Carports, pergolas and patios, and why uplift decides the design

Carports, pergolas and patios, and why uplift decides the design

These are the smallest structures we certify and they generate a disproportionate share of the problems, because the intuition about them is wrong.

A patio roof weighs very little. That is exactly why it is difficult. A heavy structure resists wind uplift with its own mass. A light one has to be held down, and every connection from the sheeting to the footing is working in tension rather than compression.

What the wind classification changes

Every site in Australia falls into a wind classification under AS 4055, based on the region, the terrain around it, how shielded the site is by neighbouring buildings, and the topography.

Two identical patios, one in a sheltered suburban street and one on an exposed ridge, are different structures. The exposed one needs larger footings, heavier members and stronger tie-downs for exactly the same span.

This is why a design bought off a supplier's standard sheet can be sound in one street and inadequate two suburbs away. The classification has to be assessed for the actual site.

The load path, upward

Sheeting to battens. Screws in tension, at a spacing that gets tighter near the edges of the roof where suction is highest.

Battens to beams. A fixing that resists pull-out, not just a nail.

Beams to posts. A connection capable of carrying the uplift, which means a bracket and bolts rather than a housing.

Posts to footings. This is the one that decides the footing size. The footing is not sized to stop the post sinking. It is sized to stop the whole structure lifting out of the ground, so its mass and the friction of the soil around it are doing the work.

That is why a carport footing is often deeper and wider than an owner expects for something that appears to weigh almost nothing.

Attaching to the existing house

The most common defect I see is a patio beam fixed to the fascia.

A fascia board carries the gutter. It is not a structural member and it cannot take the load of a roof, particularly in uplift. Once wind gets under the patio, the fascia is the first thing to fail and it takes a section of the house roof edge with it.

The correct connection goes to the rafters or the top plate of the wall, which usually means removing the gutter and fascia locally, fixing to structure, and reinstating. It costs more and it is the difference between a structure that survives a storm and one that does not.

Free standing designs avoid the problem entirely and are often the better answer on an older house where the roof structure is not worth disturbing.

Approvals in New South Wales

Many carports and patios sit within exempt or complying development if they stay inside the size, height, setback and site coverage limits in the relevant State Environmental Planning Policy.

Exempt does not mean unengineered. The structure still has to comply with the National Construction Code and the referenced standards, and a certifier issuing a complying development certificate will generally want engineering documentation or a design certificate for the wind classification of that site.

Anything outside those limits, or on a site with a heritage listing, bushfire or flood constraint, needs a development application instead.

What to have ready

The site wind classification. The proposed span and post positions. Whether it is attached or free standing. The soil, because a footing sized for sand and a footing sized for reactive clay are different.

We certify this work under structural engineering, and on the smaller jobs the engineering costs a fraction of the structure. The ones that get built without it are the ones that end up as an insurance conversation.