Splashback Rots Vertical Siding

Blair writes: I’m in a 30-year-old house built on a brick-ledge slab. The trouble is that the previous owner/builder used reverse board-and-batten siding to instead of brick. Over the years, rain hitting the ledge on the slab bounced up to rot the lower 2-4 inches of wood and likely the sole plate of the wall framing. My thought is to cut a foot or so above the rot around the perimeter, remove the old wood, and maybe install new house wrap and a z-flashing at the top with new wood that’s been treated, painted, and coated – then nailed in place. Is there anything I’m missing? And more importantly, I don’t want to have to do this again!

Steve Bliss, of BuildingAdvisor.com, responds: The problem you’re experiencing is called “splashback” and is a common problem with vertical and diagonal siding. The end-grain soaks up any water it contacts through capillary action, like a straw. The problem can be caused by rainwater sitting on or splashing off a hard surface like your brick ledge, or by melting snow in cold climates.

The rot certainly could have affected the sole plate, which may need repair as well. You won’t know until you open the wall, so a test cut in the worst area would be a good idea. Minor decay can usually be ignored and won’t grow if the area stays dry in the future. However significant decay can invite insect invitation and should be treated or replaced with treated lumber.

In general, your idea makes a lot of sense.  Z-flashing is exactly what you need here. You could replace the bottom section of siding with a horizontal trim board (called a “water table”)   – shimmed out if necessary to the thickness of your siding.

If you want to use wood, I’d recommend pressure-treated lumber or a decay-resistant species and painting all six sides. However, keeping any kind of finish on wood in frequent contact with wet concrete and splashing rain would be a challenge.

If it were my house, I would use a non-wood product like fiber-cement, expanded PVC (such as Azek), Boral, or any high-quality composite decking material. I have been happy using all of these materials to replace rotting or damaged exterior trim (including a rough cedar corner board attacked by woodpeckers). A pre-finished product would be ideal if you can find a color that’s compatible with your house design.

The key to long –term success is in the detailing. You want any water that penetrates the system to drain safely away. That means creating a water-resistive barrier (WRB) behind the siding that is lapped properly and drains to the exterior. Reverse board-and-batten could make an effective   rain-screen siding if there is a WRP with drainage behind it.

Instead of house wrap, I’d suggest going with a more durable peel-and-stick membrane such as Grace Ice & Water Shield, wrapped along the exposed section of wall and under the bottom of the water table to create a “capillary break” between the masonry and wood. (Tyvek will degrade pretty quickly if exposed to regular wetting.) The felt paper or housewrap above should lap over the membrane.

If you can’t keep the area from getting wet, as an extra precaution, I’d also suggest installing the new water table over a drainage mat such as Home Slicker (Benjamin Obdyke), placed over the membrane, and using a metal L-flashing at the bottom of the water table. The membrane would lap over the upper leg of the metal L flashing. The flashing will direct any trapped water out onto the brick ledge – similar to a weep screed if you had brick on the brick ledge. Leave at least a ¼ inch gap between the bottom of the water table and the flashing.

You’ve got an unconventional detail, here, so you need to get a little creative with your solution. The general goal is to use highly durable materials in contact with the foundation and an effective water-resistive-barrier behind the siding that drains to the exterior.

The other half of the solution is to keep the area as dry as possible. You may need to add gutters to the roof above or extend your roof edges outward next time you reroof.

Related links: Composite Trim   Flashing Membranes   Wall Flashing Details  Rain Screens

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