NH · set by county and city amendment
How deep do deck footings and fence posts have to go in New Hampshire? Below the frost line. Here's the number, and the winter behind it.
The requirement
48″
Footing bottoms in New Hampshire must reach 48 inches below grade — the frost line. Across New Hampshire it runs roughly 48″–60″ by county.
Reference: New Hampshire State Building Code (RSA 155-A; 2021 IRC), Section R403.1.4 with frost line depth entered by each municipality in Table R301.2 (2021 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the New Hampshire frost line. The hole bottom has to reach it — a few inches deeper seats the base cleanly below.
| Bag size | Exact | Buy (with spare) |
|---|
The buy column adds a 10% spare margin and rounds up. Running out mid-pour is the one mistake you can't fix.
New Hampshire gets true New England winters with weeks of continuous subfreezing air and deep seasonal cold. Frost reliably reaches four feet in the south and penetrates deeper in the White Mountains and far north.
The depth tracks how much freezing a winter actually delivers. NOAA's Air Freezing Index keeps a running score of how far and how long air temperatures sit below freezing; deeper scores mean deeper frost, and building codes translate that climate record into the footing depth above. Water expands about 9% when it freezes — when that happens under a footing, the ground lifts it. That's frost heave, and it cracks foundations, tilts fence lines, and pops deck piers. Getting below the frost line puts your concrete where the ground never freezes.
48 in is the standard requirement across southern and central New Hampshire (Nashua, Manchester and most towns); the North Country and mountain towns commonly require up to 60 in. The 2021 IRC applies statewide but each town fills in its own frost depth.
Treat this page as the starting number, not the permit. County and city amendments are where the real precision lives, and inspectors enforce the local figure — one phone call to the building department settles it.
48 inches is the working number (set by county and city amendment). Across New Hampshire it runs roughly 48″–60″ by county. Confirm the exact figure with your local building department — county amendments are common.
Bury one third of the above-grade height, never less than 24″, and never shallower than the frost line — in New Hampshire that means at least 48″. The post depth calculator checks both rules for you.
Below the frost line — 48″ minimum, bearing on undisturbed soil beneath it. Size the diameter with the deck footing calculator, then figure bags with the depth pre-filled above.
Yes — 811 covers every state, and the locate is free. Call a few business days before any post hole or footing dig and crews mark the buried utilities.