How to fill voids in exterior siding

Prevent water, critter infiltration

Q: I am trying to maintain a small house built in the 1930s. It has lapped redwood siding that is less than 1/2 inch thick with a 6-inch exposure. The siding runs under vertical boards butt-joined at each corner. Because the siding is tapered, there are triangular voids where each piece is covered by a corner board. Should these voids be filled — and if so, with what?

A: Caulk till you drop. Although this extra step isn’t always taken, a first-class job requires that all voids should be filled to prevent water infiltration, critter infiltration and to help keep the cold air out.

The siding could be beveled clapboards, rabbeted clapboards or shiplap siding. We’ve seen all of these siding styles with narrow exposures at the edge where the boards overlap on the horizontal plane of the wall. Corner boards are part and parcel of each installation.