Facade Retention On Heritage Developments

Facade retention keeps the street front of an existing building standing while everything behind it is demolished and rebuilt, and the facade is then tied to the new structure. In Sydney it usually follows a heritage listing or a conservation area, where the consent keeps the face of the building and allows a new one behind it.

The heritage streets of Potts Point, Surry Hills and Ashfield are typical ground for this work. The engineering problem is easy to state: a wall that spent a century braced by floors, a roof and cross walls is about to lose all of them, and for a while it has to stand on its own.

What the facade loses

An old masonry facade was never designed as a free-standing wall. Timber floor joists, the roof and the party walls held it in place at each level, and the brickwork only had to carry its own weight and its share of the floors.

Take those away and the wall becomes a tall, thin panel exposed to wind on both faces. Wind actions under AS 1170.2 then govern the design, along with knocks from demolition plant and vibration from the work going on behind it.

The wall survey

Before any design, the facade gets surveyed. We look at the brick and the mortar, the cracks and any lean, the lintels and arches over the openings, and whether the wall is solid brick, two leaves with a cavity, or a skin over something else.

Old lime mortar is soft, and it was meant to be. That suits the brickwork, but it limits what anchors and ties can do, so ties into an old wall get tested on site rather than assumed from a catalogue. The wider assessment is in [heritage building assessment](/blog/heritage-building-assessment-guide).

The temporary frame

A facade retention system is a steel frame designed to hold the wall until the new building can. It can stand inside the site, outside on the footpath, or on both sides.

A frame inside keeps the footpath clear but takes space the new basement and structure need. A frame on the footpath keeps the site clear, and it needs council approval to occupy the road reserve, along with the protection for people walking past covered in [hoarding design and council approval](/blog/hoarding-design-and-council-approval-sydney).

The frame grips the wall through the window and door openings, with timber packers and steel walers clamped either side of the brickwork. Its base sits on its own footings, or is held down with heavy concrete blocks where footings can't go.

Demolition in sequence

The frame goes up first. Then the building behind comes down in an order that keeps the facade braced at every stage, with floors and roof removed level by level and each level released only once the frame is carrying the wall at that height.

That sequence belongs on the drawings rather than with the demolition crew on the day, and the principles are in [demolition sequencing and structural stability](/blog/demolition-sequencing-and-structural-stability).

Basements behind an old facade

Many retention jobs add a basement, and that puts an excavation right behind a wall sitting on shallow footings of brick or sandstone. The facade footing has to be [underpinned](/blog/underpinning-adjoining-structures-explained) or held by the shoring before the dig passes below it.

The party walls on each side need the same care. They often belong to the neighbours, and they carry the neighbours' buildings as well as the facade's edges.

Survey and trigger levels

A retained facade gets watched for as long as it is exposed. Survey targets on the wall track movement and lean, crack gauges track the cracks that were there at the start, and vibration monitors sit on the wall during demolition and excavation.

Each reading has trigger levels agreed before work starts, with a set response when one is reached. A trigger isn't a failure. It is the point where the work stops and an engineer takes a look.

The permanent connection

As each new floor goes in, the facade gets tied back to it with permanent ties fixed into the brickwork and into the new slab edge. Those ties carry the wind on the facade for the life of the building, and they have to allow for the new concrete frame shrinking while the old wall stays put.

Once the new structure holds the facade at every level, the temporary frame comes away.

How we approach it

We treat the facade as a structure with its own survey, design and sequence, rather than a wall to work around. The retention frame, the demolition order, the underpinning and the permanent ties are designed together and shown on one set of drawings, with the monitoring triggers written on them.

A wall that has stood for a century is never more exposed than in the time it spends standing on its own.

ACSES provides structural engineering, including heritage assessment and strengthening, across Sydney. Talk to us about a project.

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