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Community and Q&A

Flashing Sill Board

nuthin2fancy | Posted in General Questions on
I’m planning to replace the (painted) cedar B & B siding on our 1960’s ranch style home and am somewhat confused on the subject of flashing.  The house is sited (Zone 6a) with the ridge running more or less north and south, which results in the south gable end receiving full sun.  The overhangs are 27” along the east and west elevations. The gable ends have ‘prow’ style overhangs that are 27” at the bottoms, increasing to 34” at the peak. The peaks have purlins, now wrapped in aluminum siding.
The exterior is face brick for the first 30 or so inches with a concrete brick cap where the transition to cedar begins.  Atop the brick cap is a cedar sill board, above that are the 1 x 12 vertical cedar boards and battens from there to the soffit.  Framing is 2 x 4 studs sheathed with 3/4” Celotex.  There are three 1 x 8 nailers running horizontally about the perimeter, one at the brick cap level (the sill board is nailed to this) and the other two are higher up with Celotex filling the space between the nailers.
As the included photos show, an L-shaped galvanized flashing is installed behind the siding boards and exits on top of the sill board.  20# felt paper covers the top of the flashing behind the siding.
I initially thought of using LP SmartSiding over 1/2” ZIP sheathing, but decided against this because I didn’t like the idea of the bottom edge of the siding sitting on the new (Azek) sill absorbing the rain and snow that tend to accumulate there.  My current thinking to use Azek sheets (and battens) to eliminate wood material in this high-moisture area.
Here’s where my confusion arises.  Do I install flashing as the original builder did so any intruding water is diverted to exit at the sill board?  If so, I don’t see how I can caulk this joint without trapping that water.  Without caulking, it seems an open invitation to insects as well as wind-driven water.  Forgoing flashing would let intruding water drain to the air gap between the face brick and the Celotex sheathing, which doesn’t seem desirable either.
I am nearing retirement and we intend to remain in the home for the foreseeable future.  Is this simply a case of comprising between two options or is there a better alternative?  I have extensively researched this on GBA and other sites, but am still confused.  I would think I should caulk the joint between the brick cap and sill board and joint between the sill board and the siding.  Is this correct?
Thank you.

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