I can tell you from experience that runner beans will definitely grow over 8' tall. But that's not a real concern because they'll just droop over and keep growing.

I know this is a vertical gardening question, but you may wanna try posting in the Vegetable Gardening forum. More action there.

I usually plant a row of yard-long climbing beans against my fence and allow them to climb string up the fence. This procedure works well; but I lose the back row that could be used for other crops. This coming year I want to build a planter box to mount on the top of the fence and allow the beans to trail down instead of climb.

Material: The box will be made from treated wood (though many are against use of treated wood since it has poinson in it to protect from insect damage). The poison is arsenic which occurs in minute levels in the soil. I do not believe that edible plants will absorb the arsenic.

Size: I will use 2"X10"X10' boards so the size of the box will be 10 inches deep, 10 inches across and 10 feet long (or the appropriate length between fence posts).

Mounting: I am going to attach the box to each fence post at each end of the box with wood screws and to the 2X4 cross-piece on the fence with the same screws.

Soil and Drainage: I will drill 1/2 inch holes in the bottom of the box every 6 inches to allow good drainage. In the bottom of the box I will place a layer of small rocks to support drainage without loss of soil. I will make my on soil from 1 part garden soil (purchase on sale from Home Depot, ussually about $2.50 a cubic foot), 1 part humus (purchase on sale from Home Depot, ussually about $2.37 a cubic foot), 1 part top soil (purchase on sale from Home Depot, ussually about $1.37 a cubic foot), and 1 part leaf-mold compost (free from the transfer center).

Planting: I will plant two rows of beans staggered, since I want both rows to trail over the same side of the box. The rows will be 3-iches apart and each bean4 inches apart.

Underneath the box I will plant a row of bush beans.

Charlie, are you worry about the weight of the box taking down or damaging the fence? My DH at a rule, does not want me to lean anything on the fence... we have cedar picket fence, 6 ft high.