Q. Which type of roof material is very slippery when dry?
First member to answer this question substantially correct and to the satisfaction of NCE head Tom Rausch wins a NACHI Beam www.nachi.org/nachibeam.htm
Metal
Oh, and Joe Hagarty… “manufacturer’s recomendations” is not the correct answer. 
Darrell, nope… keep guessing.
Sorry Nick…that’s the slipperest I’ve come accross? but I’m sure I haven’t seen em all. I guess I’ll find out in a few minutes though. I’m just amazed to be the first to answer…that’s a win all in it’s self! even if it’s wrong.
It’s a tough question.
Would that be clay tile roofing.
Due to the surface of a fired clay a glazed surface exists and has a low friction co-efficient. Dirt would probably assist with slide factor… I would not walk this roof even when dry not for just damage reasons but “sliding reasons”…
Wood shake or shingle roofs I have found to be very slippery when performing work in the wet months… But I guess all roof types are found to be slippery when wet…
But a “slippery when dry” roof is hard to come buy… LOL
How about asbestos-cement shingles.
Slate
I’ll give you a hint… there is a class action settlement regarding this roofing material.
Cement, Nature Guard shingles
Masonite roofing
The suspense is killing me,…I can’t think of anything as slippery as a sheet metal roof. They use them alot in the mountains and areas with heavy snow loads as it just slides right off.???
American Cemwood Cement/Wood Fiber Roofing Shakes
Aggregate.
HardiShake?
cement asbestos shingles
Patrick wins.
Actual answer provided by NCE is “Masonite Woodruf wood shingles”
WOOHOOO
Yes yes…
Something about lifespan and not meeting the H/O expectations…