By Bill Forry

If it were your house, you would've probably fired Steve O'Shaughnessy by now.

In the time that it's taken the master carpenter and his helpers to fix a single beam on their current project, some contractors could've put up a three-decker.

But this isn't any old house. And O'Shaughnessy is not your typical craftsman. He's a restoration expert charged with saving what is, by some accounts, the oldest existing structure in the city of Boston: the Pierce House on Oakton Ave., just across the street from the Thomas J. Kenny Elementary School.

Time is really not an problem for O'Shaughnessy and his crew, a rotating team of students from the North Bennett School in the North End. Time &endash; or more accurately, history &endash; is what makes their job so interesting.

O'Shaughnessy works for the Society for the Preservation of New England Antiquities (SPNEA), a non-profit that owns 35 properties in the region, which- like Dorchester's Pierce House- are treasured not only for their architectural beauty, but for what they reveal about the settlement history of the Americas. The Society acquired the Pierce House in 1968 from a 10th generation descendant of the original Pierce family. A member of the family, who lives on Cape Cod, is expected to tour the work site this week.

The Pierce House was once thought to have been built in 1650, precisely 20 years after Puritan settlers arrived in Dorchester. Now, thanks to recent tests done by SPNEA, the oldest section of the house has been dated to 1683. The house remains one of a handful of early colonial homes standing in the city and, under O'Shaughnessy's watch, its become a living laboratory for people training to keep it -and other historic structures- standing for centuries to come.

Last year, O'Shaughnessy and student trainees successfully removed the modern, asbestos skin of the Pierce House to reveal a patchwork of wide wooden planks, some of which date back to the 17th century. The goal of the current project is to restore the exterior look of the home to its early 20th century appearance, before more recent fix-up jobs changed its appearance. The guts of the building are also being re-built where needed.

"We've already buttoned the house up to some degree," says O'Shaughnessy. "We did a lot of timber frame repair. We've got a new corner post that was so badly damaged-the corner had been eaten away by insects and water damage over the years- that it was down to about one and a half inch square post."

Inside, the long beams &endash; known as girts, between the first and second floors have also been reinforced using materials and techniques that respect the original construction periods.

The Pierce House has gone through a number of transformations in its 320 year history. Initially, it was a tall, narrow house, without the sweeping salt-box style roof that is its most unique feature today. The First Period house was modeled on homes from the East Anglican section of England, much like the Blake House on Columbia Road, another period home owned by the Dorchester Historical Society.

Expansions swept the living space out on all three sides, eventually doubling the home's size over the next two hundred years. The last dramatic expansion came in 1765, when the steep, sloped roof and east wing were added.

"There were waves of earlier repairs that have gone on here and we leave those in place if they've been done properly and there's no damage," says O'Shaughnessy.

"What we will leave will resemble the way it looked in the early 1930s, both architecturally and by color scheme.

The gray-on-gray monochrome is what the paint analysis determined was there at the time."

The oldest and most well-preserved pieces of the house are not visible from the street. Inside a second-floor crawl-space is a long section of the original 1683 siding, which remains in excellent condition, since later additions protected it from the elements.

They still have the rot nails and hand-hewn wood, the earliest siding we know of from that date. And it's in excellent condition. There is an enormous amount of early evidence in the house, which makes it very important," says O'Shaughnessy.

This week, the SPNEA team is busy re-assembling the more modern siding that was taken off the east wing for repair. It is a meticulous proposition, as each piece of siding was carefully removed, numbered, evaluated for condition and, finally, reassembled in its exact location over the quilt of white-oak planks from a variety of centuries.

"We respect the methods that were used to construct the house. We use similar tools, we use a lot of modern fasteners and adhesives, but we choose very wide sheathing boards with beveled edges top and bottom. We try to retain as much original fabric as we possibly can," says O'Shaughnessy.

The rehab project is already deep into its second full season and work on the site is likely to continue for years to come. In the future, a one-car garage, which was demolished in the 1940s, may be re-built based on photographs.

"We'll continue through the fall and by mid-to-late November we'll wrap it up for the winter until spring.

The motivation is not to do it in a quick and dirty, hasty manner. We like the idea of using it as a teaching ground. It's really been great."