Bunker House: The Realisation of a Dream

For builder Neil Hipwell, the Bunker House is the embodiment of a career-long love affair with concrete.

"My dream when I started my company 14 years ago was to one day get to a point where I could build a house that combines everything I've ever learnt and everything I'm passionate about - and that's Bunker House," he says.

Overlooking Werri Beach in Gerringong on the NSW South Coast, this stunning family home reflects in every way Neil's absolute respect for concrete - not just in terms of how it lends itself to totally unique design outcomes, but it suitability as a structural solution in harsh coastal environments.

Neil is the Founding Director of Design and Construct company Futureflip, and his affinity with concrete stretches back to his days as an apprentice carpenter working on luxury waterfront homes in Sydney. 

Over the course of his subsequent career, Neil's established a reputation for exploring concrete's endless possibilities. And given that the client on this occasion is himself and his wife Krystal, it's understandable that's he's pushed the boundaries yet again. 

Concrete provides both the structure and finish to the Bunker house. It's used for slabs and ceilings, off-form walls, benchtops, bathroom detailing, floating slab stairs - even a bench seat in the backyard.

Built on a sloping site, the home is substantial - seven bedrooms, nine bathrooms and a pool, spread over three levels. However, the judicious use of sympathetic materials and natural finishes in tandem with concrete ensures it sit comfortably within its immediate environment.

In fact, the use of limestone cladding juxtaposed with sections of cedar classing on the north-facing ground level creates the impression that the building is rising, almost organically, from the earth. (The timber cladding is a nod to the cedar forests that once covered this part of the coast.)

At a glance...

Designer & Builder: Futureflip

Images: Simon Whitbread

Main Concrete Elements:

• In-situ slab floors and ceilings
• Off-form walls
• Kitchen benchtops
• Internal stairs
• External paving

Main Benefits:

• Thermal mass to help naturally regulate internal ambient temperature
• Durability and low maintenance

The upper two levels of the home make extensive use of glazing, so even though concrete provides a visual and structural framework the overall appearance is one of lightness and balance. 

The pool is cleverly incorporated into the top level, adjoining both the main living/kitchen areas and the backyard at the top (southerly) side of the sloping site. 

Considerable cost savings were achieved by utilising the second-floor slab as the base of the pool (as opposed to excavating into the yard and eating into the precious green space). The pool also acts as another level of thermal insulation for the bedrooms below. 

The mass of the concrete roof over the pool is substantially reduced by the clever insertion of four large voids. These voids create a lovely balance of shade and sunlight in the space below, while at the same time allowing light to filer into the internal living areas (including the lower levels of the house, via section of glass flooring).

A thoughtful, cost-effective approach to design is evident right across the project. The planter boxes on the outside perimeter serve a dual purpose as beams; internal wall finishes are off-form; and downpipes are concealed within walls. At the outer edges of the building the roof eaves taper away to a thickness of 70mm, reducing the scale and creating a level of fine detailing that contributes to the understated elegance of the overall design. 

Another significant cost saving was achieved during construction by recycling the ply framework. 

"Given out budget, I had to choose where I wanted Class 2 finish and where I was happy with Class 3," Neil says. 

"So, for example, we used new formwork on the upper slab to get a Class 2 finish for the ceiling, and then just flipped the boards to use them for the walls.

"We were very particular with our formwork detailing. I would give the formworker a gridline of how I wanted the boards to go. We also grid-lined and detailed all our downlights and concealed plumbing."

Neil says this meticulous approach to forward planning is the key reason the project looks so seamless.  "At the same time, I wasn't really looking for a perfect finish," he adds.

"I actually like a few knocks and bumps. It gives the surfaces a bit of character."

With the building site less than 100 metres from the beach, material durability was another key consideration on the project.

When it came to reinforcement, the majority of the higher risk areas - like splay bars, starter bars and roof steel - were done in galvanised 20s and covered with 50 MPA waterproof concrete. (A waterproof additive was used in the mix to reduce permeability, even under high hydrostatic conditions).

"Choosing concrete as the main material choice afforded a range of significant benefits," Neil says. 

"As concrete is incredibly hard wearing, it can withstand the exposure to the strong winds and constant salt associated with this site. 

"This home will last up to five times longer than a standard timber frame home in this location, which is a significant saving of cost and embodied energy." 

The decision to use of concrete also married with Neil's commitment to passive solar design. 

"We always incorporate passive solar design principles into the homes we design and building," he says. 

"For example, the splayed concrete eaves are designed to reduce the sun load during summer.

"The thermal mass of the concrete slabs and walls also help stabilise internal temperatures.

"And by sinking a large portion of the building envelope underground (like a bunker), we're using the earth as a tool to control the temperature.  We have two efficient air-con units installed. But I was there on a 40-degree day last summer and we didn't need to turn them on."