If anything, the board will shrink, rather than expand. Unless it was an air dried/seasoned piece? I’d imagine at £130, it was air dried. I know the mill the OP went to as English Woodlands and so long as they were informed as to what the board was required for, they’ll have supplied one that is reasonably stable.
The only potential issue would be cupping, or curling, of the board as it dries out any further to suit the conditions of the room. Depending upon how you orientated the board, in terms of whether the growth rings as seen on the end grain, were concave or convex, will generally give an indication of its likelihood to curl up or down. That you have fixed the front of the board very firmly to the masonry, the back edge is the one that will move, if at all.
Rule of thumb when fixing any boards, either horizontally or vertically is to orientate them so the end grain rings are in the concave, ie ‘heart out’. This will generally mean the boards will then curl in, towards what they’ve been fixed to. It’s a rule of thumb however and the wood will ultimately do what it likes in terms of twist, curl or whatever, as much depends upon where the tree grew, how it grew and what part of the tree the board was cut from.
Ideally, ask for a quarter sawn, or rift sawn board. The growth rings are then vertical in cross section and the boards are much more stable.
There are lots of ‘woodworkers’ about nowadays, very few are ‘carpenters’, or engineers who use wood. And like any engineer, if you don’t understand the material you are working with, then the luck factor increases.
Building wooden boats and green oak framing are very similar in terms of understanding how you can use the material properly to help achieve a well engineered and strong result. 😀
