The jambs of an (c 1950?) exterior door I removed have a 3/16" deep groove on the back side near the outside edge. I'm wondering what the purpose was.
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I presume it can't be a decorative feature so it's possible these boards were originally prepared for use horizontally, so that would be a drip groove. – Graphus Sep 19 '16 at 8:18
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weatherstripping of some sort? – aaron Feb 6 '17 at 13:57
Yours are rather oddly placed, so I can't say with authority that my explanation holds for them, but traditionally the grooves that run vertically on the back side of door jambs were placed there to relieve stress that could cause warping.
I'd guess that surrounding trim was tongue-and-grooved in at some point ... But that's pure guess based on what I might have intended.
