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We had a contractor add a new header beam in a load bearing wall where we opened up the space to add a closet. This was installed new about 2-3 months ago and just noticed a fair amount of horizontal splits in the header. Is this natural as the wood settled/dried, or should we be concerned? Don't want them to drywall over this if we need to do anything to fix or reinforce the structure.

Thoughts and advice? enter image description here

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Looks like seasoning checks. A tree ring's tangential moisture shrinkage rate is greater than its radial shrinkage rate. As the tree dehydrates, then, radial wood restrains the tree ring from shrinking as much as it wants to. The fissures from your images open because of the resulting stress. Sawing the tree into lumber when it's still green prevents a lot of checking. Keeping the wood's edge width at 4" and smaller also prevents a lot of the checking. It still happens, though.

Through checks are potentially an issue. Especially in the middle third of the beam, if you can stick a depth gauge into a fissure deeper than, say, 3/4", then you might have cause for concern. Since the one piece is probably paired with another piece without the checking, the probabilities work in your favor. Design values in the wood engineering code are based on the 5th percentile of strengths within a random sample of each species and grade, so even if the strength of the pictured piece is low (max strength loss from through checking is 50%), it and its pair are probably sufficient for the design load.

Unfortunately I can't quantify how probably because the lumber grading organizations don't seem very interested in publicizing their statistics under ASTM D1990 testing. Even with additional information from you on grade and species, I'm not convinced that the necessary data are available for computing probabilities. I've looked quite a bit in the past.

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