Building a fence or deck in the fall: what frost changes

Every October we get the same question: is it too late to put up a fence or build a deck this year? Usually it isn't. The wood doesn't care what month it is. What matters is the ground — specifically, whether the holes can be dug and the footings set before it freezes.

Fall building does change a few things, though, and it's better to know them before you book than to find out in March. Here's how we think about it.

The deadline is the digging, not the building

A fence or a deck is two jobs. The first is everything below grade: post holes, footings, concrete or compacted stone. The second is everything above it: framing, boards, rails, gates. Only the first one has a hard seasonal deadline.

Once the top layer of soil freezes, digging a clean, properly shaped hole gets slow and then stops being practical. Frozen clay also comes out in chunks, which leaves a ragged hole — and the shape of the hole matters more than most people realise, as we'll get to.

Carpentry is far more forgiving. Framing a deck or hanging fence boards on a cold, dry day in November is perfectly good work. So the question to ask isn't “can the whole project be finished before winter?” It's “can everything that goes in the ground be done before the ground locks up?”

Why frost depth decides whether it stays straight

In our part of southern Ontario, the working number for frost depth is four feet (1.2 m). Footings for a deck need to sit below that line, and fence posts that are expected to stay plumb should be set with it in mind too.

The reason is frost heave. Wet soil expands as it freezes. If the bottom of a footing sits inside the frozen zone, the ground lifts it. It rarely settles back to exactly where it started, so each winter leaves the post a little higher or a little more crooked than the last. That's how you get a fence line that waves and a deck that pulls away from its stairs.

Depth alone isn't the whole story. Frost also grips the sides of a post or footing and pulls upward. A hole that flares out at the top, or concrete that's been allowed to mushroom over the rim, gives the frost a lip to lift against — even when the bottom is deep enough. Straight-sided holes and good drainage around the post matter as much as the tape measure. On the heavy clay common around Fergus and Elora, which holds water and heaves hard, they matter more.

What cold weather changes on the job

None of these is a reason not to build in the fall. They're things a contractor should be planning around, and things worth asking about.

• Concrete sets more slowly as temperatures drop, and fresh concrete must not be allowed to freeze before it has gained strength. Late in the season that can mean pouring earlier in the day, protecting the footings overnight, or waiting out a cold snap.
• Days are short. A fence that takes two days in June can take three in November, simply because there's less daylight to work in.
• Fall lawns are soft. Wet ground ruts easily under a machine or a loaded wheelbarrow, so access routes need more thought and sometimes ground protection.
• Pressure-treated lumber arrives wet and will not dry out in cold, damp weather. A fall build gets stained or sealed the following spring or summer — which is the right time for it anyway.
• Wood moves as it dries. Boards installed tight in a damp fall will show small gaps by midsummer. That's normal, and a good installer spaces for it.

What to line up before the ground freezes

Most fall projects that slip into spring don't slip because of the weather. They slip because of paperwork that could have been started weeks earlier. If you're thinking about a fall build, these are the things to get moving first.

• Utility locates. Anyone digging in Ontario — homeowner or contractor — is required to request locates through Ontario One Call before breaking ground. The service is free, but it isn't instant, so allow at least a week.
• Permits. Many decks need a building permit, depending on their size, their height off the ground and whether they attach to the house. Check with your township's building department before a hole is dug; approval takes time you may not have in late fall.
• Fence bylaws. Most municipalities limit fence height and have separate, stricter rules for a fence that encloses a pool. Corner lots often have their own restrictions.
• The property line. If there's any doubt about where it is, sort that out before the posts go in, not after. A conversation with the neighbour now is easier than one in the spring.
• The layout. Decide where gates go, which way they swing, and where deck stairs land. Changing your mind is free on paper and expensive once the posts are set.

Splitting the job across winter

If the calendar is tight, there's a sensible middle path: get everything below grade done in the fall and finish the carpentry when it suits. Posts and footings set before freeze-up have the whole winter to settle, and the visible work can carry on in dry cold weather or wait for the first good week of spring.

This works well for fences in particular. It's less tidy for a deck, where you'd be looking at bare footings or an open frame for a few months, so it's a judgement call that depends on how much the yard gets used in winter.

When we'd tell you to wait for spring

Sometimes waiting is the right answer, and we'd rather say so than start something we can't do properly.

If the ground has already frozen more than a skin deep, the holes won't be clean and the footings won't be what they should be. If a deck needs a permit that hasn't been applied for and it's already late in the season, the approval may well outlast the weather. And if the yard is saturated and the only way in is across the lawn, the damage from getting equipment to the back can cost more to repair than the delay is worth.

In any of those cases the useful thing to do in the fall is the planning: settle the design, pick the material, get the permit and the quote sorted, and be first on the schedule when the ground opens up rather than joining the spring queue.

The bottom line

Fall is a perfectly good season to build a fence or a deck in Ontario. The deadline is the frost, and it applies to the holes, not the boards. Get the locates, permits and layout sorted early, make sure the footings go below frost in straight-sided holes, and plan to stain next year.

We build decks and fences across Fergus, Elora, Centre Wellington and Guelph. If you're not sure whether there's still time this year, ask — the answer depends on your ground and this year's weather, and we'll tell you plainly if spring is the better call.