Q: I live in a two-story house that has T-111 plywood siding. As you know, this siding has a vertical groove every 8 inches. Across this siding I have a 2-by-12-inch board.

My question is: Should I caulk the top of the board to keep water from coming between the 2-by-12 and the siding, or is caulking not necessary? If caulking were required, what would you recommend?

A: T-111 siding is a type of plywood siding that was used a great deal in the ’60s, ’70s and ’80s to give a rustic appearance in the day when Mother Earth News and the “back to the landers” were all the rage.

The T-111 of those days had a rough-sawn, textured surface with vertical grooves milled every 4 or 8 inches. Thickness ranged from 3/8 inch to 3/4 inch. In the West, T-111 was usually fabricated from Douglas fir, but we’ve also seen it in redwood.

In the proper location, mostly wooded areas, we think it looks quite handsome. T-111 plywood siding can be either painted or stained.

T-111 siding is still manufactured today; however, it’s most often made from engineered wood products such as oriented strand board (OSB). This leaves paint as the only option for a finish.

T-111 siding is manufactured in 4-foot widths and in lengths of 8, 10 or 12 feet. This allows a single piece of siding to cover the entire height of a wall from foundation to soffit without seams.

The siding should always be installed with the grooves vertical. If it’s installed horizontally you’re begging for a leak.

Because you have a two-story home, we doubt that the siding covers the entire height of the wall from foundation to soffit. Hence the 2-by-12 “belly band.”

As to whether or where to caulk, there’s really no easy answer. You’ll have to do a little detective work.

Try to get a look at the butt joint where the two pieces of siding come together. This may require cutting away a small piece of the 2-by-12, or perhaps you can simply pry back the board and take a look. Be careful not to split the old wood.

If the siding was installed correctly, you should see a piece of Z-metal flashing at the butt joint of the two pieces of siding. Z-flashing fits between the two pieces of siding at the end joint where two pieces of siding are butted together. Looking at it from the end, it forms the letter “Z,” hence the name.

One leg of the Z fits over the bottom piece of siding. The top piece of siding slides over the other leg. The flashing allows water moving down the wall to always be channeled outside.

The top T-111 panel should be installed about 1/4 inch off the shelf formed by the flashing to prevent moisture from wicking up into the wood. This is especially important when using engineered siding.

If the siding was installed with Z-flashing, do not caulk in the grooves. Caulk only the seam where the 2-by-12 lies flat against the siding.

Caulking here prevents water from resting between the siding and the 2-by-12. The flashing will channel the water moving down the unobstructed grooves away from the top of the 2-by-12 down the wall.

If there is no Z-flashing at the joint of the siding, you potentially have a serious problem. Without Z-flashing the only thing between water infiltration in the siding seam is the caulk on the top of the 2-by-12. In that case, be diligent in regularly caulking the entire seam where the 2-by-12 meets the siding. Inspection and recaulking once a year is absolutely necessary preventive maintenance.

As for the type of caulk, if the siding is painted, use a good acrylic painter’s caulk. If the siding is stained, use a clear silicone caulk made for exterior use.

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