Dear This Old House:
I need help with replacing the rotted bottom edge of white pine board-and-batten siding. It was resting on a 2x8 that trapped water. This needs to be corrected, and I'm weighing the pros and cons of various trim materials to put in its place. Can you help?
—James Becker, via email
Board-and-batten siding has an appealing rustic look, is quick and easy to install, and can make use of inexpensive materials such as white pine and spruce that are not typically put on exteriors.
But because this siding is installed vertically, it is very vulnerable along its bottom edge. That's where the wood's thirsty end grain lies exposed, eager to suck up any moisture that comes its way. Often this edge has no trim at all, and that's fine as long as the siding sits at least 6 inches above the ground and the end grain is primed or given a water-repellent coating. That way, if any moisture manages to get in the end grain, it has a way to escape and the wood can dry out. Unfortunately, that 2x8 water table captured water and kept the end grain wet. (I suspect the installer didn't coat the siding's end grain, either.)
Okay, that's the diagnosis. Here's the fix.
Prep the old siding
To get rid of most of the rot and allow you to remove the 2x8, make a horizontal cut through the siding 1½ inches above the 2x8's top edge. Tack a batten to the water table horizontally to guide the saw and ensure a straight 90-degree cut. Set the blade at a 5-degree bevel and a 1½-inch cutting depth. Rest the saw's shoe on the batten, making sure the blade is beveled up, and make the cut.
Remove the 2x8 and check the freshly cut edge of the siding for soft spots. If you find any, take another run at it with the circular saw 1 inch above the first cut. If that cut exposes soft rotted wood, replace the affected boards with new ones. Trim the house wrap or builder's felt protecting the sheathing so it is even with cut end of the siding.
For the replacement water table, the combined thickness of the board and the batten means the material has to be 1½ inches thick, which limits your options. If your siding is stained, you probably want the water table to be natural wood too. In that case, your choices are Accoya and western red cedar.
Accoya is plantation-grown radiata pine that's been acetylated—treated with acetic anhydride, a derivative of vinegar, to remove the wood's free hydroxyls, which sustain mold and wood-eating insects. It's sanded smooth and can be painted. Install it with stainless-steel nails and seal the end cuts with a primer or a water-repellent clear coat.
Western red cedar contains extractives that give the cinnamon-colored wood its color and natural resistance to rot and insects. You can get it rough-sawn or smooth, depending on the look of your siding.
If your siding is painted, both Acoyya and western red cedar can be painted to match. But in this case you have a third option: cellular PVC. This foamed thermoplastic looks like primed pine, but it won't rot, won't suffer insect damage, and won't warp, cup, or split. The only company that makes cellular PVC that's the same thickness as a 2x is Versatex. It carries a 30-year warranty. The key thing to know about PVC is that unlike wood, it expands along its length as temperatures increase and shrinks along its length as they go down. You keep the joints tight when installing in hot weather so they have the chance to shrink as it gets cooler. And in cool weather, you leave the joints open so they can grow tight as temperatures warm up. Follow the manufacturer's guidelines to the letter.
If you made one cut in the siding 1½ inches above the top of the water table, then order 2x10s to replace it. If you had to make two cuts in the siding then order the same number of 2x4s as 2x10s. These 2x4s will shim the water table up against the siding.
Guarding against water
Before putting any material back, take these steps to ensure the rot won't return. If the siding is painted, brush the cut ends with a generous coat or two of an exterior-grade oil-based (alkyd) primer. If the wood is stained or left natural, treat the cut ends with a water-repellent wood preservative such as Wolman's Woodlife Classic.
Next, cover the sheathing that was exposed when you removed the 2x water table with a strip of self-adhering waterproofing membrane, such as Vycor Plus (grace.com). It should be tucked about 3 inches under the existing wrap. You'll probably have to back out some siding nails temporarily to do that. Then slip some metal deck-ledger flashing under the existing wrap. Tap the siding down tight against the flashing to hold it in place, leaving a ⅛-inch gap between the flashing and the battens. The flashing protects the top edge of the water table and ensures that any water that gets behind the siding is routed away from the sheathing.
Ledger flashing is sold in 8-foot lengths. If the repair is longer than 8 feet, run a vertical bead of caulk near the end of one strip and overlap it with the next strip by 6 inches. That will stop water from getting around the ends of the flashing strips.
Take the 2x10 water table you've chosen and rip a 15-degree bevel along one edge. (If you need to add the 2x4, rip a parallel bevel along the 2x10s opposite edge.) With the bevel facing out, snug the water table against the ledger flashing, then nail it to the sheathing at each stud location with stainless-steel nails. If you're using cellular PVC, you can also screw the trim in place using the Cortex hidden fastening system and cover the heads with matching PVC plugs. Using a piece of scrap wood, tap the flashing down against the water table's bevel. There will be a gap between the siding and the water table that will allow the pine siding to dry out after it gets wet.
If you need 2x3s to finish this job, rip a 15-degree bevel along the bottom edge of the water table before installing it. Then brace the 2x3 edge-up on a workbench, set the circular-saw blade to a 2 ¼-inch depth and a 15-degree bevel, and cut a kerf into the top edge as close as possible to the board's corner. Now place the 2x3 on the flat with the kerfed side facing up, and make a rip cut ¼ inch from the side opposite the kerf, to reveal the bevel's full width. Cut a shallow kerf in the side opposite the beveled face about a half inch from the thinner edge to prevent water from migrating along its underside.
Gun a thick bead of caulk to the back edge of the 2x3, then install it bevel up against the water table by toe-screwing from the underside into the sheathing and the studs. Use stainless-steel screws and prime or seal the end grain; and you won't have to worry about the edge ever rotting again.