A common problem with cement board siding products (and some other types of horizontal lap siding) is that older installation guidelines called for caulking the joint instead of flashing the joint.
This has proven problematic over time as caulking the joint results in unsightly surface appearances due to the different textures as well as resulted in failure of the caulk joint and associated ongoing maintenance.
Since newer installation guidelines do not recommend caulking this joint but instead recommend leaving a small gap and flashing behind the ends of the siding, I think it is more prudent to add flashings when it is possible to do so.
If the corners of the siding boards have been face nailed, it can be a little more difficult (though not impossible for experienced siding installers).
Otherwise adding flashings is typically a piece of cake!
Here is what the detail look like.
In this first picture we see what a typical siding joint looks like on a home that was sided with cement board siding and there is no flashing behind the butt joints. The caulk has failed and water can find its way behind the siding.
The following series of pictures will detail how to make a simple flashing that can be slid up behind the joint. It requires no nails.
Depending on your reveal of your siding the flashing length will be ¼ inch less than the reveal. Dog ears will be cut on both sides approximately ¾ inch from the bottom and then folded to create a “stop” that will locate itself on the top of the underlying row of siding but not be so long that it will run into the nails that hold the siding of the row being flashed.
Note that the flashing does not go up high enough to hit the underlying nail but is still high enough to behind the next row of overlying siding.
Once the joint is flashed, it really will not look much different than before it was flashed but water cannot get behind the siding.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board