In my experience it is not the thickness but the design of the subroadbed.
5.2mm (nominal 1/4") Laun can be used if you adequately support it.
Consider using a spline variation:
For a single track 0 gauge cookie cutter raised road bed, mount 1 x 4 risers up to the exact needed height. Then run 1 1/2" wide slit strips of the same laun on a vertical plane fastened parallel along the top of the risers on each side for the subroad bed support. Use spacer tie in pieces of 1 x 4 X 1 1/2 as spline support/spline gauge blocks between risers. Do not have more than 12"-16" of run without a riser or spacer block. Predrill wire management holes in all 1 x 4 risers and spacer tie in pieces
Place rough trimmed 8' long panels of laun on the top of the laun rails and pencil trace up from the bottom sliding lengthwise against a 1/2" spacer off the vertical lattice so as to arrive at a 5" wide subroadbed.
Flip the penciled panel over and carefully cut out that one panel and smoothly sand the edges. Repeat this over and over around the layout. Carefully position all joints between risers so as to provide space for the bottom splice plates .
Note, mock up each and every laun subroadbed joint so as to fasten each bottom splice plate on ONE END with Tightbond III
Run a bead of premium urathan construction adhesive along the top edge of all the 1 1/2" lattice strips. Then carefully place all the pre cut, pre joined subroad bed on the adhesive and let dry over night. A brad gun comes in handy here. Light duty clamps can also assist.
You will have an 10 minute or so open window for position adjustments. Just make sure you have well spread out flat weighted surfaces holding everything in place while drying. Better to use more adhesive than too little.
You will screw up somewhere, you will miss some spots, you will push some adhesive off the top edge. That's OK. Just use enough to allow for a safety margin.
Remember, urathane cinstruction just has to touch both surfaces up to 3/8" apart. Tightbond III like all wood glues needs to be clamped together for a minimum of 30 minutes. Actual spec is 175 lbs per sq. inch clamping pressure.
In less than an hour take a thin piece of cardboard, like a business card, then fold it in half and plow it lightly along the glued crevice to scoop up excess nasty looking glue. Make it pretty.
Use gloves. Clean spilled adhesive immediately off skin with mineral spirits soaked rag then wash hands with soap and water. Do this right away with each glue/skin contact.
Practice subroadbed assembly with some scrap pieces. This will become one of the strongest structual components of any layout. Typically I would use either cork or Homobed roadbed on top. Always make the subroad bed wider than you might think. This will help to attach any scenery terrain material. You could get away with occasional 32" straight spans using this method. Try to keep most risers at 16".
If you feel uncertain with the glue you can screw it or shoot it with 1/4" crown X 1" long staples. IMO, glue is best. Do not screw into end grain.
There are several other ways to develope strong subroad bed.
Perhaps the easiest way is to simply use a 7 ply minimum 3/4" (23/32") Birch/Maple shop grade plywood. Shop grade has a finish that is not furniture stain grade but does have the same strength qualities.
Going to a quality 3/4" thickness panel IME, is the big step. 3/4" seems to provide the strong construction margin the average D.I.Y. needs.
Avoid buncha pieces. Never piece at the very top or very bottom of a grade.
There is no one right way to build good benchwork.
Just run away from the "That'll do" or the "That's good enough" crowd.
Cheating on your layout is like cheating on your spouse, it will catch up to you in one way or another. tt
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