but the foam is supporting it in that plane. I've seen splits on wood boards but put this down to the timber not being cured before use or not being sealed properly.

my poorly put point (plus i was sounding out decrep's thinking as i acknowledge your experience) was sealed wood is not as unideal as some think, yes wood is weak along grain but strong in many others not just relatively two planar direction of glass.

I'd choose construction like keef's board anyday because my 110kg has punted boards like that very hard without issue yet snapped light glass sandwich boards as the hardfoam is poorly supported & once that breaks the glass offers poor overall strength.

How sure are, that there is only 1 thin layer of glass in the construction of the boards you've ridden?
Have you actually sanded into them and had a look?

Because the sandwich foam is only there to provide compression strength, it doesn't have much tensile strength in any direction, so I can't see it, "supporting the wood".

Obviously most of the tension load is along the board, and the timber will probably cope well with this, (depending how thick it is).

But I can't see it handling any cross wise tension.

I have repaired a couple of timber boards and they definitely had a layer of glass each side of the sandwich foam, even if there was no glass on top of the timber.

sorry i wasn't referring to single layer wood as that isn't a sandwich & that would be just asking for trouble. I was referring to glass, dcell, wood.