a Deck?
Deck footing depths depend on the size of the deck and its height off the ground. A free-standing deck up to 200 ft² and within 20” of the ground within 3 feet of its perimeter, doesn’t require footings. Larger decks or those with greater elevation require a footing that rests at least 12” below the undisturbed ground level.
Decks that are attached to a frost-protected structure should be at least 6 inches below the local frost line. However, if the footing rests on bedrock, it can even be above grade level. Your local building department will be able to assist you with how deep your deck footings need to be.
How Deep Should Deck Posts Be in the Ground?
Deck posts support the full weight of the deck and everything that is on it, so you don’t want them to sink or wobble. Posts supporting free-standing decks should be set into the ground at least 12” below grade on undisturbed soil and often require diagonal bracing to minimize lateral movement.
If the deck is attached to a frost-protected building, the base of the post should be 6” below the local frost line and rest on gravel, a poured concrete footing, or be encased in concrete. Posts can also sit on buried or exposed bedrock but may require lateral and/or diagonal bracing depending on elevation.
Minimum Depth for Deck Footings Code Requirements
The minimum depth for deck footings depends on deck size, loads, and whether it is free-standing or attached. Section R507.3 and its subsections in the 2021 IRC address the minimum depth of footings for free-standing and attached decks. The top of footings for free-standing decks resting on beams and posts should be at least 12” below the surface of undisturbed allowable soils.
The top of footings for decks attached to frost-protected structures should be 6” below the local frost line. Footings on bedrock and those using alternate approved methods are an exception to the depth requirements.
Pro Note Local building codes may differ from the current IRC, so always check with your local building department if building a deck that is more than 20” above grade level or is attached to a building.
Deck Footing Frost Depth Map
Frost level can vary by inches or feet depending on climate and geography, so there isn’t a one-size-fits-all map. There are dozens if not hundreds of frost depth maps showing average levels across the continental U.S. and Canada and all identify that the further north you go, the deeper the frost line. Unfortunately, that doesn’t help much if you’re looking for a specific ZIP or Postal Code.
Most maps that show frost levels on continental scales are general, whereas frost depth maps for smaller political areas tend to be more specific. Unfortunately, it may take some digging to find a good map for your location.
The best place to get frost-depth information is your local building department. They will be aware of local conditions and usually provide it upon request.
A quick search of your State or Province’s frost depths will reveal more accurate information, but not as detailed as your local government can provide. The type and depth of soil, amount of ground moisture, and temperature ranges affect frost depth, so there will always be pockets that require different depths.
Remember, the frost line is the minimum depth and the top of the footings should be 6” below that line. Additionally, frost doesn’t normally affect bedrock, so you don’t need to go deeper unless you want to.
To illustrate this, a continental US frost depth map identifies that frost levels in Minnesota range from less than 70” in the south to more than 100” in the north, with the State average being 80”. The Minnesota State map for frost depths identifies the depths in the southern half of the State as 42” and the northern half being 60”.
However, looking at the Frozen Soil Profile for Koochiching County in northern Minnesota identifies the yearly range between 48” and 78” over the past 18 years, with the average being about 61”. So, the local building department may take that information along with soil information and adjust the footing depth.
The Average frost depth for Alaska is 100”, but bedrock may be as deep as you need to go. Warm climate states have an average frost depth closer to the surface, with California’s being 5”, Florida 1”, North Carolina’s 10”, and Texas 10”.
States that experience colder and longer winters have deeper frost levels. For example, Idaho’s is 31”, Maine 74”, Michigan 55”, Montana 61”, New York 50”, and Wyoming’s is 56”. Beware the average, though, and check with the local building department.
Footing Depth for Freestanding Deck
Freestanding deck footings do not have to go below the frost line. Those up to 200 ft² and within 20” to 30” (depending on the local codes) of the ground within 36” of their perimeter, don’t require a footing.
Larger decks or those further off the ground require the top of footings to be at least 12” below the surface of undisturbed soil. You can use footings even if not required, especially if you want to protect wood components from moisture, but it isn’t necessary.
Pool Deck Footing Depth
Pool decks may be at ground level or elevated to be slightly above or even with the top of the side of an above-ground pool. If the deck is attached to the pool, the footings should go below local frost levels.
If the deck is more than 20” off the ground, the top of the footings should be 12” below the surface of undisturbed soil. However, a pool, whether above or below ground, will exert force on the ground due to the weight of the water, so dig footing holes with care.
What Happens When You Build Deck Footings Not Deep Enough?
If deck footings aren’t deep enough or on proper soil or bedrock, then the deck will respond to ground shifts or movements. Most decks will lift, drop, dip, settle, or collapse if the ground is prone to frost movement or the soil is not strong enough to support the loads. If the deck requires footings to be below the frost line, then ensure they are.
It’s easy to adjust and level a ground-hugging freestanding deck, but very difficult to repair a deep footing for a deck 30” or more off the ground. Decks attached to heated structures require footings to be below the frost line to prevent damage to the structure. So, before building a deck, make sure to identify the local frost depths and if you need to dig down below them, then make sure you do.