We've blogged before about DIY cardboard furniture but at the time there weren't actually instructions. Here's a guide on turning cardboard into lumber, for furniture!
While recycling cardboard curbside is good, the energy it takes to process it is frustratingly large. Turning it into something usable, like furniture or even weed block is a great way to reuse the cardboard without the energy expenditure used to reprocess it.
The basic idea is to make a large, solid piece of cardboard with many layers with alternating corrugations for strength. The layers are glued together (the author uses wheat paste which he says is available for less than $1 per gallon), making sure to keep everything flat and even. Once the glue dries, cut to size and use just as you would regular lumber!
See these photos and complete instructions at by the author at href="http://www.instructables.com/id/Cardboard-Lumber/">Instructables.
All photos from Instructables.