Dig Post Holes Below the Frost Line
This is the hardest physical work in deck building. Each post hole must reach below your local frost line—typically 36-48 inches deep—to prevent the deck from heaving when the ground freezes. The good news: once the holes are dug and footings poured, the heavy labor is behind you.
Quick Summary
Typical depth
36-48 inches
Typical diameter
12 inches
Time per hole
15-45 minutes
Why Frost Line Depth Matters
When water in the soil freezes, it expands. This expansion creates an upward force called frost heave. If your deck footings are above the frost line, the freezing soil will push them up during winter. When spring comes and the ground thaws, the footings don't settle back evenly—they stay crooked.
The problem: Shallow footings lead to a deck that tilts, separates from the house, and becomes unsafe. Fixing frost heave damage often means starting over completely.
Frost Line Depths by Region
Northern States (MN, WI, ME)
42-60 inches
Mid-Atlantic (PA, NJ, OH)
32-42 inches
Upper South (NC, TN, VA)
18-24 inches
Deep South (FL, TX, LA)
0-12 inches
Always verify with your local building department. Frost depths vary by microclimate and can differ from neighboring areas.
Proper Hole Dimensions
Depth
- •Below frost line + 6 inches for gravel
- •Typical range: 36-48 inches in most regions
- •Check local code for exact requirement
Diameter
- •12 inches minimum for 4x4 posts
- •16 inches recommended for 6x6 posts
- •Rule of thumb: 3x post width
Pro tip: Make the hole slightly wider at the bottom than the top (bell-shaped). This creates a footing that resists being pulled up by frost. The flared bottom is especially important in areas with severe freeze-thaw cycles.
Choosing Your Digging Tool
Manual Post Hole Digger
The classic clamshell design. Works well for a few holes in normal soil. Very labor-intensive for deep holes or clay.
Best for: 1-4 holes, loose/sandy soil, shallow depths
One-Person Power Auger
Gas-powered with handles you hold while standing. Can be difficult to control if auger hits a rock—may spin you around.
Best for: 4-8 holes, experienced operators, rock-free soil
Two-Person Power Auger (Recommended)
Larger motor with handles for two operators. Much safer and more powerful. Handles rocks better without spinning operators.
Best for: Multiple holes, clay or rocky soil, deep holes
Rental cost: $50-100/day at most equipment rental centers
Step-by-Step Digging Process
1. Verify Hole Locations One More Time
Before you start digging, double-check each marked location:
- •Confirm no utility markings are within 18 inches
- •Re-measure spacing between holes
- •Verify alignment with your string lines
2. Cut Through Landscape Fabric
If you installed landscape fabric, cut an X at each post location:
- •Make the X slightly larger than your hole diameter
- •Fold the fabric flaps back while digging
- •You'll tuck them around the concrete footing later
3. Start Digging
Begin at the center of your marked location and work outward:
Manual Post Hole Digger Technique
- 1. Plunge handles down to bite into soil
- 2. Squeeze handles together to grab dirt
- 3. Lift straight up and dump soil to the side
- 4. Keep walls as vertical as possible
Power Auger Technique
- 1. Position auger vertically at center point
- 2. Engage throttle and let weight do the work
- 3. Lift auger every 6 inches to clear dirt
- 4. If you hit a rock, reverse and try a different angle
4. Check Depth Frequently
Use a tape measure or marked stick to check depth as you go:
- •Mark your required depth on a straight 2x4 or stick
- •Check every 6-12 inches of digging
- •Go 2-3 inches deeper than required (for gravel base)
5. Flare the Bottom
Once you reach depth, widen the bottom of the hole to create a bell shape:
- •Use a digging bar or clamshell digger at an angle
- •Make bottom 4-6 inches wider than the top
- •This creates a footing that resists frost heave
Why this matters: A flared bottom creates an upside-down mushroom shape. Even if frost tries to push the footing up, the widened base anchors it in unfrozen soil below.
6. Add Drainage Gravel
Place 2-4 inches of gravel at the bottom of each hole:
- •Use crushed stone or pea gravel
- •Compact the gravel by tamping with a 2x4
- •This provides drainage and a stable base for concrete
Dealing with Common Problems
Hitting rocks
Small rocks: pry them out with a digging bar. Large rocks: try to dig around them or use a rock bar to break them up. If you can't remove a boulder, you may need to shift the post location a few inches (verify with inspector).
Hitting tree roots
Cut roots cleanly with loppers or a reciprocating saw. Avoid cutting roots larger than 2 inches diameter—this can damage the tree. If major roots are in the way, consult an arborist or consider relocating the post.
Water filling the hole
High water table or recent rain can fill holes. Let holes drain overnight if possible. For persistent water, add extra gravel to the bottom. You can pour concrete into a few inches of standing water—it will displace the water.
Hard clay soil
Clay becomes much easier to dig when moist. Soak the area with a hose the night before digging. Use a power auger with a clay bit. Work in short bursts to prevent overheating the auger motor.
Frequently Asked Questions
How deep should deck post holes be?
Post holes must extend below the frost line plus 6 inches for gravel drainage. This typically means 36-48 inches in most regions. Check your local building code for the exact frost line depth.
How wide should the holes be?
At least 12 inches for 4x4 posts, 16 inches for 6x6 posts. A good rule is three times the post width. The bottom should be flared wider than the top to create a bell shape.
What is frost heave?
Frost heave occurs when water in soil freezes and expands, pushing structures upward. Footings above the frost line get pushed up in winter and don't settle back evenly. This causes decks to become unlevel and can damage the structure.
Should I use a post hole digger or power auger?
For 1-4 holes in loose soil, manual is adequate. For more holes, hard soil, or deep holes, rent a two-person power auger ($50-100/day). Two-person augers are safer and more powerful than one-person models.
Ready for the Next Step?
With holes dug and gravel in place, you're ready to pour the concrete footings that will support your deck posts.