Q: How do I waterproof the shingled roof of a framed addition where it meets an existing wall of a log home?

A: Robert W. Chambers, a consultant and log-home construction specialist, responds: The new roof of the addition will have step-flashing, installed in the typical fashion with a piece of metal flashing added as each course of shingles is put down. But the step flashing itself does not attach to the log building. After the addition roof is completed, counterflashing attaches to the existing wall of the log building, overlapping the step flashings by at least 2 inches. The height of the step flashing and counterflashing depends in part on whether future log-wall settling is expected (more on that later).

To install the counterflashing, cut a kerf, or reglet, into the logs to capture the top part of the counterflashing. The reglet needs to be deep enough to completely bridge the “valleys” between the logs, and be cut with a slope of at least 30 degrees for drainage. The best way to cut the reglet is with a circular saw, using a straight edge to guide the cut. If the reglet is not straight, sliding the counterflashing into it could be problematic. If the space is too tight for a circular saw, an oscillating multi-tool with semi-circular blade might work.

The counterflashing tucks into the reglet and stays in place by friction, with no pins or nails needed for attachment. The vertical leg of the counterflashing should sit snugly against the step flashing (but not be attached to it in any way). The counterflashing may be caulked to the reglet, but do not use caulk where the step flashing and counterflashing meet.

Log homes take up to five years for the logs to reach the equilibrium moisture content for local humidity conditions. Up to that point, the logs will settle or shrink dramatically—6% is the industry standard, which means that a 9-foot wall will settle as much as 6 1/2 inches! Your conventionally framed addition will shrink very little by comparison. So if the log home you’re adding on to is more than five years old, you shouldn’t have to worry.

If the log home is less than five years old, then you must leave a gap between the bottom of the counterflashing and the new roof that is at least as big as the settling that is still expected. You will need to work out the dimensions of the flashing to maintain the 2-inch overlap when you install it, but still have room for the bottom edge of the counterflashing as it settles closer to the addition roof over time.

If settling is expected, the addition should not be attached to the log building, unless you allow for unrestricted movement of the log building. It is safer to build the addition as a freestanding structure, holding the addition structure away from the logs by several inches. And be aware of any protruding parts of the logs such as knots that may move down and interfere with the addition as the log structure settles.

If you’ve never worked with a log home before, or if you are unsure about the settling that may occur, I’d recommend consulting with a log-home specialist in your area. Also, there are a couple of good publications that can help you better understand the dynamics of log home construction: Effective Practices & Methods for Log Home Construction, available through the International Log Builder’s Association, and Log Construction Manual.