Frost depth in Ottawa: how deep deck and fence footings need to go

How deep do deck footings need to be in Ottawa?

Article 9.12.2.2 of the Ontario Building Code sets the minimum depth of foundations below finished ground level, and it does it with a table rather than a number. Two things pick your row: what the soil is, and whether it drains well down to the depth frost reaches. A deck foundation contains no heated space, so the columns that apply are the ones for unheated foundations. The heated-basement columns are written for houses.

Read the clay row closely, because it is the one most backyards fall under before anyone has dug. It covers clay and also soils not clearly defined, and it reads: 1.2 m but not less than the depth of frost penetration. That is two tests in one line. The footing goes to 1.2 metres, and if frost at that spot reaches deeper, the footing follows it down.

The Code never prints a frost depth for Ottawa. Article 1.1.3.2 says only that the depth of frost penetration shall be established on the basis of local experience. The most local statement of that experience we could find is the City's own. Its homeowner checklist for a residential sundeck carries a sample cross-section, and the dimension it draws from finished grade to the bottom of the footing is 4 feet, on an 8-inch concrete pier over a 24-inch by 6-inch footing. It is a sample drawing and not a by-law, and it lands on the Code's 1.2 metre floor rather than below it.

The rock row reads no limit in every column. A footing that bears on sound rock needs no particular depth, because rock does not heave. Where limestone sits close to the surface, this is the row that applies, and our Kanata deck builder page covers what that looks like on a real lot, including the awkward case where one end of a deck finds rock and the other end does not.

One more sentence in the article matters. It lets the table's depths be decreased where experience with local soil conditions shows lesser depths are satisfactory, or where the foundation is designed for lesser depths. In practice that route runs through an engineer's design or a building official's knowledge of the site.

Source: Ontario 2024 Building Code Compendium, Division B, Article 9.12.2.2 (Publications Ontario PDF) · O. Reg. 163/24, Building Code, on e-Laws · City of Ottawa: homeowner's sundeck checklist and sample plan (PDF)

The deck that does not need frost footings at all

Sentence (7) of the same article switches the depth table off for decks and other accessible exterior platforms that meet every one of five conditions. The deck is not more than one storey. It is not more than 55 square metres, about 592 square feet. The distance from finished ground to the underside of its floor joists is not more than 600 mm. It does not support a roof. And it is not attached to another structure, unless it can be demonstrated that differential movement will not adversely affect the performance of that structure.

Building departments call this a floating deck. It sits on pads or blocks at grade and is allowed to move with the ground. Carleton Place's building department, in a deck guideline written for the 2024 Code, puts the trade-off plainly: a floating deck may require seasonal adjustment of its columns because of frost heave.

The attachment condition is where people get caught. A low deck fixed to the house is not a floating deck, however small it is. The house stands on foundations below frost, and a deck that rises and settles with the ground each winter pulls on its connection to a house that does not move. That is the differential movement the Code is talking about.

The City's permit test and the Code's floating-deck test also measure different things. The City asks whether an attached deck's walking surface is more than 600 mm above grade. The Code measures to the underside of the joists, and it rules out attached decks entirely. So a small attached deck can fall outside the City's permit test and still fail the floating-deck conditions. Our Ottawa deck and fence permit guide covers the City's side in full.

Source: Town of Carleton Place: Part 9 decks guideline, OBC 2024 edition (PDF)

Why frost can lift a footing that is already deep enough

Over the winters of 1966 to 1968 the National Research Council ran a field experiment on its own property in Ottawa, in a post-glacial marine clay commonly called Leda clay. E. Penner and W. W. Irwin set steel pipes 3.5 inches across to a depth of 5 feet, anchored them to the bedrock beneath, and measured how hard the freezing ground pulled upward on them.

The answer was up to about 6,000 pounds on a single pipe. The test plot, kept clear of snow, heaved roughly 3 to 4 inches every winter. Their conclusion, published in 1969, is the sentence worth remembering if you own a deck in this city: placing foundation footings below the depth of frost penetration does not ensure stability of a structure against frost heaving if the soil is allowed to freeze to the foundation wall or column.

The mechanism is called adfreezing. Wet soil freezes onto the side of a post or pier, the frozen layer heaves, and it tries to carry whatever it is frozen to. The paper's working method treats the force as spread over the area where frozen soil touches the column, so the part of a pier that sits inside the frost zone matters as much as where its bottom ends up.

Two practical points follow. A pier that flares out near the surface, the mushroomed top you get when concrete overflows a form at grade, gives the freezing layer more to hold and a ledge to push against. And the footing at the bottom is wider than the pier for a reason, like the 24-inch footing under an 8-inch pier on the City's sample plan: that is the part bearing on soil below the frost.

Source: NRC: Penner and Irwin, Adfreezing of Leda clay to anchored footing columns (1969)

What Ottawa's ground temperature records show

Environment and Climate Change Canada publishes soil temperature normals for its Ottawa CDA station at the Central Experimental Farm, measured at 50 cm, 1 m, 1.5 m and 3 m down. The soil records run from 1981 to 1998 and are published within the 1981 to 2010 normals.

At 50 cm the monthly average slips just under zero in February and March. At 1 m it never averages below 2C. A 1.2 m footing therefore sits well below where the monthly averages put freezing at that station, and that margin covers what an average hides.

The first thing it hides is a hard winter. A monthly mean smooths a two-week cold snap into the rest of the month. The second is snow. Snow insulates the ground, and ground kept clear of it freezes harder; the NRC's Leda clay plot, cleared of snow, heaved 3 to 4 inches a year. Beside a deck, that is the shovelled path to the back door and the edge of the driveway.

We have not turned these readings into a frost depth for your yard, because one station, one soil and one surface cannot supply that. What they do show is the shape of an Ottawa winter underground, including a lag that matters for timing, which is the last section of this guide.

Source: Environment and Climate Change Canada: Ottawa CDA soil temperature normals at 1 m (data) · Environment and Climate Change Canada: Canadian Climate Normals

Sonotube and concrete pier, or helical pile?

The two systems people compare are these. The first is a poured concrete pier: a hole below frost, a footing pad at the bottom, and a cardboard form tube filled with concrete up to grade. Sonotube is the brand name most people use for the tube. It is the system on the City's sample sundeck plan, and its footings are sized under the prescriptive rules in Part 9 of the Code.

The second is a helical pile: a steel shaft with one or more helical blades welded to it, screwed into the ground by machine until it reaches a set torque. Several pile systems carry evaluations from the Canadian Construction Materials Centre, part of the National Research Council. Two of them are Techno Metal Post under CCMC 13059-R and Postech under CCMC 13102-R. Both are evaluated against the National Building Code of Canada 2015 for decks and porches within the scope of Part 9, and the CCMC states that in most jurisdictions its evaluation is sufficient evidence for approval by Canadian authorities.

The conditions inside those evaluations are the useful part, because they separate a pile done properly from a pile done fast. Under 13059-R a registered professional engineer must determine the number and spacing of the piles in all cases, the installer must be certified by the manufacturer and carry a certification card, and each pile is labelled with the evaluation number. Capacity is read from the torque needed to turn the pile, averaged over the last 600 mm of installation; 1,356 N m (1,000 ft-lbs) corresponds to an allowable 20 kN (4,500 lb) in compression. The shaft is wrapped in a ribbed polyethylene sleeve that the evaluation describes as a frost sleeve, there to isolate the pile from being jacked up by annual frost heave. That is the NRC's Ottawa adfreezing problem, answered in the product itself. The Postech evaluation, 13102-R, requires its piles to be screwed in to below the frost line.

Helical piles have limits too. The 13059-R load table covers granular, cohesive and silt soils, and rock is not in it. On a recently backfilled site the table does not apply at all, and allowable loads have to be confirmed by testing on site, which matters in newer subdivisions and regraded yards. Carleton Place's building department lists metal helical piles among foundations not covered in Part 9, to be designed by a qualified professional under Part 4, which lines up with the engineer the evaluation already requires.

Fill is a question for both systems. A poured footing has to reach undisturbed soil, and a pile on backfill has to be tested. Neither one lets you skip finding out what the deck is standing on.

Source: NRC CCMC 13059-R: Techno Metal Post · NRC CCMC 13102-R: Postech Screw Piles

Fence posts follow the same physics, not the same table

A fence that does not enclose a pool needs no building permit in Ottawa; the City's Fence By-law governs where it goes and how tall it can be. The Building Code's foundation table is written for buildings and decks, so nothing in it sets a depth for a fence post.

Frost does not make that distinction. A post set into the freezing layer gets gripped and lifted the way the NRC's pipes were, a little each winter, and a fence shows it as a line going out of true and a gate that stops latching. We set fence posts below the frost line and work out the depth on site, and our wood fence page explains how.

Why fall is the last easy window for footings

Ottawa's air turns before its ground does. At the Central Experimental Farm station the normal daily mean is 8.4C in October, 2.0C in November and -5.6C in December. Freezing degree-days, a running tally of how far below zero the days have been, go from 33 in a normal November to 190 in December, and a normal December has 17 days on which the high never gets above zero.

Underground the lag runs the other way. Soil at 50 cm still averages 5.2C in November and 1.8C in December at that station, and at 1 m it is 8.3C and 5.1C. So in a normal December the subsoil is not the obstacle. The surface and the concrete are. Every hole starts in the top layer, which is the first to freeze once the days stay below zero, concrete placed in a cold hole has to be protected from freezing while it gains strength, and the City's footing inspection has to happen before backfill, booked 48 hours ahead, in the shortest days of the year.

Helical piles change one part of that. The CCMC evaluation describes them as immediately ready for loading after installation, so there is no curing to protect. The engineer's layout, the certified installer and the machine are all still needed, so the planning does not get any shorter.

So if you want a deck standing this year, its footings go in during the fall or they wait for the spring, when the whole trade is busy at once. Our guide to the best time to build a deck in Ottawa covers that calendar. If the build is for next summer, the winter is still good for design and the permit application, so the footings can go in as soon as the ground allows.

What to check on the drawings before anyone digs

Whoever builds your deck, the paperwork should show the footings, and you can read them yourself. The City's sundeck checklist asks for a foundation plan showing footing size and location, and a cross-section showing footing depth and construction. So the depth is a number drawn on the plans, measured from finished grade, and not something settled on site after the hole is dug.

Read that number against the Code. On clay, or ground nobody has identified, it should be at least 1.2 metres (4 feet). A shallower footing needs a reason written down: the footing bears on rock, the deck qualifies as a floating deck, or an engineer designed the foundation for less.

A helical pile foundation comes with its own paper trail, set out in the CCMC evaluation, and every item on it is something you can ask to see. A professional engineer determines the number and spacing of the piles. The installer carries a certification card from the manufacturer. Each pile is labelled with the evaluation number. And a certificate attesting to the conformity of the installation and the allowable loads for the piles must be provided.

For poured piers the check is the City's footing and foundation inspection, which happens before backfill. A hole filled in before the inspector has seen it can mean uncovering the work again.

Where we come into it

Every project we take on is designed in 3D and AutoCAD before a board is cut, and those drawings become the plans the City's checklist asks for. We prepare the permit drawings and the application as part of the build. We have built in Ottawa since 2016. We are licensed, insured and family-owned, and every build carries a written workmanship warranty.

If a deck you already own shifts every spring, that is a footing question before it is a board question, and it is worth having looked at before anyone replaces boards. For a new deck, call (613) 902-6137. We look at what is under your yard before we price what goes on top of it.