Twice the Fun

Orthopedists use X-rays to diagnose bone and joint disorders. Dentists use X-rays to locate cavities. Mark Lind, senior project designer for Austin, Texas-based CG&S Design-Build used X-rays to gain insight into the structural integrity of a concrete house. Prior to encountering this concrete house, built in the 1950s, Lind had never seen a concrete house. He had the rare opportunity to work on it not once, but twice, with two separate owners and on two separate stories.

The home’s previous owners called upon CG&S Design-Build to renovate their house, including the kitchen, family room and a glass-block master bath added onto the side of the house. Shortly after completion, the owners sold the house to a couple with two children who desired a larger space than what the 1-story house offered. “They wanted to add on, and we looked at pushing out back, but there would have been a lot of site excavation involved,” Lind explains. Instead, the owners opted to build a second story that incorporated a master bedroom, master bath and closet, and a rooftop deck with an outdoor fireplace. In addition to stellar views of the Austin skyline, the east-facing second floor also offers acoustical privacy for the mother, who is a physician and often needs to sleep during the day.

Second-floor Flotation

The second-floor addition was not concrete. “Not only could we not support the weight on top of the roof, but in terms of building a new insulated structure it was easier to build it out of wood framing and insulate it with wet-blown expanding polyurethane insulation,” Lind says.

Although concrete has strength in compression when used as a column or wall, it is weak in tension as in a roof such as the one that the house sported. “Because this was built in the 1950s, we weren’t positive how much steel reinforcing the concrete roof had, and the engineer cautioned us against building directly on top of the slab. He didn’t think the roof would hold the weight of the second floor,” Lind notes. Instead, following the engineer’s suggestion, Lind and his team built the entire second story on oversized floor joists that span from the front concrete wall of the first story to the rear concrete wall. “The whole second floor is supported on what are essentially 22-inch-high bridges that take the weight off the flat concrete roof,” Lind says.

When planning the stair tower, the structural integrity of the house was a mystery because there were no existing drawings. The tensile strength of the roof and its structural characteristics really depend more on how much steel is in the concrete rather than how much concrete is there, Lind explains. “We tried to determine how much steel was in there to give the engineer some certainty so that when he ran calculations for cutting a chunk out of the ceiling for the stair tower we knew what would happen to the rest of the roof,” he remembers. “We wanted calculations done first to be able to predict the outcome instead of cutting and facing the consequences.”

In the absence of drawings, Lind had to find other means to determine how much rebar was in the concrete. “There are companies that have ground-penetrating radar and can look at pipes in the ground,” he says. “Similarly, a local company had the equipment and ability to X-ray through the surface of concrete and locate the steel. It’s pretty amazing,” Lind says. Although the image was blurry, it relayed enough information to determine the spacing of the steel located in the concrete slab.

Concrete Challenges

Working with concrete does not come without its challenges. Whereas wood-framed walls accommodate cutting through them and easily reframing the opening, concrete is more monolithic. “It’s a bigger deal when you have to cut a hole in a concrete wall as opposed to cutting studs out and taking drywall out,” Lind asserts. “We had to be very careful about how and where we cut and how we reinforced it. You just don’t have to be that careful or conscientious when you’re dealing with wood studs. Concrete is a specialty. There was a learning curve with us.”

Many engineers who work on residential construction aren’t too familiar with concrete structural systems, Lind asserts. Fortunately, he had an engineer that was knowledgeable about the material and was instrumental in determining where the CG&S team could remove concrete and what would be required to reinforce the area around the stair tower. The team hired a specialty concrete cutter who removed a piece of the existing concrete roof to accommodate the 2-story stair tower. They then inserted steel-beam fasteners into the exposed section of concrete to help reinforce the area. The stair leading up is an open steel stair, making it more of a sculptural element.

One particularly unique aspect of the house is its butterfly-shaped roof, which the homeowners suggested. The roof matches the 1950s design of the house and also facilitates rainwater collection. Rainwater collects on the center of the roof and runs in two downspouts down the south side of the house to holding bladders designed for the half basement. The rainwater is intended to be used for watering the landscape.

Seamless Design

Lind did not want to make the second-floor addition totally independent of the finishes from the earlier renovation, and the homeowners agreed. Clear-coated medium-density fiberboard is present on both floors. The shape of the staircase echoes some of the design from the island in the kitchen, as well as the vanity and master bath. “Much of the detailing was taken directly from what we had done downstairs,” Lind explains. “Hopefully the whole house reads as a uniform feel in aesthetics and materials.”

One requirement of the owners was to make the home as green as possible. “Concrete is a really terrible insulator,” Lind asserts. “It uses a different thermal principle. It has a high capacitance, which means it can store temperatures and delay the release of those temperatures throughout time. It’s the complete opposite principle in physics from insulating.” Insulation is measured in R-value and that is what the Austin Energy Green Building Program rating for which Lind was applying measured. To combat the poor R-value of concrete, Lind added insulation on top of the roof to compensate. In addition, the 5-kilowatt solar array from the first renovation was disassembled and relocated on the roof of the new stair tower.

The homeowners also requested a three-sided glass shower, which became a pivotal element to the master bath. Bamboo flooring and LED lighting complete the green design features of the second floor. Low-VOC materials and a tankless water heater helped achieve a five-star rating under the Austin Energy Green Building Program, as did appropriately sized mechanical systems to reduce the heavy air-conditioning load, rainwater storage and an 8-foot overhang on the eastern side of the house to shade the home.

“This house was a lot of fun,” Lind recalls. “I was really sad to leave the house after the first remodel phase we had done because I thought I’d never work on a concrete house again. Just a few months later, the new owners came back to us to do the second phase. So, in the end, I did get to work on a concrete house a second time, after all. That was a rare opportunity.”