How deep fence posts go in the Illinois Valley, and why

What the frost line is, and why a post has to clear it

The frost line is how far down the ground freezes (opens in a new tab) in a normal winter. Municipal codes and footing tables in this part of the state are written against it, which is why a fence built to a southern-Illinois habit does not survive here.

Soil holds water. Water expands when it freezes. Everything inside the layer that freezes gets pushed around by it, every year, without exception.

Two things happen to a shallow post. The first is grip. Frozen soil bonds to the sides of the post and lifts it as the ground swells. The second is what forms underneath. Ice builds in lenses below the base and jacks the post up from the bottom.

Come spring the soil settles back down. The post does not come all the way with it. Loose fill runs into the void underneath, and next winter the post starts higher than it started the year before.

A post set below the frost line has its bottom sitting in ground that never freezes. The frozen layer can grab the shaft, but it has nothing to lift against. That is the whole mechanism, and it is why depth is the single decision that outlives every other one on a fence.

Frost does not drive to the same depth everywhere in the five counties. Soil type, drainage, exposure, and how much snow sits on a run all change how far down it gets in a given winter. That is why the depth for your yard belongs in your scope of work, written down, rather than read off a table.

What a heaved post looks like in year three

You will not see it happen. You will see the results, and they arrive in a predictable order.

• The gate scrapes in February. It swings clean again in July. The following February it scrapes worse, and by the third year it scrapes in July too.
• The latch stops catching. The gate post has moved and the strike no longer lines up. People start lifting the gate to close it.
• The top rail is no longer straight. Stand at one end of the run and sight down it. A heaved post shows as a hump in a line that used to be dead flat.
• A gap opens under a picket. Not the whole run. One panel, in one spot, where a single post came up.
• The concrete collar sits proud of the grass. You can see the top of a footing that used to be buried.
• On vinyl, the panel pulls out of the routed post. The rail is only held by the depth of that routed slot, and a post that lifts takes the slot up with it.

None of that is a materials failure. Every one of those fences was built with fine material. They were set in the layer that moves.

Xcel’s practice, and where the number belongs

Two things, and only one of them belongs on a web page.

• The practiceXcel sets fence posts below the frost line.
• The numberIt goes in your scope of work, not here. A depth printed on a website is not a depth anyone can be held to afterwards.

So ask for it. Ask how deep the posts go and whether that clears the frost line, and have the answer written into the scope before you sign. The estimate is free and Jake comes to the property, so the question costs you nothing to put to him. It is the right thing to ask any fence contractor in this region, this one included.

A contractor who answers that question without hedging has told you most of what you need to know about the rest of the job.

Rocky ground, and the change order that arrives after you sign

Parts of the Illinois Valley have notoriously rocky ground. It is not uniform and it is not predictable from the street. One yard augers like butter and the yard two doors down stops the machine cold.

Here is how it goes wrong. A contractor from outside the area prices the job off linear feet and a soil assumption. The crew arrives, digs the first few holes fine, and then the auger bites, stops, and starts bouncing. Work halts. Somebody calls the homeowner. The conversation that follows is about money, and it happens after the contract is signed, which is the worst possible time to have it.

Xcel prices rock in before quoting. The ground gets read during the walkthrough, and the quote is built on what was found out there. The number you were given is the number.

That is worth being precise about, because it is a different promise than the one most contractors make. Plenty of quotes carry a rock clause that tells you what you will be charged extra if rock shows up. That clause is a warning. Pricing the rock in before the number is written is not a warning. It is the reading happening before the price instead of after it.

What to ask before anyone digs in your yard

Six questions. Ask them of every contractor you are considering, and write down what you get back.

• How deep do the posts go, in inches, and does that clear the frost line? Get the number into the scope of work. Said on the phone, it is not a specification.
• What goes in the hole with the post, and how is it backfilled?
• What happens to the price if you hit rock? Ask this before signing. The answer costs you nothing now and can cost a lot later.
• Who calls in the utility locate? Digging in Illinois requires one. The statewide one-call service is JULIE (opens in a new tab), reached at 811, and the request goes in before anything goes in the ground.
• How are gate posts handled differently from line posts? A gate post carries a swinging load every day of its life. It is the post most likely to move and the one you will notice first.
• Do I need a permit or a survey? Many towns and HOAs want a sign-off before a fence goes up, and those approvals stay in the homeowner’s name. Know what your municipality asks for before you schedule anything.

Deck footings are a separate question

The frost-line logic applies to anything structural set in the ground, and deck footings in this region are governed by your local building code (opens in a new tab) rather than by a fence rule of thumb.

The below-frost-line practice described on this page covers fence posts. It is stated that way because that is what it covers. If you are planning a deck, ask about footings as its own question when you talk through the build, and do not let a fence answer stand in for a deck answer. A deck footing carries a live load down into the ground. A fence post holds a line.

Related questions

Xcel Outdoor Living builds fences and decks across Bureau, LaSalle, Putnam, Marshall, and Lee counties, Illinois. Call 815-579-2323 to walk your fence line.