A.If you are using a solid header or one made up of 2x10s or 2x12s, the wood can shrink 3/8 inch across the grain as it dries from the 19% moisture content permitted in "dry" lumber to the 9% to 11% moisture content found in a heated house. Undoubtedly, the header in question was well along that drying curve before the wall was closed.
The shrinkage can be reduced or prevented by using drier lumber for headers, preferably at about 12%. But lumber that dry may be difficult to find. Laminated veneer lumber beams are usually more stable than ordinary lumber. An alternative is to use a plywood box beam as a header, since plywood has already been dried and shrinks much less than...
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