How much clearance does a deck need off the ground, and where does the water go?

Where a deck actually rots

Not on the surface. Five places, in order of how often they show up:

1. The top of the joist, under the gap between two deck boards, where water sits on a flat surface all day. Joist tape exists for exactly this.
2. The ledger and the rim board behind it, where water has run down the wall because there was no flashing.
3. The bottom of a post, where the end grain sits on concrete that wicks water out of the ground.
4. The bottom of a stair stringer, sitting in soil.
5. Anywhere the wind cannot reach, which is the whole argument for clearance.

Clearance is about drying, not about looks

Wood decays when it stays wet. Wet wood dries when air moves past it. A deck 8 in off the ground with 2x10 joists has almost no air under it, so anything that gets wet stays wet, and the code answer to that is to require preservative treatment on everything within 6 in of the ground.

Treatment is a defence, not a solution. A treated joist in still, damp air lasts longer than an untreated one and still lasts less long than a joist that dries. If you have the height, use it.

Leave yourself a way in

This is the item people skip and regret. On the pier block and pad foundation that Edmonton accepts for low decks, the deck will move seasonally, and the requirement that comes with that acceptance is that the posts must be reachable so the platform can be re-levelled. Two ways to satisfy it:

• A minimum 2 ft by 2 ft access way to under the deck, where the joist bottoms are at least 24 in off the ground.
• Some removable decking over each post.

Removable decking costs four screws and a marker line at build time. Cutting it in later costs a circular saw, a set of boards that never quite match, and an afternoon.

Where the water goes

Deck boards are laid with a gap so water and debris fall through rather than sitting on the joist. About 1/8 in is the working figure. Treated lumber bought soaking wet will shrink as it dries and open gaps on its own, which is why some builders lay wet boards tight.

The grading matters as much as the gap. Slope the ground away from the house under the deck, not into it, and keep the space free of the leaves and needles that turn into soil against the posts.

Under deck water management is a membrane or a panel system below the joists that collects the water and drains it away, giving you dry storage or a dry patio underneath. It is a real upgrade, and it moves a maintenance problem: the system itself has to be reachable and cleanable, or it becomes the thing holding water against the frame.

Ventilation, and the gas line rule

Air has to be able to move under the deck. Solid skirting all the way round turns the underside into a damp box. If you skirt for looks, use something with gaps in it, or leave openings at opposite ends so there is a cross draught.

There is also a specific Edmonton note: a deck over any gas line should be vented on at least two sides, and the gas meter has to stay accessible for maintenance and replacement. Talk to the gas utility before you frame over a line.

The water you are bringing to it yourself

Four things put more water on a deck than the weather does, and all four are free to fix at design time:

• A downspout discharging beside a post.
• A dryer, bathroom or furnace vent blowing warm moist air under the deck.
• Planters sitting directly on the boards, which hold a wet footprint for a decade.
• A lawn sprinkler pointed at the fascia.

The five minute spring check

1. Look for the flashing lip over the ledger. Then push a screwdriver into the ledger near each end.
2. Look at the base of every post for dark, soft wood, and for soil that has built up against it.
3. Push hard on the guard at the middle of each run.
4. Clear the gaps between boards where they have packed with needles.
5. Check that the ground under the deck still slopes away from the house.