I have a span of 11'2" unsupported in the middle and it is quite stable.

The secret was to make the whole board and its supports as stiff as possible so that there is very little tendency to sag in the middle.

My board is 3/4" ply supported on L girders fixed to the wall. These are made up of two lengths of 2"x1" softwood. This gives support to the rear and each of the two ends. To provide strong support along the length of the span away from the wall I installed two L Girders that run the entire length of the span and that rest on the end wall girders. To make these as stiff as possible I made these from 3"x1" timber for the web and 2"x1" for the top. I stiffened these girders further by affixing two lengths of Spur shelving supports as shown on the photograph. I happened to have these as spare from a dismantled bookcase, but they are not that expensive in the context of the total cost of building a model railway.

The width of the board is 3'

The girders also provided support for lighting for the lower layout.

To hide wiring for the upper layout I made use of cable trunking fitted to the underside of the baseboard.