Extremely sturdy. This plastic is more like the Trex brand of patio planks.  About 78 thick. Hubby put together both chairs in under an hour. They rock well, although a little bumpy.  My only complaint is that my feet dont reach the floor when sitting all the way back.  Id live to get a foot stool to match each chair.  They are plenty wide, and Im 200 lb, 52. I would never get the wood version now that Ive seen these.