I was thumbing through my copy of "The Gardener's Guide to Cactus" by Scott Calhoun, and on page 22 under "Cactus soil mixes for containers" it has Steve Plath's Ultra-Drain growing mix. The book says, "The owner of Signature Botanica nursery outside Phoenix, Steve developed his Ultra-Drain mix after discovering that growing certain species of Mammillaria in straight calcined clay worked wonders where a normal cactus mix failed. "I could water them every day and not rot them," Steve said. Calcined clay is a clay that is fired to 1200-1400 F (650-760 C). It looks like light tan gravel but is extremely absorbent. Most people are familiar with calcined clay because it is the main (and often only) ingredient in kitty litter and bagged oil-spill absorbents."
Has anyone tried growing any sempervivum in calcined clay? Sounds like it might be just the thing, especially in areas with soil that is unsuitable or where rot is a problem.
Because of living in Seattle where there is so much water, I did begin using a mix of almost 50% pumice. And that seems to be doing well this year, although it is only October...if it doesn't work I'll try the calcined clay for next year 
Photos whenever you'd like of your yard James please 
a dream come true! Love the reds in the second photo really pretty! 
for the clay you use, what is the brand and where do you get it from?
