Deck Building Calculator

The two most common triggers are height and size, plus attachment. Many Ontario municipalities require a building permit once a deck is more than about 24 inches, or 600 millimetres, above grade, and a deck larger than roughly 108 square feet, or 10 square metres, often needs one as well. On top of that, a deck attached to the house generally requires a permit regardless of its size, because the connection to the building is a structural matter. There are exemptions for small, low, free-standing platforms in many places, but the thresholds and the fine print vary from one municipality to the next, so the numbers in this calculator are a planning guide rather than a guarantee. The cheapest and simplest move is to call your local building department before you dig, describe the deck you are planning, and ask directly whether a permit is required, because getting that answer early costs nothing and prevents the far more expensive experience of being told to stop work or rebuild something that was not approved.

Around 24 inches, or 600 millimetres, above grade is the point where guards, meaning railings, generally become mandatory under the Ontario Building Code, and that single requirement changes the character of the whole project. Once guards are involved, the deck is treated as a more significant safety element, which usually means the building department wants to see it, which in turn usually means a permit and inspections. Below that height, a deck is often considered low enough that a fall is not a serious hazard, so the requirements are lighter, though you still have to comply with zoning and any local rules. The practical takeaway is that the jump from a low platform to a deck at or above 24 inches is not just a few more boards, it is a step up in code obligations, guard construction, and paperwork, so it is worth deciding early whether your design sits below that line or clearly above it rather than hovering right at it.

Guard height in Ontario depends on how far the walking surface sits above the ground below. For most residential decks the minimum guard height is 900 millimetres, which is about 36 inches, and that applies where the deck surface is not more than about 1,800 millimetres, roughly 6 feet, above the adjacent grade. Once the drop exceeds that height, the required guard rises to 1,070 millimetres, about 42 inches. In addition to height, the openings in the guard matter: no opening may allow a 100 millimetre sphere, about 4 inches, to pass through, which is the rule that governs baluster spacing and is often checked directly by inspectors. There can be additional considerations at stairs and in specific configurations, and some inspectors apply a non-climbable interpretation to horizontal elements, so if you want to avoid a rebuild it is wise to design a clearly compliant, clearly non-climbable guard from the start rather than testing where the line is.

Deck footings in Ontario must bear on undisturbed soil and extend below the local frost depth so that seasonal freezing and thawing cannot heave them, and in much of the province that means going down on the order of 1.2 metres, roughly 4 feet, though the exact depth depends on your region and soil and is set by your municipality. Skimping here is one of the most common and most damaging deck mistakes, because a footing that stops above the frost line will lift in winter and settle in spring, and the deck will move, rack, and pull on any ledger connection to the house. The two typical systems are a poured concrete pad with a sonotube pier above it, and a manufactured form such as the BIGFOOT system that combines the bell-shaped base and the tube. Deck blocks and patio stones are sometimes acceptable for very low free-standing platforms but are commonly rejected for elevated or attached decks, so unless your building department says otherwise, plan for real frost-depth footings.

Both approaches are valid, and the right choice depends largely on the condition of your wall and how confident you are in the connection. A properly designed ledger, bolted into the structural framing of the house and correctly flashed, transfers load cleanly and is used on countless decks, but the ledger is also where a large share of deck problems and failures originate, almost always because of water getting behind it and rotting the framing, or because it was fastened to something that is not structural. Brick veneer, for instance, is not structural and cannot carry a ledger. If your wall is masonry or veneer, if the framing behind it is questionable, or if you cannot flash the connection properly, a free-standing deck that carries its own load on its own posts and footings is often the safer choice, because it removes the water path into the house entirely. For anything but a simple, well-understood wall, it is worth getting the connection detail right on paper before you build.

Often yes. While 16 inches on centre is a common joist spacing for traditional wood decking, many composite and PVC decking products require joists at 12 inches on centre, and the spacing can be even tighter when the boards run diagonally rather than straight across. This is not a matter of builder preference or opinion, it is a manufacturer requirement tied to the product warranty, and installing composite over joists that are spaced too far apart can void that warranty and lead to boards that feel bouncy or sag over time. Because the required spacing depends on the specific product, the board thickness, and the installation direction, the correct step is to check the manufacturer’s installation instructions for the exact decking you intend to buy before you frame, since the joist layout has to be built for the decking, not the other way around. Planning this early avoids the unpleasant discovery, after the framing is done, that your beautiful composite boards need joists you did not install.

Exterior decks demand corrosion-resistant hardware throughout, and the cost of all those individual pieces adds up far faster than most people budget for. Modern pressure-treated lumber is more corrosive to metal than older formulations, so the fasteners and connectors in contact with it have to be hot-dip galvanized or stainless steel, typically with a heavy G-185 coating on connectors, and cheaper zinc-plated hardware corrodes quickly and can fail. On top of the deck screws, a deck needs joist hangers, post bases, beam connectors, structural bolts and washers, and often hurricane or tension ties, each of which is inexpensive on its own but significant in total across an entire deck. It is genuinely common for the hardware and fasteners line to be one of the quiet surprises in a deck budget, which is exactly why this calculator includes a hardware allowance rather than letting it hide. Buying the correct corrosion-resistant hardware is not the place to economize, because the connectors are what hold the structure together.

This calculator is a planning and budgeting tool, and it is genuinely useful for getting an early sense of materials and cost and for understanding which decisions drive the price, but it is not an engineered design and it will not always match what an inspector requires. Real approval depends on details this tool does not resolve, including the correct joist and beam sizing for your exact span, species, and load, the soil bearing capacity at your site, the zoning setbacks that govern where the deck can sit, and the specific attachment and guard details your municipality enforces. Loads like a hot tub or a roof over the deck change the structure substantially. The right way to use the numbers here is as a starting point that helps you plan, price, and have a more informed conversation with your designer or building department, and then to confirm the final design against your municipality’s prescriptive deck tables or an engineered drawing before you build. Treat it as a smart first estimate, not a stamped drawing.

Usually yes. Guards are required once the walking surface is more than 600 mm above finished grade, and most municipalities require a building permit for elevated decks. Confirm with your local building department.

Often yes, especially if it is attached to the house. Many municipalities use 10 m2 and/or 600 mm as common triggers, but attachment can still require a permit. Confirm locally.

Typically no if it is maintenance only – swapping surface boards – and you are not changing the structure, size, height, stairs, or guards. Structural changes usually trigger a permit.

That is zoning, not the Building Code, and it varies by municipality. Check your local zoning and setback rules before you dig footings.

Usually not advisable. Easements and utilities can restrict structures, and septic systems need clearance for function and future replacement. Get locates and confirm restrictions with the municipality or utility.

Commonly a site plan (deck location to lot lines), a framing plan (joists, beam, posts with sizes and spacing), footing details, guard and stair details, and ledger connection details if attached. Your municipality often has a checklist.

Sometimes for very low, free-standing platforms. Many municipalities do not accept deck blocks for elevated decks or decks attached to a house. Verify locally.

There is no single code diameter – load, soil bearing, post spacing, and deck size all matter. Use your municipality’s prescriptive details and tables, or an engineered design.

Post spacing depends on beam size, number of plies, species and grade, joist span, and loads. Use municipal prescriptive tables where available, otherwise engineer it.

Helical piles are common, but permit requirements still apply if the deck triggers a permit. Some municipalities require pile specs and/or engineered layouts.

Brick veneer is not structural. Ledger loads must transfer into structural framing. If the connection is questionable, a free-standing deck is often the safest route.

Yes. Flashing is critical to prevent water intrusion and rot. Use durable, compatible flashing that directs water out and away – do not rely on caulking.

It depends on span, spacing, species and grade, and loads. Use municipal span tables or an engineered design, especially for longer spans or heavy loads such as a hot tub or a roof.

16 in on centre is common. Composite and thinner decking often require 12 in on centre. 24 in is less common and must match the span tables and the decking manufacturer’s requirements.

Use rated connectors at ledger connections, where joists bear without full bearing, and at post and beam connections – joist hangers, post bases, and ties. Use exterior-rated, PT-compatible hardware.

Guard height changes with deck height above grade. Many municipalities enforce 900 mm (about 36 in) at lower heights and 1,070 mm (42 in) once the drop exceeds about 1,800 mm (6 ft). Confirm locally.

Openings must not allow a 100 mm (4 in) sphere to pass. Special conditions can apply at stair triangles and other locations.

Handrail requirements depend on the number of risers and the stair configuration. Municipal guides give the clearest local interpretation – follow them.

Use PT-compatible fasteners and connectors – hot-dip galvanized or stainless where needed. Avoid standard zinc fasteners, which corrode quickly with modern PT lumber.