Deck questions

Permits, drawings and inspections

• Do I need a permit to build a deck?
• How high can a deck be before it needs an engineer?
• What drawings does a deck permit application need?

  A site plan, a plan view and a section are normally enough for a simple rectangular deck. Edmonton asks for drawings whenever the deck is multi-level, an irregular shape, longer than 18 ft out from the house, wider than 30 ft, over 55 square metres (592 square feet), built of engineered lumber or steel, carrying a hot tub, fireplace, roof or gazebo, or on a top of bank site. Ask your own authority what its threshold is.

• Can I build the deck first and get the permit after?

  You can apply after the fact in most municipalities, but you lose the two things a permit is for. The reviewer cannot check what is now buried, and the inspector will often ask for work to be opened up. Expect a higher fee and a real chance of rebuilding a footing or a ledger connection you cannot see.

• Does a hot tub on the deck change the permit?

  Yes. A hot tub is an extraordinary load, so the joists, beam, posts, foundation and every connection need an engineer stamped design. Setting the tub on the ground and framing the deck around it usually avoids that, and Edmonton says so directly, but every service point on the tub still has to stay reachable. The electrical circuit is its own permit.

• What does the inspector actually look at?

  Footing depth and bearing before the concrete or the post goes in, then the framing: ledger fastening and flashing, beam bearing on top of the posts, joist hangers and their specified fasteners, guard post connections, and the stair rise and run. Most municipalities want to see the holes open, so book the first inspection before you backfill.

Footings, frost and the ground

• How deep do deck footings have to go?
• Can I use deck blocks instead of digging footings?

  For a low deck in Edmonton, yes, and the City says so: pier blocks on pads over well drained, undisturbed or compacted coarse grained soil have proved adequate. The catch is that the deck will move seasonally, so you have to be able to get under it and re-level it. Plan a 24 in by 24 in access way, or removable decking, before you build.

• Are screw piles allowed under a deck?

  They work well, but the National Building Code (Alberta Edition) does not define them, so they are not a prescriptive solution. That means an engineer designs them. For a deck over 1.8 m (5 ft 10 13/16 in) high that design has to be submitted with the permit application anyway.

• How big does the footing pad have to be?

  Size follows the load and the soil, so there is no single number. A 24 in by 24 in pad at least 4 in thick is the common Edmonton starting point for a low deck. Once the deck is more than 1.8 m (5 ft 10 13/16 in) high, or over 55 square metres (592 square feet) and needing a permit, an engineer sizes it and stamps the design. Footings that do not bear on rock or on coarse-grained soil that drains well go below frost, about 1.2 m (47 3/16 in), or are designed, once the deck is over 55 square metres (592 square feet) or its joists are more than 600 mm (23 9/16 in) up.

• Do I have to call before I dig?

  Yes, and it is free. Alberta One Call marks buried utilities. A deck may sit over a gas line so long as the line is not put at risk and the meter stays reachable, and the City asks that a deck over any gas line be vented on at least two sides. Check with the gas utility before you frame over one.

Attaching to the house

• Should the deck be attached to the house or freestanding?
• How many bolts does a ledger board need?

  The National Building Code does not prescribe ledger fastening, which surprises people. Edmonton points builders at the International Residential Code for best practice instead. Home inspectors use 1/2 in lag screws or through bolts with washers, staggered, at least 2 in from the top and bottom edges and 5 in from the ends, spaced by the joist span. Get the spacing from the source your authority accepts, not from a rule of thumb.

• Does the ledger have to be flashed?

  Yes, every time, even under a roof overhang. Without flashing, water runs behind the ledger and rots the rim joist it is bolted to, which is the connection that fails. Do not use aluminum flashing against pressure treated wood or galvanized fasteners: the two corrode each other.

• Can I attach a deck to a cantilevered floor?

  Not without an engineer stamped design, unless the original house builder installed a ledger there and it is marked on the engineered floor layout used to build the house. A cantilevered floor was never designed to take a deck hanging off it.

• Why do decks fall off houses?

  Almost always at the ledger. Inspection bodies put roughly nine in ten deck collapses at the point where the deck separates from the house, usually because the ledger was nailed rather than bolted, was never flashed and rotted the rim joist, or was bolted to a surface that was never structural.

Joists, beams and posts

• How far can a deck joist span?
• What size beam and post does my deck need?
• How far can a joist cantilever past the beam?

  The Canadian Wood Council deck span guide gives a maximum cantilever with each joist size: 8 in on a 2x4, 16 in on a 2x6 and a 2x8, and 24 in on a 2x10 and a 2x12, measured from the centre of the beam. The joist still has to be within its allowable span as well, so a long cantilever costs you span.

• Can the beam bolt to the side of the posts?

  No, not as a prescriptive solution. The beam bears on top of posts that are at least as wide as the beam. Alberta gives no direction for a split beam with members bolted either side of a post, so that detail needs an engineer, or full blocking down to the foundation at every post.

• Where can I join a beam that is not long enough?

  In a 2 ply beam on three posts the single joint goes over the middle post. If it cannot, use a 3 ply beam built up the way the code shows. In a 3 ply beam on three posts a joint may fall near the quarter point from the middle post, one per span.

• How do I build up a 2 ply or 3 ply beam?

  Nailed: two rows of 3 1/2 in nails no more than 18 in apart, plus nails within 4 in to 6 in of the end of every piece. Bolted: 1/2 in bolts with washers no more than 4 ft apart, and within 2 ft of each end.

• How far can a beam cantilever past the last post?

  The Alberta code does not prescribe it, so strictly it is engineering. The rule of thumb Edmonton reports as having worked is one and a half times the beam depth past the centre of the post: 9 in on a 6 in beam, 12 in on an 8 in beam, 15 in on a 10 in beam, 18 in on a 12 in beam.

• Can I use I joists or LVL on a deck?

  Yes, but they leave the prescriptive tables behind. Engineered wood, steel, masonry and concrete framing all trigger plan review, and you submit the manufacturer or engineer specifications with the application.

Guards and railings

• How high does a deck railing have to be, and when do I need one?
• How far apart can spindles be, and what is the no climb rule?
• Can a built in bench count as the guard?

  Yes, if it is measured honestly. A fixed perimeter bench can serve as the guard where the guard height is met measured up from the seating surface, not from the deck, and the openings below the seat meet the same limits. In practice that makes for a tall back, which is why most builders keep the bench and the guard separate.

• Do glass and cable railings need anything extra?

  Yes. A manufactured guard of metal, vinyl or glass has to show it meets the code through a CCMC evaluation report, a testing report, or Alberta engineer stamped drawings, and you follow the installation instructions exactly. Glass has to be factory marked tempered or laminated safety glass to the named Canadian standard.

• How should guard posts be fastened?

  With lag screws or through bolts, not nails, and not a notched post relying on the notch. Add rim blocking behind a side mounted wood post so the guard can take its design load, and use a 2x8 joist or deeper so there is enough face to bolt to.

• Does a low deck need a guard if there is a retaining wall beside it?

  It can. If the surface within 1.2 m (47 3/16 in) of the deck edge drops away more than 600 mm (23 9/16 in), or slopes away steeper than 1 in 2, the code measures to that lower surface. A 500 mm (19 11/16 in) deck at the top of a retaining wall is treated as a high deck, needs a guard, and needs a building permit.

Stairs, landings and handrails

• How tall can a deck step be and how deep does the tread have to be?
• When does a deck stair need a landing?
• When does a deck stair need a handrail, and how high?
• How wide do deck stairs have to be and how far apart can stringers go?
• Why is my bottom step always the wrong height?

  Because the tread thickness was not taken off the bottom riser when the stringer was laid out. It is the single most common stair error, and it shows up as a top or bottom riser that is out by the thickness of a tread. The rise may only vary 5 mm (3/16 in) between adjacent steps and 10 mm (7/16 in) across the whole flight, so it fails.

• Can I build a spiral or curved stair off a deck?

  Yes, to tight tolerances. Tapered treads need at least 150 mm (6 in) of run at the narrow end, and measured 300 mm (12 in) in from the centre line of the handrail at the narrow end the tread has to be a uniform 255 mm (10 in) to 355 mm (14 in) deep, with the same rise tolerances as a straight flight.

• Do deck stairs need lighting?

  Exterior stairs serving a house are expected to be lit, and inspectors call it out. Beyond code, it is the cheapest safety item on the deck: a light at the top and one at the bottom, or lit risers, removes the step you cannot see in November.

Materials and durability

• Which parts of a deck have to be treated or naturally decay resistant?
• What decking can I use and how far apart do the joists go?
• Can I lay the decking diagonally?

  Yes, but the joists move closer together. A board laid at 45 degrees crosses more space between joists than its spacing suggests, so you either tighten the joists or pick a board rated for the longer unsupported length measured along the board. Most builders drop from 16 in to 12 in centres.

• Which fasteners can I use with treated lumber?

  Fasteners have to be compatible with the treatment. Hot dipped galvanized or stainless is the normal answer, and the treatment manufacturer publishes what its product needs. Plain zinc plated screws and aluminum flashing corrode quickly against modern treated wood.

• Is cedar as good as pressure treated?

  For the surface, cedar is a fine choice and is naturally decay resistant. For the parts in or near the ground it is not the equal of factory treatment, and cedar sapwood is not decay resistant at all. Cedar also spans slightly differently from spruce, so use the span tables for the species you actually buy.

• Do I have to seal or stain the deck?

  You have to make it resistant to the weather, one way or another. Factory treatment does that for the framing. A surface finish is what keeps the boards from cupping and greying, and it is the difference between a deck at fifteen years and a deck at thirty.

• What does a composite decking product have to prove?

  That it meets the code, through a CCMC evaluation report, a testing report, or Alberta engineer stamped specifications, submitted with the permit application. After that you follow the manufacturer joist spacing and floor preparation, not the lumber tables.

Water, clearance and upkeep

• How much clearance does a deck need off the ground, and where does the water go?
• Should I leave gaps between the deck boards?

  Yes. A gap of about an eighth of an inch lets water and debris through instead of sitting on the joist. Wet treated lumber shrinks as it dries, so boards installed soaking will open up on their own, which is why some builders lay them tight.

• Can I put a roof over the deck later?

  Only if the deck was built for it. A roof is an extraordinary load: it needs an engineer stamped design for the deck and its connections whatever the deck height. If a sunroom is in your future, build the foundation for it now and keep the drawings, because the City is not obliged to hold them for you.

• How far does the deck have to be from overhead power lines?

  Edmonton requires at least 3.5 m (11 ft 6 in) of clearance from the deck walking surface up to electrical wires. It is easy to miss on a second storey deck or when a service drop crosses the back yard, and it is not a detail you can fix after framing.

• How often should a deck be inspected?

  Walk it every spring. Look at the ledger and its flashing first, then the post bases, then the guard posts, then the stair stringers: those four carry the risk. Anything that has been standing thirty years, or has been through an owner who added a hot tub, is worth a professional inspection.