I have dug footings in soft sandy soil, hard clay, and half-frozen ground, and I have fixed decks that heaved and shifted because the footings were never done right in the first place.
Quick answer: Deck footings are the concrete foundations underground that hold up your posts. They spread the weight of the whole deck across a wider area of soil so the structure does not sink or shift. Footings must sit below your local frost line depth, or freezing ground can push them right back up out of the earth.
What Are Deck Footings ?
Deck footings are the underground foundation pieces that hold up your posts. They sit below the ground, out of sight, but they carry the entire weight of the deck above them.
Here is why footings need to be wider than the post sitting on top of them. Soil can only hold so much weight in one small spot before it starts to compress or sink. A footing spreads that weight out over a bigger area of dirt, so the ground can support it without the post slowly sinking over time.
I’ve found that footings are the part of the job people most want to rush. They are underground, nobody sees them once the deck is built, and digging is hard, sweaty work. But footings are the base of everything else you build on top. Get them wrong, and every other part of the deck inherits that problem.
Footings are the very first structural step in a bigger project, so make sure to read our complete Deck Building guide to see how this step fits into the whole process from digging to railings.
The Golden Rule of Footings: Finding the Frost Line
The frost line is the depth in the ground where soil freezes during winter. This depth changes a lot depending on where you live. Cold northern areas often have a frost line several feet deep, while warmer southern areas may have a very shallow frost line or none at all.
Here is why this matters so much. When soil freezes, the water inside it expands. This creates a strong upward force called frost heave. If your footing sits above the frost line, that expanding, freezing soil can physically push the footing upward, right out of the ground. This can lift and warp your entire deck over just one or two winters.
Digging your footing below the frost line puts the concrete underneath that freezing zone. The ground below the frost line generally stays at a stable temperature year-round, so your footing stays completely still, no matter how cold it gets up top.
You must check your local building department for the exact frost line depth in your area before digging. This number changes by region, and it directly affects how deep every footing hole needs to go.
Deck Building 101: The Complete Guide
Standard Footing Size & Depth Guide
| Deck Load Level | Typical Footing Diameter | Notes |
|---|---|---|
| Light load (small deck) | Around 8 inches | Always confirm depth against your local frost line |
| Standard residential load | Around 10-12 inches | Common size for most attached decks |
| Heavy load (large or multi-level deck) | Around 16-18 inches or larger | Wider footing spreads greater weight |
These sizes shift based on soil bearing capacity, which is how well your specific soil can hold weight. Sandy or loose soil often needs a wider footing than dense, firm clay soil for the same amount of weight.
Poured Concrete vs. Pre-Cast Blocks: Choosing Your Foundation Style
Pre-cast deck blocks are solid concrete blocks made in a shape that a post can sit directly on top of. They are simple and fast to use. From my experience, these work fine for low, freestanding platform decks or small island decks that sit close to the ground and are not attached to the house.
Poured concrete piers, using cardboard Sonotube forms, are the standard choice for larger, taller, or attached decks. You dig a proper hole below the frost line, drop a Sonotube into it, then pour concrete into the tube to form a solid pier the exact size and depth you need. This method gives you real control over depth, width, and strength, which is important once you are carrying a heavier structural load.
Helical screw foundations are another option. These are large metal screws driven deep into the ground with special equipment. They work well in certain soil types and can be faster to install than digging and pouring, though they usually cost more and may need a specialized contractor.
Footing Types Comparison
| Footing Type | Best For | Notes |
|---|---|---|
| Pre-cast concrete blocks | Low, freestanding platform decks | Fast and simple, limited depth control |
| Poured concrete piers (Sonotube) | Attached or larger residential decks | Full control over depth and width |
| Helical screws | Certain soil types, faster installs | Often needs specialized equipment |
Layout Accuracy: The Batter Board and String Line Trick
Before you dig a single hole, you need an accurate layout. Batter boards are simple wood stakes set up outside the corners of your planned deck, with a horizontal board connecting them. You stretch builder’s string between the batter boards to mark your exact deck lines above the ground.
Once your strings are set, check them for square by measuring both diagonal lines across your layout. If the deck footprint is square, both diagonal measurements will match exactly. This step matters just as much for footings as it does for framing, since your footing holes need to line up straight underneath your future beams.
Mark each footing location where the strings cross, then double check every measurement before you start digging. It is much easier to fix a layout mistake with a piece of string than after a hole is already dug and concrete is already poured.
Footing Inspection & Layout Verification Checklist
[ ] Batter boards set up outside all planned corners
[ ] String lines stretched tight between batter boards
[ ] Diagonal measurements checked and matched for square
[ ] Footing locations marked directly under planned beam lines
[ ] Local frost line depth confirmed with your building department
[ ] Footing size confirmed against your expected deck load
[ ] Underground utility lines marked before any digging begins
Step-by-Step Guide to Digging and Pouring Deck Footings
Step 1: Locating Underground Utilities and Clearing the Workspace
Always call your local utility locate service before digging any holes. This step protects you from hitting buried gas, electric, or irrigation lines.
Step 2: Marking Your Hole Locations Accurately Using String Grids
Use your batter boards and string lines to mark exact footing locations. This layout stage connects directly to the rest of your project, so make sure to read our complete Deck Building guide to see how this step fits into your overall plan from the very start.
Step 3: Digging Down Using a Manual Post-Hole Digger or Renting a Power Gas Auger
Dig each hole to your required depth below the frost line. A manual post-hole digger works fine for a few holes in soft soil, but a power auger saves a lot of effort on larger decks or harder ground.
Step 4: Adding a 2-Inch Layer of Crushed Gravel at the Bottom of the Hole for Drainage
Gravel at the bottom of the hole lets water drain away from the base of the concrete, instead of pooling underneath it. This helps prevent frost-related movement and keeps the footing more stable long-term.
Step 5: Dropping Cardboard Tube Forms (Sonotubes) into the Holes and Leveling Them
Set your Sonotube into the hole, then check that it sits level and at the correct height before you pour any concrete.
Step 6: Mixing and Pouring Fresh Concrete, Tapping the Tubes to Remove Trapped Air Bells
Mix your concrete according to the bag instructions, then pour it into the tube in stages, tapping the sides of the tube as you go. This releases trapped air pockets, called air bells, that can weaken the concrete if left inside.
Concrete Pouring & Safety Checklist
[ ] Wear safety glasses and gloves when mixing concrete
[ ] Confirm gravel base is in place before pouring
[ ] Check Sonotube is level before adding concrete
[ ] Pour concrete in stages, tapping tube sides to release air
[ ] Avoid pouring concrete in freezing weather conditions
[ ] Keep the pour area clear of trip hazards and loose tools
[ ] Allow proper curing time before adding structural weight
Step 7: Setting Adjustable, Galvanized Heavy Metal Post Bases into the Wet Concrete Top
Press a galvanized post base into the wet concrete while it is still workable, checking that it sits level and lines up with your layout. All the weight from your beams above will travel straight down through this connection, so check our complete Deck Beams guide to see how that weight moves from the top of the structure all the way down to this concrete footing.
Common Footing Blunders Beginners Make
Leaving loose, un-tamped dirt at the bottom of the hole. Loose soil compresses under weight over time, letting the footing sink slowly after the deck is built.
Pouring concrete in freezing winter weather. Fresh concrete needs proper temperature conditions to cure correctly, and freezing weather can weaken the final strength.
Skipping the gravel drainage layer. Without gravel, water can pool right under the concrete, which increases the risk of frost-related movement.
Putting wooden posts directly inside wet concrete instead of using metal stand-off bases. Wood sitting directly in concrete stays damp much longer, which speeds up rot right at the base of your post.
Frequently Asked Questions
What do I do if I hit rocky ground while digging? You may need to adjust your hole position slightly or use a heavier digging tool, but always maintain your required depth below the frost line.
How do I handle a footing hole that keeps filling with water? This usually means a drainage issue in your soil, so consider adding extra gravel at the base and pouring concrete as soon as conditions allow.
What ratio do I use for mixing bagged concrete? Most bagged concrete comes with a printed water-to-mix ratio on the packaging, so always follow the manufacturer’s exact instructions.
Can I use ground screws instead of traditional concrete footings? Yes, in certain soil types this can be a faster alternative, though it often requires specialized equipment or a contractor familiar with the system.
How long does concrete need to cure before building on top of it? Concrete typically needs several days of curing time before it reaches enough strength to safely support structural weight, so always allow proper curing time.
Do I need a footing under every single post? Yes, every support post needs its own properly sized footing to safely distribute the load into the ground.
What happens if my soil is very sandy instead of firm clay? Sandy or loose soil often has a lower bearing capacity, so footings may need to be wider than they would in dense, firm soil.
Can I reuse old footings from a previous deck? This depends on their condition, depth, and whether they still meet your current frost line and load requirements, so have them checked before relying on them again.
Is it okay to pour footings on a slope? Yes, but sloped sites often need extra care with hole depth and level checks, since the ground height changes across the footprint.
How many footings does a typical attached deck need? This depends on your beam spans and post spacing, which are calculated based on your overall deck size and load.
Final Thoughts
Deck footings are the part nobody sees once the project is finished, but they carry everything above them. Dig below your frost line, use a proper gravel base, and give your concrete the time it needs to cure before building on top of it.
If you want to see how footings fit into the whole build, check out our complete Deck Building guide, and take a look at our complete Deck Framing guide for how your posts connect on top of this foundation. Got a question about your own footing project? Drop a comment below. I read every one.
