About half an hour into the project, I started asking myself: What the hell am I doing creating a play space for my children 16 feet above the ground?

My son Joshua, age 9 3/4, got it into his head this summer that he wanted a treehouse -- it was probably something he was reading. When he brought it up, the kid in me swooned slightly. A cozy perch way up in the branches had a visceral appeal. He must have picked up on that.

The next thing I knew, Josh brought a how-to book home from the library. We used it one night for bedtime reading, and I realized that with skills I picked up as a theater techie in my early 20s, everything there was within my abilities. He searched the Web and showed me sites where you can order designs and plans for 100 bucks or so. Yuck -- more off-the-shelf commercial entertainment for kids, I thought. It seemed so un-treehouse-ish. But the part about treehouses being perfectly safe ifproperly constructed made a favorable impression.

We live at the end of a dead-end street, with a half-acre yard bounded by an urban brook. Josh and I scouted land across the brook, in the wooded patch owned by an elderly man with a striking resemblance to Santa Claus. If we found a suitable tree, we figured we'd ask him for permission to use it and take it from there. None of those ideal three-trunk combinations the book was full of presented themselves. I took another look at the few trees in our yard, and somehow visions of a three-dimensional structure started to superimpose themselves on my mind's eye as I examined a double-trunk ash by the brook. A sturdy bough extended from one of the trunks, but it was rather high.

Sunday morning found me at the top of an aluminum extension ladder with a tape measure. Later on I was at the lumberyard discussing the relative merits of pressure-treated 2 x 6s and 2 x 8s with a sales clerk. How much weight were these floor beams supposed to support? How long were the spans? Should I anchor them to the tree with 6-inch or 8-inch lag bolts? How safe is safe when it comes to your kids playing in trees?

Those suckers were heavy, and in order to rest one corner of the triangle on the extending branch, I had to haul them high off the ground, take measurements, come down to drill holes and cut angles and then haul them up and wrestle them into place. Josh climbed up to get a load of the view -- "Whoa," he said with a sly smile. Looking down on the stream side was almost like being in a skyscraper.

By this time I truly was questioning my sanity -- though I couldn't afford to admit that. I concentrated on making sure that all the pieces were securely fastened and then some. From how many different angles could I bolt the beams without killing the tree? What about when the tree grows? The Web site, I remembered, assured me that it wouldn't crumple up and spit out this uninvited appendage.

Later in the day a neighbor with kids from across the street visited. "It's high," was her comment, but her face betrayed the sense of magic the structure (whose outlines were just emerging) conjured. She wanted to know what I was planning to do for walls. That question had crossed my mind a few times, and something on the order of the Maginot Line was coming into focus. She assured her 6-year-old son that he would be allowed to play way up there. My 3-year-old daughter also has discernible designs on that play space.

I devised ever more ways of overbuilding, securing, and doubly securing the lofty platform on which the treehouse and conceivably thousands of fantasies and hours of play would rest. My self-doubt transformed into pleasure.

What was going on? The feeling that I was doing something tangible to protect my children -- one more bolt here and another screw there -- was satisfying, but that alone couldn't explain my joy. I was also offering them a slightly dangerous environment detached from our home, an environment that I am smart enough to realize will become marginally accessible to adults once their busy hands complete the work I started.

The world is a dangerous place, and I take my share of calculated risks. Facilitating a play world more than a dozen feet above the earth's surface, I came to see, fits with an approach to parenting that involves exposing my kids to "managed danger." The treehouse is an apt metaphor. It's high, yes, but it's solid. It is detached from our house, but I can see it from the kitchen window. Mischief will no doubt be plotted there, but hey, if a cool little clubhouse attracts my children's friends to our yard -- rather than to who knows where -- at least I'll know the conspirators.

It also occurred to me that of all the things I do this year, this treehouse will probably become my most enduring legacy. As a reminder of my weekend up a tree (it could probably double as a bomb shelter), that house might well outlast me. And then there are the minds and hearts of my children. The somewhat treacherous, funky little shack high above a stream that Dad built is likely to be talked about and embellished well past my years.

A friend recently shared a pet theory about childhood. "Once they reach 10," she said, "everything they do is in some way part of their preparation for leaving." Josh will pass that threshold in a few months. I am so glad that circumstances conspired for us to fulfill his 9-year-old's fantasy of having a treehouse. If occupying it becomes one of the early steps of flying our nest to figure out his own, I'm glad to have established myself on the right side of inevitability.

Halfway through helping me put up the platform (he fetched hammers, levels, wrenches, and bolts when I forgot to lug them up the ladder with me), Josh told me he had decided what he wants to be when he grows up: an architect and a builder. He couldn't have thanked me more eloquently.