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buggers said:... its not important how thick the caseing layer is as long as it holds moisture and gives some structure to fruit off of

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1. DepthThe correct depth to apply the casing layer is directly related to the depth of the substrate, Greater amounts of substrate increase yield potential which in turn puts more stress on the casing layer. Prolific first and second flushes can remove a thin casing or damage it's surface structure, thereby limiting future mushroom production. A thin casing layer also lacks the body and moisture holding capacity to support large flushes. ASA GENERAL RULE, THE MORE MUSHROOMS EXPECTED PER SQUARE FOOT OF SURFACE AREA, THE DEEPER THE ASING LAYER. (emphasis in original)
Agaricus growers use a minimum of one inch and maximum of two inches of casing on their beds. Substrate depths of six to eight inches are cased 1.25 to 1.5 inches deep. Substrates deeper than 8 inches are cased 1.5 to 2 inches deep. Nevertheless, experiments in Holland using casing depths of 1 inch and 2 inches demonstrated that the deep casing layer suppored higher levels of microorganisms and produced more mushrooms. (See Visscher, 1975). To gain the full benefits of a casing layer, an absolute minimum depth on bulk substrates is 1 inch. For fruiting on sterilized grain, the casing need not be as deep as for fruitings on bulk substrates. Shallow layers of grain are commonly cased .75 to 1 inch deep.

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buggers said:

i tried a 1 1/4" caseing layer on 5 inches of poo and it never colonized after two weeks... its not important how thick the caseing layer is as long as it holds moisture and gives some structure to fruit off of

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Isn't that a little contradictory?    If it means the difference between colonizing in two days, and not colonizing after two weeks, I'd say that casing depth is very important.

Anyhow, 1/2" has always worked for me really well.  Anything above 1/2" and you can be in for a long wait, with no real benefits.  Remember to apply it as evenly as possible.