Structural basics: bearers, joists, posts and footings

Every deck that does not sag, sway or pull its ledger out of a wall is doing a quiet piece of physics. People walk. Furniture sits. Rain pools for an hour. A party gathers in one corner. The boards take that load and hand it to the joists; the joists hand it to the bearers; the bearers hand it to the posts; the posts hand it to the footings; the footings hand it to the ground, which was there first and will be there after the oil is gone. If you can keep that sentence in your head, the rest of the craft is detail.

This is not a span table and it is not an engineering certificate. Typical talk about sizes is how builders learn to see. It is not how a certifier signs a high deck on a clay bank. Read it as a way to become a better client of the people who will draw the stairs and the people who will specify the timber.

The load path, named from the top

Decking boards are the only members most owners can name. They are the wearing surface and, structurally, a series of small spans between joists. Their job is to spread a footfall and to drain. They are not the frame. A thick board will not save a skinny joist. A handsome species will not save a wet bearer. Choose boards for durability, slip, look and the way they take a fastener; then let the frame be a frame.

Joists are the regular rhythm under the boards. They run, in the common Australian job, across the shorter direction of the bay, sat on or hung from the bearers. Spacing is a conversation with the board: a thin or more flexible decking wants joists closer together; a stiff hardwood will tolerate a wider spacing if the table and the grade say so. The joist is also where the connector lives — the hanger, the screw, the secret of whether the underside looks like a finished room or a hardware drawer.

Bearers are the heavier members that collect the joists and carry them to the posts. They are the sentence of the structure: long, few, and unforgiving if they are undersized or poorly seated. A bearer that is spliced on a post, or continuous over it, is a different animal from a bearer that stops short and hopes. In conversation, this is where people start waving at “what we did last time”. Last time was a different span, a different load and a different soil.

Posts take the vertical load down, and they take some of the horizontal story as well — wind on a high deck, people leaning on a rail, the slow racking that turns a rectangle into a parallelogram. A short post in a garden bed is a stool. A long post on a falling block is a column, and columns buckle, rust at the base, and need bracing you can explain.

Footings are the conversation with the ground. Concrete in a hole is the usual Australian answer. The size, the depth, the reinforcement, the way the post meets the concrete — these change with soil, slope, frost in the high country, and the presence of a sewer you did not know about. A footing that is a bag of premix in a scrape is not a footing. It is a pause before movement.

The walking surface is the architecture people photograph. The load path is the architecture that is still there after the photograph.

Spans as a way of seeing, not a way of signing

Builders carry span tables the way cooks carry temperatures. They are useful. They are not cuisine. A table assumes a grade of timber, a stress rating, a spacing, a load, a deflection limit, and a world in which the member is dry enough and straight enough to be the member in the photograph. Change the load — a roof, a kitchen, a stone-topped bench, a crowd on a narrow bay — and the table is a different document. Change the timber — a pretty but ungraded stick from a salvage yard — and the table was never about you.

Typical talk is still worth having. It teaches proportion. A joist that looks like a decking board is not a joist. A bearer that is asked to span a room like a floor joist may need to be two members, or a laminated one, or steel. A post that is asked to stand two metres out of the ground on a windy ridge wants a section and a base that a garden job will not give it. Use those instincts to know when to stop and ring the engineer. Do not use them to avoid the engineer.

Steel appears in good timber decks without apology. A steel bearer can take a span a hardwood cannot, or keep a depth the door needs. A steel post can meet a footing with a bracket you can inspect. Mixed construction is how Australian outdoor rooms have always been built when the architecture asked for a long, level plane over uneven ground.

Ventilation is structural

The underside of a deck is a climate. In a wet summer it can be a closed tank of warm air, leaf and the smell of soil. Members that were sized as dry timber then live as wet timber. Connectors that were specified as galvanised then live as a salt and tannin experiment. Termites, as the companion essay says, prefer the climate you accidentally designed.

Ventilation is not a lifestyle extra. It is how the frame remains the frame the table assumed.

• Leave a way for air to move under the walking surface, even on a low deck.
• Do not seal the perimeter with a garden wall and call it tidy unless you have designed a dry, inspectable box.
• Keep irrigation from spraying the joists.
• Give water a way off the boards and a way off the bearers. A bearer with a dip is a gutter.
• On a roofed deck, the subfloor still wets from driving rain and from the garden. Roofed is not dry.

A fully lined outdoor room is a different structure and a different condensation problem. If you want a ceiling under the joists, you are no longer only building a deck. You are building a soffit, and you will need the discipline of a soffit: fall, drip, and a way for the dark to breathe or be sealed on purpose.

Connections, corrosion and the small parts that fail first

The Australian climate is hard on metal. Coastal salt, indoor pools a suburb inland, tannin from hardwoods, and the ordinary wet-dry cycle will find a cheap connector and unmake it. The materials conversation includes fasteners because a treated pine member with the wrong nail is a corrosion cell, and a hardwood with the wrong screw is a split.

Name the connectors as carefully as the timber.

Joist hangers should be the right size, the right durability class for the exposure, and nailed or screwed as the manufacturer drew, not as the leftover box allowed. A hanger with three nails in it is a decoration.

Post bases should keep end grain out of a puddle. A post dropped into concrete and left to rot at the collar is a classic Australian failure, as old as the weekend job. A bracket that lifts the timber, or a detailing that lets the post sit dry on a pier, is the grown-up version.

Ledgers and fixings into the house should be designed, flashed and not asked to carry a roof they were not told about. The house attachment is a structural joint and a waterproofing joint and a termite joint. It is allowed to be expensive.

Coastal and poolside jobs want stainless or a specified equivalent, not a hopeful galvanised coating. Inland is not a free pass if the deck is permanently wet or packed with treated timber. Ask. Write the answer on the drawing.

Falling ground, and when carpentry becomes a section

Australia is full of blocks that look flat on a listing and fall a storey by the back fence. The design of that fall is the subject of the sloping-block essay; the structure of it is this one. A deck that begins as a threshold at the kitchen can become a high structure with a long post and a stair that is a project of its own. Height changes everything: the barrier, the approval pathway, the slenderness of the posts, the need for bracing, the way a person feels when they stand at the edge.

Lateral stability is the part sketches forget. A table can size a beam for gravity. Gravity is not the only load. People lean. Wind hits a screen. A rectangular frame wants to become a diamond. Bracing, shear in the decking plane, and posts that are held at the base rather than balanced on it — these are the reasons a high deck on a slope is an engineering letter, not a longer version of the low one.

Soil is the other half of the slope. Fill is not ground. Expansive clay is a seasonal machine. Rock is a gift and a drilling invoice. A footing schedule that assumes a suburban loam will not travel to a ridge. This is why the first site visit is not about board colour. It is about a level, a probe, and a look at what the neighbour’s retaining wall is doing.

On falling ground the stair is not an accessory. It is how the load path meets the garden, and how a person meets the height. Draw it with the frame, not after.

What a finished drawing set looks like

A designer deck arrives on site as a set of decisions that can be built twice the same way. Member sizes and grades. Spacings. Connector schedule. Footing diameters and depths, or a note that they are by engineer. A section through the house attachment. A note on ventilation and inspection. A stair and a barrier that belong to the same structure. None of this is bureaucracy for its own sake. It is how the load path stays the path you intended when the party stands in the corner and it rains.

Bring a sketch, not a photograph of someone else’s backyard. Ask the designer to talk you through the path from board to soil. Ask where they would stop and call an engineer. Ask how the frame will be inspected in ten years. Then let the timber be timber: the pleasure of a surface that sounds right under a chair, sitting on a structure that does not need to advertise itself. The anatomy drawing is not a romance. It is the reason the romance lasts.