In Mark Muldrow's backyard, there's a perfectly good tree house.
It has a "secret doorway" that swings on broken hinges, wide gaps in the mismatched wall boards and a sturdy roof that works great for two young boys to perch on.
But this lonely structure sits unused in the shadow of its upgrade -- a 10- by 12-foot, fully insulated structure with a uniquely engineered double roof, triangular windows to capture passive heat, carpet flooring, finished interior, vinyl siding, electricity, cable TV, a loft and a "floating" deck for entertaining.
Future plans include a solar panel and a small wind generator to create a "stand alone" facility.
It's paradise for Muldrow's sons: Chris, 13, and Jack 12.
"It is nice," said Chris. "It's a retreat."
But the tree house project has also involved lessons in physics, carpentry, ecological stewardship, creative design and frugalness shared between Muldrow and his sons.
"It's really about raising boys," said Muldrow.
The trio has been working for two years on the project.
"It took us a year to save the money for it," said Muldrow, who lives near Long Lake south of Spicer.
His sons did odd jobs in the community and sold honey from their beehive to help pay for materials. By using discounted end pieces and re-used materials, they kept the cost to about $1,500. It would've cost at least $4,000 if everything had been purchased new, said Muldrow.
Winter months were spent creating a design and researching how to make the tree house sturdy and safe. They consulted with a neighbor who had construction experience, as well as the county zoning administrator who provided some additional advice on how to "beef up" the construction.
"It's all over-built," said Muldrow.
The house is perched in an old oak tree that was struck with blight before the tree house was built. As a result, the branches have been severely pruned and there are no leaves.
"We mourned the loss," said Muldrow, and then, after being assured the tree would remain sturdy for at least another 20 years, got on with recycling it and putting it to use as the base for the tree house. The unusual look of the tree has resulted in the moniker "stump house," said Jack.
There were challenges in engineering the structure.
By taking the dimensions of the deck and house, and potential cubic feet of snow on a given winter day, the father and sons figured out how much weight would need to be distributed on beams that were anchored to the massive tree trunk.
"It was a learning process for the kids," said Muldrow.
"It was fun building it," said Chris. "I'm glad we did it. I'm definitely glad we did it."
To give the impression the deck is "floating" in air, "the whole deck is hanging off the tree," said Muldrow.
Other than the tree, there are no ground supports for the deck. Instead, it's anchored with heavy-duty beams that run the length of the entire structure, with 10-ton cables that crisscross the beams and the tree and steel supports that are attached to the trunk with eight-inch lag bolts.
The tree house has a double roof with a two-inch gap in between the layers to provide a "chimney effect" for heating and cooling.
The walls of the house are "insulated very tight," Muldrow said. The tree house has kept the kids warm even during a 16-below winter sleepover adventure.
Although the house is built with tight construction, Muldrow said they had to allow for flexibility and shifting caused by wind.
He's confident that in the advent of any strong winds the tree house will remain secure in the tree while his own nearby home will be blown away.
In addition to building the tree house together, the father and sons are learning about keeping bees and hope to harvest and sell honey from their six hives this year. They eat food from their family garden and bake -- and sell -- homemade bread that's baked in their outdoor wood-fired bread oven.
The fullness of the lessons, the work and time the father and sons spend together are represented by a sturdy, yet precarious-looking balanced-rock sculpture in the front yard.
By carefully placing the rocks on top of each other, the sculpture is beautiful and balanced.
"It's how you build your life," said Muldrow.