glue

[bigegg]

how is 110+ yr old wood going to move that readily? i didn't want any screws as its a likely start place for water ingress.

wood moves - it absorbs water from the air and swells - then it dries out and shrinks - it *will* be a problem if you glue it

if you don't want screws, a flexible construction adhesive will be a better choice

 

[KemppiFrog]

I find Sika flex 11FC is pretty good.It is a PU adhesive designed to stick roof tiles together and remains slightly flexible. Stick to just about anything, including your skin The PU foaming types require close gaps and the foam that fills big gaps is very weak.
Epoxy is my glue of choice, but, a bit toxic and not very cheap