Jon,
A drawing for an early (ex- SM&AR) M&SWJR horsebox shows tongue and grooved planks. Thickness is not given, but they scale out at 3/4in..
However, it looks appreciably thicker in photos of that particular prototype; the drawing shows the planking was cut flush to the body all round (no overhang) with a moulded section pinned over the joint- along top of sides and ends. Presumably these were to hold roof canvas in place- and perhaps guttering as well.
So even though the roof described a simple arc on this vehicle, the boards did not overhang the sides or ends, and the roof gives the impression of being quite thick.
More usefully, a couple of MR drawings are clearly annotated and show the roofs covered with 1in. thick tongue and grooved planks.
One, an arc roofed variant, does in this case show the 1in. planking simply overhanging sides and ends, with no moulding strips or guttering.
I guess I'm suggesting that 1in. plank thickness seems ok, but the roof edges may have looked thicker on some vehicles due to some form of moulding or guttering strips around the roof/ body join.
Thoroughly enjoying your progress with this model,
Yours,
Jamie