Composite vs. pressure-treated decking

Start with the ledger

When a deck collapses, it's usually the ledger that fails. That's the board where the deck attaches to the house, and when it pulls away, there are usually people standing on the deck. It's the most important safety detail in the whole structure, and you can't see it once the deck is finished.

If you've just moved into a house with a deck, look underneath it. These are the problems to look for:

• Nails instead of bolts. The classic failure. Nails slowly pull out under repeated loads.
• No flashing. Water sits behind the ledger and rots the rim joist from the inside.
• A ledger fastened to brick veneer. Veneer isn't structural and can't hold up a deck.
• A ledger fastened over the siding. It leaves a gap, puts all the load on the fasteners, and traps water.
• No lateral connection. The deck holds its weight fine but has nothing stopping it from pulling away from the house.
• Posts sitting on soil, or soft wood at any connection.

Deck failures happen most on decks a previous owner built without a permit. Checking yours takes an afternoon and costs nothing.

A freestanding deck on its own posts avoids the ledger entirely, and on a difficult wall it's often the better choice. It needs more footings, which is a small cost for removing the riskiest part of the structure.

The decking itself

Pressure-treated lumber is the default for good reasons. It's cheap, every lumberyard has it, and any carpenter can work with it. The cost that doesn't show up in the quote is the staining: a full clean and new coat every couple of years, either as your weekend or as a contractor's invoice.

Capped composite has a core of wood fibre and plastic with a protective shell bonded around it. It doesn't rot, doesn't need staining, and holds its colour much better than the early versions did. It costs about twice as much installed, and it does need some care. You wash it instead of refinishing it.

Footings, railings and permits

In the US, the IRC exempts decks under 200 sq ft that sit no more than 30 inches above the ground and aren't attached to the house. Attaching it to the house is what usually triggers the permit.

Design a deck you'll use

Decks often get sized by what fits instead of what you'll do on them, which is how people end up with a big deck and one small table.

• Furniture takes more room than you think. A dining table with the chairs pulled out needs about 12 by 12 feet on its own.
• Leave a path. Keep a clear route past the furniture to the stairs and the door. People stop using a deck they have to squeeze across.
• Mind the door threshold. The deck surface has to sit far enough below the inside floor to keep water out, which affects the framing depth and the number of steps.
• Think about the sun. A west-facing deck with no shade is too hot on summer evenings, which is exactly when you want to use it.
• Plan the stairs early. They take a lot of space, and where they land shapes the yard around them.
• Run wiring and gas while it's framed. Lights, an outlet or a gas line for the barbecue are easy while the frame is open and a pain afterwards.

A low deck is cheaper and simpler. Above a certain height a deck needs railings, and above a higher one it usually needs a permit and bigger footings. A deck one step down from the door often avoids both and looks better against the house. If the ground allows it, consider that before defaulting to a raised deck. If you do build high, decide early what happens underneath: storage, a drainage system, or just open access. It's much cheaper to plan than to add later.

Questions homeowners ask