Necessity/value of using cardboard or weed barrier under gravel shed foundation?
If you don't choose to use geotextile, you may (depending on soil conditions) find that over time your gravel migrates into the ground, and the fine soils of the ground migrate up through your gravel layer - that is one thing geotextile is used specifically to prevent.
Weed barrier or cardboard are not geotextile, even if one roll of black plastic &quot;cloth&quot; looks much like another. And weed barrier is of very limited use against weeds, in my experience - new organic material (leaves, etc) get into the spaces in the gravel along with weed seeds and you have new weeds growing on top of the weed barrier in short order, unless you are going to vacuum all the rocks up, wash them and replace them regularly - I don't consider that to be a viable use of my time anywhere, much less under a shed.
Expect your larch in contact with the ground/gravel to rot. Being totally biodegradable oriented means you'll be replacing parts that biodegrade. If you'd like to do a bit less of that but are (apparently) also avoiding concrete, consider elevating the larch on somewhat larger rocks - done right it's lasted a fairly long time in some old barns I've met. The trick is to keep the wood dry, and water likes to wick...