Soil Depth

I am designing my very first flower garden which follows the outside contours of a chain link fence.
One eight foot section will require a flower box since it lays on top of an old foundation-no soil.
My question is how high should a flower box be ? Would 11 inches of soil be adequate? 8 ft long , 33 inches across and 11 inches high. I am sure it would depend on the type of flower. I am interested in a perennial ,mostly shade, in zone 4. Any recommendations for the amount of soil depth? or a flower for that depth?

Since you haven't built the box yet, why not first determine what you want to plant, then you can make the box and the soil in it most suited to the plants. I'd guess a foot or a little more would be enough for most things, plus a few extra inches of height for a thick layer of mulch in winter, but it would depend on the plant.

I think you're probably going to want a good couple of feet for perennials...think of this bed as being sort of like a really wide container that doesn't have very good drainage in the bottom. You're not going to want to plant things that will have their roots down near the bottom of your "container" because that area will tend to stay too wet for many plants. If you're just planting small annuals you can probably get away with something shallower, but for perennials you need to plan for roots that are going to go down deeper. You might want to consider renting a jackhammer and removing the old foundation from that area (or paying someone to come do it for you)--then you can plant anything you want and don't have to worry.

Thank you everyone, that is helpful information

Having a flower box along that stretch of foundation could make a nice display. Make sure you drill holes in the bottom of the box and put a layer of stones on the bottom for drainage. Put the box on some feet to get it off the cement a little. You'll get better drainage and will allow some air flow under the box so the wood won't rot. Along with perennials, add some trailing annuals such as Wave petunias that will flow over the box and you will have color after the perennials quit blooming. As far as box depth for perennials, I would say a foot should do it, but you may have problems in the winter keeping the perennials alive. Although I left some forgotten daylilies in little 1 gal pots outside all winter, and they made it. Much to my surprise. Make sure you send some pictures this summer. Have fun.

After reading all the helpful posts I believe I will use this box for annuals instead of perennials.
That way I can keep the 11 inch depth which allows me to use the wood I have on hand. In addition, in the future , I can move the box and dig up the old foundation. I also will be able to change plants from year to year. The photo shows the other side of fence , opposite of where the box will go . I posted it to show what I am trying to achieve.