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Q.When installing cedar clapboard, should gaps between the siding and trim be caulked?

A.Corresponding editor Paul Fisette responds: Clapboards should be installed tight against the trim boards, and the joints should be left uncaulked. Omitting the caulk allows any water that penetrates the joint to drain reasonably well and promotes drying.

Although the use of caulk does not cause any disasters, it can elevate the moisture level of the sheathing. I have taken moisture readings of sheathing on many homes, and on average the sheathing underneath caulked corners has a higher moisture content than that under uncaulked corners, and a higher moisture content than the sheathing a couple of feet away from the corner. I recommend that vulnerable areas of the sheathing (like corners) should have a double layer of asphalt felt or house-wrap under the trim boards.

Hot southern exposures, wide temperature swings, and wet-dry cycling take a toll on caulked wood joints. In most cases, caulk quickly loses its bond from one of the wood surfaces and leaves a crack where water can enter. Yet even cracked caulk can slow the drying process.

Well-detailed caulk joints require several fussy details. For caulk to work well, the end grain of the clapboards should be painted, to prevent absorption of the caulk solvent. Moreover, a bond breaker (backer rod) should be installed in the gap against the sheathing to avoid three-sided adhesion. This backer rod ensures that the caulk will adhere only to the siding and the trim board, so that the caulk can stretch without tearing.

A caulked joint works best when the gap measures a uniform 1/4 inch. But technical specs for most building sealants indicate that joint movement should not exceed 50%. Since a 6-inch trim board can shrink and swell by 1/4 inch as a result of normal exposure to the elements, you would have to leave a 1/2-inch gap between the trim board and clapboard ends to accommodate 50% movement. Who’s going to do that?

Finally, uncaulked joints require less maintenance. Few homeowners have the time to dig out old cracked caulking on a regular basis.