Footings and framing

Most people choose a deck by its boards, but it's the structure underneath that determines whether a deck is safe and how long it lasts. Footings, posts, bearers and joists carry the loads from people, furniture, planters and sometimes spas down into the ground.

This guide explains the basics in plain language. It isn't a design guide: your deck's structure should be designed or checked by a qualified person, and your building surveyor will inspect it.

The parts of a deck structure

• Footings: usually concrete pads or piers in the ground that spread the deck's loads into the soil.
• Posts or stumps: vertical members that sit on the footings and support the bearers. They may be timber, steel or concrete.
• Bearers: the main horizontal beams that sit on the posts.
• Joists: smaller horizontal members that sit on or hang from the bearers and support the decking boards.
• Ledger: where a deck is attached to a house, a ledger is fixed to the house to support one end of the joists.
• Bracing: elements that stop the deck swaying, especially important for higher decks.
• Decking boards: fixed to the joists.

Footings

Footing size and depth depend on the soil, the deck's height, its loads and the spacing of posts. Reactive clay soils, common in parts of Melbourne, can move as they wet and dry, which affects footing design. Your builder or engineer will specify footings, and your building surveyor will usually inspect them before concrete is poured.

Before digging, check for underground services. Before You Dig Australia is a free referral service that passes your enquiry to asset owners, who respond with information about their pipes and cables.

Timber treatment and ground clearance

Timber near the ground is exposed to more moisture and decay. Business Queensland explains that H3 treated wood suits uses kept off the ground but exposed to weather, while H4 suits wood in contact with the ground or continually damp. WoodSolutions notes that H4 isn't intended for critical structural elements in ground contact, which need H5 or a Class 1 durable hardwood, and that close-to-ground situations may call for higher protection.

Many builders use steel or concrete posts, or stirrups that hold timber posts above the ground, to reduce moisture problems at the base.

Spans and spacing

Bearer and joist sizes depend on how far they span and how far apart they're spaced. Decking boards also have maximum spans between joists, which vary by product. Composite boards often require closer joist spacing than timber. Your builder or engineer will use appropriate span tables or calculations.

Our deck calculator estimates joist numbers for a simple deck at a chosen spacing, useful for understanding a quote.

Attaching a deck to the house

The Victorian Building Authority says a structural engineer can make sure a deck's design meets building codes and that, if the deck is fixed to a building, the building can support the structural loads. The connection must be properly fixed and flashed so water doesn't get into the house wall. Poorly attached ledgers are a known cause of deck failures, so this detail matters.

Ventilation and drainage

Air needs to circulate under a deck so timber can dry. Low decks with little clearance, or decks enclosed with screening, can trap moisture. Make sure water drains away from the house and from under the deck.

Fixings and connectors

Bolts, screws, nails, joist hangers and brackets must suit outdoor use and be compatible with the timber treatment. Corroded connectors can weaken a structure over time. Your builder should follow manufacturers' recommendations.

Inspections

Building surveyors commonly require inspections at key stages, such as before footings are poured and once framing is complete. Don't let the builder cover the frame with boards before a required inspection.

Termites

Termites are active in many parts of Victoria. Timber decks, especially those close to the ground, can be at risk. Using appropriately treated or naturally durable timber, keeping the area under the deck dry and clear, and avoiding direct contact between untreated timber and soil all reduce the risk. If your house has a termite management system, make sure deck construction doesn't bridge or compromise it. Regular inspections by a licensed pest manager are a good idea in termite-prone areas.

Protecting the house wall

Where a deck meets the house, water management is crucial. The deck surface should generally be below the internal floor level or designed so water can't flow inside. Flashing should direct water away from the wall and the ledger. Weep holes and subfloor vents in brick walls should not be covered, because they let moisture escape and air circulate under the house. Ask your builder to explain how they'll handle these details.

Melbourne's climate and your deck

Melbourne's weather is hard on outdoor timber in its own way. Hot, dry spells with strong UV can dry and grey boards quickly, while long, damp winters keep shaded parts of a deck wet for weeks. Decks on the south side of a house, under trees or close to the ground may stay damp much longer than sunny, well-ventilated ones. Think about where your deck sits, how much sun each part gets and where leaves will collect, and plan materials, ventilation and maintenance accordingly. A deck that dries quickly after rain will almost always last longer.

Thinking of doing it yourself?

Building a deck yourself can be rewarding, but it's a significant project. You'll still need a permit where one is required, and the deck must meet the same standards as one built by a professional. Be realistic about your skills, tools and time, especially for footings, framing and balustrades, which are critical to safety. Many owners take a middle path: they have a registered builder construct the structure and handle the permit, then do finishing work, such as oiling, themselves. If you're considering an owner-builder approach, check the Building and Plumbing Commission's current requirements first.

Keep a deck file

Keep a simple folder, paper or digital, for your deck. Include the building permit and inspection records, plans and engineering details, the contract and any variations, product names and colours for boards and coatings, warranty documents and receipts. Add photos taken during construction, especially of the frame and footings before they're covered. This file is invaluable when you need repairs, make a warranty claim, sell your home or resolve a dispute, and it saves the next owner a lot of guesswork.

Spas, planters and heavy loads

A spa full of water and people, large planter boxes filled with wet soil, a masonry barbecue or an outdoor kitchen can put far more load on a deck than furniture and people alone. If you're planning anything heavy, tell your designer and builder at the start so the footings and framing can be designed for it. Adding a spa to an existing deck without checking the structure is risky. A structural engineer can assess whether a deck can carry the extra load.

Rain, runoff and drainage

Think about where water goes when it rains. Water falling on the deck passes through the gaps between boards to the ground beneath, which should drain away from the house rather than pooling. Roofed decks need gutters and downpipes connected to the stormwater system. Avoid directing runoff onto neighbouring properties. On sloping sites, the ground under the deck may need shaping or drainage. Standing water under a deck encourages decay and pests, so get drainage right from the start.

Key points

• Footings, posts, bearers and joists carry the deck's loads.
• Footing design depends on soil, height and loads; check for underground services first.
• Match timber treatment to exposure, with extra protection near the ground.
• Attached decks need careful design, fixing and flashing.
• Allow ventilation and drainage, and don't cover framing before required inspections.