Spacing of joists on L-girder benchwork?

[exodus]

I've been reading Linn Westcotts' How to Build Model Railroad Benchwork (1996 Edition) for the Nth time, in preparation for building my around-the-walls dream layout. Nowhere in his description and directions for building L-girder benchwork can I find a recommendation (unless I have missed it) as to how far to space joists on top of the girders. By checking the spacing of joists on the layout designs he gives at the end of chapter 5, I find they are spaced on average about 20" apart. However, some spacing is significantly more or less. On my middle-of-the-room 9'x10' flat-top test layout I spaced them at regular 18" intervals (how I arrived at that, I don't remember).

I realize that spacing of joists will vary depending on what they are supporting, but obviously 6' will be too distant and 4" too close. However, does anyone have a suggestion as to what might be a good average spacing (perhaps 20"?) and how much one might reasonably vary from that (say 6" or 12"?). I don't want to waste lumber by spacing at 18", if I can space them at 24." Spacing of the girders is not an issue for me, it's spacing of the joists which I'd appreciate feedback on.

Thanks, Malcolm

[kenneo]

Malcolm --- try reposting in the model RR forums. You'll get better and faster answers there.