Sheathing, tarpaper and clapboards

I'm in the process of replacing the clapboards on my 1840s Greek Revival house. Strangely, there is no sheathing on the walls: The clapboards are nailed directly to timber frame of the house. Do I need to add some kind of sheathing before I re-side? And should I use old-fashioned tarpaper or switch to one of the more modern house wraps?

Sheathing is that layer of wood between the frame of a building and its exterior skin. It was once made from 1-inch thick boards, then 1/2-inch thick plywood and more recently oriented strand board or flakeboard.

Sheathing makes an important structural contribution to buildings that are framed with dimensional lumber. Without bracing of some kind, a frame made from 2x4s or 2x6s isn't very rigid. OSB, plywood or boards help prevent the building from racking and flexing.

Timber frames are much stronger

Before "stick framing" with dimensional lumber became the norm in the late 19th century, houses were made from heavy timbers held together with a variety of specialized joints.

Dovetail, mortise-and-tenon and elaborate scarf joints helped to make a very rigid frame. Additionally, individual pieces - the sills, beams, rafters and purlins - were typically of heavier stock than the dimensional lumber we use today.

As a result, builders didn't need sheathing -- even though many used it nonetheless.

I saw this first hand a few years ago when I replaced a few feet of rotten sill on my own house, which dated from about that era. It, too, had no sheathing. The damaged sill was under a vertical corner post that ran from the sill to the eave two stories above. I thought I'd have to support the post while I removed the damaged sill. But as I started picking away at the rot I realized the post wasn't moving.

Before long I had taken out all of the bad sill and there was nothing but air between the bottom of the corner post and the granite foundation. Nothing sagged. The frame was that strong.

Sheathing does have the advantage of providing an extra layer of protection from wind-driven moisture. But from a structural point of view you don't need it unless - of course, you live in an active seismic area. In that case, you should check with the local building office.

Besides, adding that layer of wood could greatly complicate your life by bumping the siding out beyond the edge of all your door and window casing. That's a tough problem to solve unless you replace the trim, too.

Tar paper or house wrap?

Tar paper -- also known as building felt -- is tar impregnated paper. It is an age-old choice as a weather barrier under roofing and siding. On walls, it has been eclipsed in new construction by various brands of house wrap, a woven or spun-bonded plastic.

This underlayment is an important line of defense against moisture penetration. It allows the passage of water vapor, so moisture that gets into the wall cavity has a way of getting out, but it blocks wind-driven rain, melting snow and the like.

So which should you use? Well, Paul Fisette, a building scientist at the University of Massachusetts, and his students conducted tests on a number of brands of house wrap along with 15-pound felt. He concluded that Tyvek and R-Wrap brands did the best job among house wraps -- but that traditional tar paper also is effective.

In the end there may be no clear winner. Just make sure you use one of them.

Equally important, don't forget to back-prime the siding before you install it and to dab primer on all cut ends. Wood siding is a permeable surface. Left bare, it will absorb water. Coating the back of the clapboards will prolong their life, make paint stick better on the outside and help to keep wall cavities drier.

For more on what Fisette's study uncovered, visit the Web site for the university's Building Materials and Wood Technology center at http://www.umass.edu/bmatwt