what boards are best.

[miketrain]

4 years with sundela boards was a nightmare just been told 2 use marien ply 4 years ov work gone starting again my layout was 14 bi 8 in a garden shed . .

[travellingmatt]

I've used 9mm OSB, (oriented strand board) which is the stuff made of wood shavings. Not very flat, so I used a 3mm MDF where the track is located, but is seems stable so far.

Very cheap in comparison with ply, and more stable than MDF or chipboard.

Matt

[Smiler]

I just used 4 ply mike... very sturdy.

[Flashbang]

Hi
IMO you won't beat Marine quality or Birch Plywood.
However, if you're unable to locate these boards, then the next best is to use WBP (Water & Boil Proof) ply. But do carefully check each sheet over before purchasing for unwelcome undulations in its surface or warping etc.
As for thickness of the boards, that depends on the main bracing timbers underneath the layout and their size. I use 9mm ply. However if the area is very well supported then you may get away with 6mm thickness. I brace on a 400mm (15" inch) square grid. On 6mm board I wound reduce the framing to a 300mm (12" inch) square grid. All the bracing being on a minimum PSE (Planed Square Edge) timber of 44 x 21mm (Approx 2" x 1"), though you can go thinner so long as the timber is deeper i.e. 18 x 66mm etc. All timber being fitted narrow edge up to the underside of the baseboard.

The next suitable solid sheet material for a baseboard surface to consider is perhaps MDF. MDF will need sealing with paint, varnish or PVA etc all round and including all edges, especially if used in a possibly damp shed environment.

[travellingmatt]

MDF warps/sags badly under it's own weight in damp conditions, unless VERY rigidly supported.

Matt