When I need long runs of 1x baseboard, I don’t splice my boards with 45° scarf joints. Instead I cut the boards square, letting the breaks fall where they may regardless of stud layout, and I join the pieces with small splines.
I use a 1/4-in. slot-cutting bit in my router to cut a 5/16-in.-deep dado in the end of each board. Care has to be taken here to avoid cutting through the top edge of the baseboard stock. Then I spread glue on a 1/2-in. by 1/4-in. spline that is slightly shorter than the length of the dado, insert it in one of the dadoes, and tap another board onto it. The result is a perfect, tight-fitting, no-fuss joint.
With this method, you save time by not having to cut, adjust, and recut the mitered joint, and you save material by not having to break the baseboards over a stud. You also avoid the inevitable splitting when you nail mitered joints together without predrilling them.
Robert Prasch, Portland, Ore.
Edited and Illustrated by Charles Miller
From Fine Homebuilding #25
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Depending on the height of the base, it might be faster and simpler to use a biscuit joiner.
Or, for a stronger joint, use a domino loose tenon.
Expansion and contraction are always an issue. The old masters told me that a 45 degree scarf was useless while a well secured 30 degree joint worked very well. Time has proven this out. A spline is fine unless the material shrinks beyond the strength of the joint