Above is the Hobbit home’s front facade. Built to Passive House standards, the home’s features include low air infiltration; super-insulated walls, roof, and floor; high-performance windows and exterior doors; energy recovery ventilation; and ADA accessibility (inside and out).
Above is the Hobbit home’s front facade. Built to Passive House standards, the home’s features include low air infiltration; super-insulated walls, roof, and floor; high-performance windows and exterior doors; energy recovery ventilation; and ADA accessibility (inside and out).

For the past six years, when Jim Costigan wasn’t working on high-rise construction and battling long commutes in and out of New York City, he was building his dream—a “Hobbit” house. This civil engineer, self-proclaimed Lord of the Rings nerd, and father of four teenagers spent his weekend “down-time” building an earth-sheltered home in Pawling, N.Y., some 70 miles north of NYC.

Along with sons Ethan, Jude, and Terence, daughter Georgia, and wife Jo, Jim built what can best be described as a heavily-insulated arched highway overpass built to Passive House standards. One of the home’s most impressive features is its cast-in-place concrete roof.

Help from an engineering wizard. To achieve his vision of creating a large vaulted room with no intermediate supports, Jim had to revise his initial plans of building a uniformly thick concrete arch clear-spanning 32 feet and supported by two long parallel walls—the load calculations weren’t working out. Jim sought out help from “concrete genius” Nat Tocci, P.E. Nat called for a tapered arch, ranging from 9 inches thick at its center to 16 inches over the walls. He also beefed up the concrete walls to 16 inches from 10 inches and thickened the slab to 6 inches from 4 inches, adding continuous rebar run through the slab to tie the walls together (the slab acts as a collar tie).

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In addition to tapering the roof slab and thickening the walls and floor, the engineering “wizard” called for the wall-to-arched-slab intersection to be poured monolithically (the roof slab was poured in stages to prevent cold joints from occurring in the tapered slab’s more structurally critical thickened portions). First, they poured the thickened edges (10 feet up the roof) along the length of the building, then finished up with the remaining center section. Continuous “shear keys” were formed in both concrete walls (at the footing and roof-to-wall junctures) to prevent lateral movement. Of note, the structure was designed to be free-standing, supporting all lateral loads without factoring in the backfilled soils (due to the hard-to-quantify characteristics of soils). These design tweaks helped eliminate the need for additional wall buttresses, making the footings and walls easier to form and install. Click here to see a floor plan drawing.
Tim Healey In addition to tapering the roof slab and thickening the walls and floor, the engineering “wizard” called for the wall-to-arched-slab intersection to be poured monolithically (the roof slab was poured in stages to prevent cold joints from occurring in the tapered slab’s more structurally critical thickened portions). First, they poured the thickened edges (10 feet up the roof) along the length of the building, then finished up with the remaining center section. Continuous “shear keys” were formed in both concrete walls (at the footing and roof-to-wall junctures) to prevent lateral movement. Of note, the structure was designed to be free-standing, supporting all lateral loads without factoring in the backfilled soils (due to the hard-to-quantify characteristics of soils). These design tweaks helped eliminate the need for additional wall buttresses, making the footings and walls easier to form and install. Click here to see a floor plan drawing.

The Hobbit Hollow crew. Jim assembled a crew made up of his wife, kids, and a rotating cast of their school friends in need of part-time jobs. The kids worked hard; assembling the formwork was no easy task (the wall forms alone weigh 90 pounds per panel; 160 in all were used). To form the roof, the crew cut a series of arch-shaped “ribs” out of plywood and laid them out upright on a temporary flat deck (the deck was built at wall height using OSHA planking supported by 4x4 stringers on 150 metal post shores). Run perpendicular to the ribs, 2x4 purlins were inserted into slots cut into the plywood on roughly 12-inch centers. HDO plywood was then laid down over the framing to form the roof’s interior face. See slideshow above.

The Hobbit Hollow crew. Top left: Jim Costigan (dark-blue shirt) and son, Ethan, guide the pump hose to fill the footing forms (sons Jude and Terence and family-friend Albert look on). Top right: Terence Costigan and friends Mikey, Peter, Taylor, and Zack break down and load one hundred and sixty 90-lb. Symons concrete forms. Bottom left: Daughter Georgia and son Jude shape the decorative blocks out of scrap high-density overlay (HDO) plywood on a site-built router table. Bottom right: Jim’s wife, Jo Costigan, cleans the roof deck in preparation for setting the roof slab’s top reinforcing grid down.
The Hobbit Hollow crew. Top left: Jim Costigan (dark-blue shirt) and son, Ethan, guide the pump hose to fill the footing forms (sons Jude and Terence and family-friend Albert look on). Top right: Terence Costigan and friends Mikey, Peter, Taylor, and Zack break down and load one hundred and sixty 90-lb. Symons concrete forms. Bottom left: Daughter Georgia and son Jude shape the decorative blocks out of scrap high-density overlay (HDO) plywood on a site-built router table. Bottom right: Jim’s wife, Jo Costigan, cleans the roof deck in preparation for setting the roof slab’s top reinforcing grid down.

Mystical patterns. The Hobbit Hollow crew made the four skylight-well forms with 2x4s and HDO plywood and created the decorative patterns by screwing beveled shapes of scrap HDO plywood to the forms. The decorative blocking needed to be beveled in order to easily release the forms after curing. A two-person routing station was set up on-site to shape the hundreds of pieces—one person manned the router (with 45-degree chamfer bit), the other screwed the blocks into the various-shaped router jigs.

Top left: Georgia and Jude Costigan build forms one of the home’s four skylight wells. Top right: Terence Costigan installs can lighting centered in a decorative block. The lighting was wired with poly-coated BX cable—both the can lighting and BX cable were rated to be set in the concrete roof. Bottom: Note the patterns on the finished ceiling in bedroom (bottom left) and kitchen skylight well (bottom right).
Top left: Georgia and Jude Costigan build forms one of the home’s four skylight wells. Top right: Terence Costigan installs can lighting centered in a decorative block. The lighting was wired with poly-coated BX cable—both the can lighting and BX cable were rated to be set in the concrete roof. Bottom: Note the patterns on the finished ceiling in bedroom (bottom left) and kitchen skylight well (bottom right).

In total, the crew installed tons of rebar joining the footings to the walls and the walls to the arched roof. Eighty-five yards (342,000 lb.) of concrete was needed to complete the roof—and the forms held without a blowout.

For more information about this, visit Jim’s blog at myhobbitshed.com.