Understanding NJ Frost Line Requirements for Decks — Why It Matters

If you’re planning a deck in South Jersey — maybe in Marlton, Cherry Hill, Deptford, Voorhees, Mullica Hill, Berlin or any of the towns we serve — here’s something many homeowners overlook: how deep your posts go into the ground matters as much as what they’re made of.

❄️ What is the “Frost Line”?

The “frost line” (or “frost depth”) refers to how deep the ground freezes during cold weather. When water in the soil freezes, it expands — and that expansion pushes upward. Any structure whose concrete footings sit above the frost line is vulnerable to shifting, heaving, or becoming unstable once the ground freezes and thaws.

That’s why most municipalities in New Jersey — including those in South Jersey — require deck footings to go below the frost line.

📐 What the Code Requires

• Deck footings must go at least 36 inches (3 feet) below grade before concrete is poured.

• Footings must also be sized properly — usually at least 12 inches in diameter, or larger, depending on how heavy the deck and load will be.

If you skip or cut corners — like using shallow concrete blocks or “deck blocks” sitting just on top of soil — you’re gambling with frost heave and future instability.

🌨️ What Frost Heave Looks Like (And Why It’s Dangerous)

When frost gets under a deck that isn’t properly founded:

• Posts can shift or lift as the ground freezes and thaws.

• Deck boards may warp or become uneven.

• Stairs, railings, or even the deck ledger (where the deck attaches to the house) can loosen — putting safety at risk.

• Worst-case: structural failure — especially if heavy weight (grill, hot tub, furniture, multiple people) is on the deck.

Because many deck failures come from bad footings rather than bad boards, what you don’t see matters more than what you do.

✅ What You Should Do Before You Build

1. Make sure your builder digs footings 36″ or deeper — not just “below grade.”

2. Confirm footings are wide enough (12″ diameter minimum, or more depending on deck size/load).

3. Ensure footings are poured on undisturbed or well-compacted soil, not loose or recently backfilled dirt.

4. If building in a flood zone or on unstable soil — let your contractor know. Sometimes extra measures or engineered footings may be required.

5. Always get a proper permit and inspection — municipal inspectors often verify frost-line compliance before you pour concrete.

   
   

🏡 What This Means for South Jersey Deck Owners

South Jersey winters may not feel brutal compared to North Jersey, but frost still reaches deep enough to cause real problems for shallow or poorly built footings. A deck that seems “fine for now” can start to shift, tilt, or sag after a couple of freeze–thaw cycles — especially if the footings weren’t installed correctly or the framing wasn’t designed to handle movement.

At C-JAM Construction, we build with the long game in mind. Every project begins with proper footing depth and soil prep (see our Footings Guide), and continues with a framing system designed to stay solid season after season (covered in our Deck Framing Blog). Whether you’re in Cherry Hill, Mullica Hill, Voorhees, Marlton, Berlin, or anywhere else in our South Jersey Service Area, your deck gets the same code-compliant, structurally sound foundation.

If you're ready to make sure your next deck is built to withstand South Jersey winters — from the ground up — call us at 856-302-0249.