There are various reasons to sheet mulch – for weed and grass suppression as well as building healthy soil with nitrogen-rich compost. For more information, this is a good article by Tricia Diggins written for Ecological Landscape Alliance, “Sheet Mulching.”
In our case, we wanted to use cardboard for sheet mulching to suppress weeds and grass and add a layer of cedar wood chips on top. We were not building soil so we did not wet the cardboard or add any horse manure on top.
Knowing for many months that I was going to use this technique in front of the garden and garage, I collected cardboard from work, bringing home 2-3 boxes per day. The garage was overflowing with cardboard.
When we were ready in early August, Will removed any tape and layered the cardboard like a puzzle so no weeds will growth through cracks and slits in the cardboard.
We have so much cardboard, that we are using it in the garden