You make some interesting points Roger. So I've done a little bit of digging and found this article
https://www.popularwoodworking.com/wp-c ... vement.pdfI found it interesting, I hope that you do too as I think it answers some of your questions. I've always assumed that gaps around a floor would be necessary as it's the advice that is always given by all suppliers of wooden flooring. I guess that it has to be a bit of a catch all due to the variances in the fitting of the floor. I think perhaps, we are both right in our own way.
The article suggests, albeit in America, that "most climate-controlled houses change
3 percent to 4 percent MC during a year". I'm not sure how we would compare in the UK - my gut instinct is that we would have a higher variance in humidity, but I've no basis for this thought!) The co-efficient of movement for redwood is .00101 for quarter sawn and .00229 for flat sawn.
So if we work through your example of 100mm wide boards in a 4m wide room with the formula provided (width of board x realtive change in MC x co-efficient of movement), we get
Redwood100 x 3 x 0.00101 = 0.3mm per board, or 12mm movement for the room as a best case scenario.
Alternatively,
100 x 4 x 0.00229 = 0.916mm per board, or 36.64mm for the whole room worst case.
White Oak - just for a bit of fun100 x 3 x 0.00180 = 0.54mm per board, or 21.6mm movement for the room as a best case scenario.
Alternatively -
100 x 4 x 0.00365 = 1.46mm per board, or 58.4mm for the whole room worst case.
Interesting, I think, and you can see where just through seasonal changes you might get up to 36mm/58.4mm (Redwood/Oak) of movement in what's not a massive room. Overall, I think it bears out the advice given that you should have an expansion gap. However, if you have a room with constant climate control and well seasoned boards you should be OK.
Hope you all found this interesting!
Just as a slight aside - I dont spend my time looking up this sort of stuff for fun! I've been wondering for a while how to calculate the gaps for T&G panels in doors for a while (it's what I do!) and now I know some of the answer