Hi anngio, The members of our gardening forums are very informative and are always glad to share their knowledge. You might try posting your question on either the Square Foot Gardening forum or the Container Gardening forum. I hope this helps!

I would not underpin a concert slab into a block foundation wall, especially in northern Ohio. The contractor that told you he could not guarantee against cracking is being very honest and his plan is solid. The type of concrete is very important. Not all grades of concrete are the same. Cement to concrete is like flour is to a cake. Not all cake recipes are the same, similar but not identical. The grade of concrete is dependent on what raw materials are available locally to you. Whether hardeners are added as well as a myriad of other factors including the type of cement used and what other aggregates are used to produce the product where you are. Educate yourself as to what grades of concrete are best for you in your climate zone and what is available from your area. ASTM (American standard testing method) if I recall my acronyms correctly, strength is a number that is important. It tells the hardness of the final product. Moisture content is a number as equally important. These numbers are what the final products quality aim is and the real number is after analysis is done after production. All concrete will crack, eventually. We are in the same zone as northern Ohio and use a 1/4 " per foot slope away from the building and to the lowest level at one end. You would be surprised that some crews mistakenly slope to the house, oops. The more of a slope away the better. You might want to consider a colour tint in the concrete that is easily done.

Thanks for the endorsement. 18 years of continuous change. The pergola is entirely DIY - design, fabrication and assembly. VERY careful measurements. Drilled holes at the corners, then made the notch cuts with a circular saw as far as possible and finished with a hand saw. Cut the narrow edge using a saber saw. Cleaned corners using a chisel. Following assembly, I installed a lag screw at each nested connection. The ogee ends were also cut using a saber saw. My double outside rails are not notched into the post, rather lag bolted using two, 1/2" bolts (2 each direction, 4 at each corner). Post are 6 x 6 pressure treated then skinned with cedar. Been standing over 7 years though the footings were set as part of the patio build in 1998. Good luck with yours.

Back to the original question about changing from ply to cement board... Was the "contractor" originally planning on tiling directly over plywood? If so, thats not typically an approved method. He would need either some sort of backerboard like durock or something like Ditra. I'm pretty sure this guy is taking you to the cleaners. As for the Nuheat mat, if you're talking about just installation, I would think that is quite pricey. All he would have to do is put down a thin layer of thinset and put the mat down in it. Possibly put a layer of SLC over top of it if he's not using Ditra. He would also have to connect the electrical which depending on the state of your house could be pricey since they should be on their own circuit. I'd keep an eye on this guy given the information you've provided. Dan